High iodine groundwater in the lower Kuitun River in Xinjiang: Evidences from stable carbon isotopes characteristics

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Degradation of organic matter through microbial activities is an important process of iodine enrichment in groundwater systems. The stable carbon isotope ratios of groundwater have certain indicative significance for understanding the formation process of high-iodine groundwater. This study aimed to explore the role of microbiological processes in enriching iodine in high-iodine groundwater downstream of the Kuitun River in China and employed stable carbon isotopes to assess the influence of organic matter biodegradation on groundwater iodine enrichment. The results showed that all groundwater in our study area exhibited reducing conditions and was weakly alkaline, primarily consisting of slightly saline water with dominant anions and cations being Cl - and Na + , respectively. The concentration of I - in groundwater ranged from 51.66 to 552.79 µg/L, with an average of 177.68 µg/L. Approximately 61.54% of the groundwater was highly enriched in iodine. Dissolved inorganic carbon (DIC) concentration in groundwater ranged from 22.97 to 100.85 mg/L, primarily due to microbial degradation of organic matter and weathering dissolution of silicate minerals, primarily consisting of HCO 3 - . DOC concentration ranged from 2.01 to 4.22 mg/L, mainly originating from C3 plants. In reducing environments with abundant organic matter in aquifers, microbial involvement in organic matter decomposition and reducible dissolution of iron minerals were the primary hydro-biogeochemical processes leading to the release of solid-phase iodine in aquifers and its migration into groundwater. The model for the origin of high-iodine groundwater in the study area was of the burial-dissolution type. Iodine Groundwater Stable carbon isotope Dissolved organic carbon Dissolved inorganic carbon Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Iodine (I) is a trace element that significantly impacts human health. The iodine content of groundwater has a direct influence on the intake of drinking water by residents. Adequate supply ensures the normal functioning of the human thyroid gland, while iodine deficiency or excessive intake can lead to serious metabolic disorders (Sun et al., 2017 ). Since the 20th century, high iodine phenomena in regional groundwater have been reported in a variety of locations worldwide. This phenomenon has become increasingly prominent, leading to a public health safety issue. In response, various experts and scholars have begun to investigate the phenomenon of high iodine in water sources and conduct research on high iodine groundwater (Guo et al., 2015 ; Kassim et al., 2014 ; Voutchkova et al., 2014a ). The Chinese government has classified the limits of high iodine groundwater according to the national standard "Delineation of Highly Iodized Water Sources and Highly Iodized Disease Areas" and the industry standard "Delineation of Iodine Deficient Areas and Iodine-Adequate Areas", which classify groundwater into low iodized groundwater (I - <100 µg/L) and highly iodized groundwater (I - ≥100 µg/L) according to the concentration of the iodine. High iodine groundwater is distributed in numerous countries across the globe, including Denmark, Switzerland, Chile, Argentina, Canada, Japan, China, etc. (Álvarez et al., 2016 ; Li et al., 2014 ; Pearce et al., 2013 ; Togo et al., 2016 ; Voutchkova et al., 2014a ; Voutchkova et al., 2014b ; Voutchkova et al., 2017 ). The extremely high concentration of iodine was observed in groundwater in coastal areas of Japan, with the highest concentration reaching 34,000 µg/L (Togo et al., 2016 ). It has been reported that high iodine content in groundwater has been identified in several provinces and cities in China, including eastern coastal regions (Hebei, Tianjin, Shandong, Fujian, Jiangsu), central regions (Henan, Shanxi, Anhui), and northwestern inland regions (Xinjiang, Shaanxi, Inner Mongolia) (Ma et al., 2022 ; Wang et al., 2022 ). These areas are primarily located either in arid to semi-arid inland basins (e.g., Datong Basin, Taiyuan Basin, Hetao Plain, Tarim Basin, etc.) or coastal regions (such as the North China Plain, the Huaihe River Plain) (Alvarez et al., 2016; Li et al., 2014 ; Li, Z., et al., 2022 ; Sun et al., 2021 ; Tang et al., 2013 ; Xue et al., 2022 ; Zhang et al., 2013 ). Among them, the iodine content of groundwater in the North China Plain ranged from 0.88 to 1106 µg/L. Notably, approximately 48.2% of sampling sites exceeding 100 µg/L, which was mainly distributed in the III and IV pressurized aquifers in the coastal zone (Li et al., 2017a ). In the Datong Basin, the iodine content of groundwater varied from 14.4 to 2180 µg/L. Approximately 44.8% of groundwater samples exhibited iodine concentrations exceeding 100 µg/L, which was mainly distributed in the groundwater discharge area in the center of the basin (Li et al., 2014 ; Li et al., 2016 ). In the downstream area of the Kuitun River in Xinjiang, the groundwater iodine content ranged from 13.96 to 574.85 µg/L, with 38.46% of groundwater samples classified as high iodine groundwater, and the overall groundwater I - concentration increased gradually from south to north (Chao et al., 2024a ). From the eastern coastal regions of China to the central basins and plains, extending to the arid inland basins in the northwest, high iodine occurrences were observed in aquifers, and the causes of high iodine groundwater varied in different regions. In groundwater systems, in addition to the prevailing pH, redox environments and water-rock interactions, organic carbon and microorganisms are also important factors influencing the transformation, transport, and release of iodine morphology in hydric soils/sediments. All of these complex hydrologic-biogeochemical processes can affect the transport, release and enrichment of iodine in groundwater (Wang et al., 2022 ). Microbially mediated reductive dissolution of organic matter and iron oxides is widely recognized as a crucial process in the formation of high iodine groundwater (Wang et al., 2021 ). During this process, dissolved organic carbon (DOC) serves as the primary carbon source and energy for microbial metabolism, thereby affecting the redox reactions and transformation of elements to some extent (Yu et al., 2015 ; Yu et al., 2018 ; Xie et al., 2013 ; Zhou et al., 2018 ). The isotopic composition of DOC (δ 13 C DOC ) can indicate the source of organic carbon in groundwater and reflect microbial metabolic activities. Dissolved inorganic carbon (DIC) is an important product of organic matter degradation by microbial activity, and its stable isotope composition (δ 13 C DIC ) can be used to determine its source in groundwater, revealing the microbial degradation process of organic matter in groundwater (Zhang et al., 2021 ). Carbon isotopes in groundwater systems are primarily used for distinguishing organic matter sources and indicating microbial metabolism of organic matter (Zhang et al., 2021 ). Therefore, utilizing stable carbon isotopes in groundwater to characterize the process of organic matter degradation under microbial activity and its impact on iodine enrichment holds significant indicative value. Research on high iodine groundwater have primarily focused on the North China Plain and the Datong Basin in central China, with relatively fewer studies conducted in the arid inland basins of northwest China. The downstream area of the Kuitun River in Xinjiang constituted the primary distribution area of high iodine groundwater in the arid inland basin of Northwest China. This groundwater was largely distributed in the deep confined aquifers, which was different from the distribution in other regions, where the high iodine groundwater was buried in the phreatic aquifers or the shallow confined aquifers. Previous studies have indicated that 73% of the high iodine groundwater in the downstream area of the Kuitun River was buried between 170 and 200 m (in deep confined aquifers). The reductive dissolution of iron oxides in the aquifer and competitive adsorption between HCO 3 - and I - were the main factors affecting the iodine enrichment in the groundwater, and additionally, the weakly alkaline reducing environment, deep sediment layers, and sluggish flow conditions in the groundwater provided favorable conditions for iodine enrichment (Chao et al., 2024a ). Organic matter in groundwater served as nutrients and electron donors for microbial activity, exhibiting strong adsorption capacity for iodine. Under reducing conditions, the metabolic activity of anaerobic microorganisms was enhanced, and iodine complexed with organic matter will be released into groundwater with the degradation of organic matter (Li et al., 2017b ). Therefore, the microbial-mediated degradation of organic matter holds significant importance in studying the enrichment of iodine in groundwater. However, the role of organic matter biodegradation in iodine enrichment in groundwater remains unclear in the study area. Consequently, this study took the deep confined groundwater in the downstream area of the Kuitun River as its research object. It was based on the hydrochemical characteristics of groundwater and stable carbon isotope analysis to identify the degradation process of organic matter under the influence of microorganisms on iodine enrichment in the high iodine groundwater. This will further enhance understanding of the genesis mechanism of high iodine groundwater in Kuitun, Xinjiang, providing theoretical guidance for the protection and effective utilization of groundwater. 2. Material and methods 2.1 Study area The study area is located in the Kuitun River Basin, situated in the middle section of the Tianshan Mountains and the southwestern part of the Jungar Basin in Xinjiang, China (Fig. 1 ), and this basin segment lies deep within the interior of the Eurasian continent and experiences a temperate continental arid desert climate. The long-term average temperature is relatively low at 7.3°C, with an annual average precipitation of 165 mm, and whereas, the evaporation rate is high, reaching up to 2080 mm annually, significantly surpassing the precipitation (Qiao et al., 2022 ). The downstream area of the Kuitun River is relatively sluggish, flat and low-lying topography, with stagnant groundwater runoff and strong evaporation. This results in the area acting as a groundwater discharge area. The geomorphology of the area is mainly alluvial fine-soil plains and alluvial lake plains. The aquifer structure in the study area comprises a multi-layered confined aquifer system, with an upper phreatic aquifer and lower confined aquifers. The depth of the phreatic aquifer generally is less than 10 m, with aquifer thicknesses typically falling within the range of 10 to 30 m. The buried depth of confined aquifers was greater than 30 m, and the buried depth of the confined aquifer water level gradually increases from south to north. In the downstream plain area, groundwater receives lateral recharge from the Jotun Ailisheng Desert, the alluvial plains of the Beishan Mountains, and the impacted fine-soil plain of the Kuitun River. Vertical recharge comes from agricultural irrigation water and rainfall infiltration. The lateral runoff recharge is the most important source of pressurized water in the middle and deep parts of the downstream plains area on the horizontal side, and the vertical infiltration recharge is mainly for the phreatic aquifers. During the Quaternary period, the Kuitun region has consistently remained at the center of sedimentary zone, resulting in the formation of thick sedimentary layers primarily composed of mud and clay. The Quaternary formations are widely distributed across the plain area, with outcrops in the hinterland consisting of alluvial, aeolian, lacustrine, or composite origins, underlain by buried lacustrine and glacial-lacustrine deposits. The lithology of the aquifer in the downstream area comprises a multi-layered structure, from top to bottom consisting of sub-sandy loam, gravelly loam, fine sand, and sandy gravel, with localized occurrences of sub-clay weakly permeable layers (Chao et al., 2024a ), as shown in Fig. 2 . 2.2 Sample collection and pretreatment Previous studies have shown that high iodine groundwater in the Kuitun area was mainly concentrated in the downstream region of the Kuitun River (Chao et al., 2024a ; Wang et al., 1986 ). Therefore, this study was conducted in July 2023 to investigate and collect samples from groundwater wells already in use in the downstream area of the Kuitun River. A total of 13 groups of groundwater samples were collected, with well depths ranging from 90 to 200 m. Among them, 12 groups were identified well depths greater than or equal to 100 m, which were primarily used for agricultural irrigation. One group had a depth less than 100 m, serving as household wells for domestic water supply and yard irrigation. According to the structure of the aquifer downstream of the Kuitun River, all the groundwater collected in this study is deep confined aquifers. The well probe was cleaned before collecting water samples, and then the sampling bottles were rinsed with clear groundwater for three times before collecting groundwater samples, which were sealed and labeled for classification. Cation analyses (major elements and trace elements) were acidified to pH < 2 with appropriate amounts of concentrated nitric acid of superior purity and stored away from light; anion analysis and isotopic determination were performed on filtered water samples directly after collection. All water samples were collected without bubbles in the bottle and stored at 4°C. Additionally, the depth, latitude, and longitude of each groundwater sampling point were recorded on-site, and pH and Eh were measured using a multiparameter portable instrument (HI 8424, HANNA). 2.3 Chemical analysis of groundwater 2.3.1 Conventional water chemical indicators Groundwater cation analysis followed the Chinese National Standard for groundwater quality analysis method (DZ/T 0064-2021) using flame atomic absorption spectrophotometry, and the detection limits for Na + , K + , Ca 2+ , and Mg 2+ were all 0.1 mg/L. HCO 3 - and CO 3 2- were determined using the dual-indicator-neutralization titration method, while Cl - was determined using the silver nitrate titration method, both with detection limits of 1 mg/L. SO 4 2- was determined using the barium chloride titration method with a detection limit of 5 mg/L. The Total dissolved solids (TDS) value was obtained by the summation of the concentrations of eight ions, including Na + , K + , Ca 2+ , Mg 2+ , HCO 3 - , CO 3 2- , Cl - , SO 4 2- (Dai, 2006 ). Fe was determined using a TAS-990 atomic absorption spectrophotometer. I - in groundwater was determined using the T6 New Century UV-Visible spectrophotometer according to the groundwater quality analysis method (DZ/T 0064-2021). The iodide starch spectrophotometric method was employed, and the detection limit was set at 10 µg/L. The measurement range was between 10 and 500 µg/L. Groundwater samples exceeding this measurement range needed to be diluted before analysis. 2.3.2 Stable carbon isotope The determination of DIC, δ 13 C DIC , DOC, and δ 13 C DOC in groundwater was performed using the ISOPRIME100 isotope ratio mass spectrometer from Elementar and the total organic carbon analyzer ISO TOC CUBE. The analysis was conducted by Chengdu Baihui Biotechnology Co., Ltd. in China. Determination of DIC and δ 13 C DIC : 8 drops of anhydrous phosphoric acid were added to a 12 ml headspace vial, which was then sealed with a cap. Under helium gas conditions with a flow rate of 100 mL/min and purity > 99.999%, an automatic sample introduction needle was used to evacuate each sealed sample vial for 300 s. This process aimed to eliminate the influence of residual air in the sample vial on the measurement results of the carbon isotope ratio of the sample (Atekwana and Krishnamurthy, 1998 ; Wassenaar et al., 1989 ). After the evacuation process, 0.2 mL of groundwater sample was added to the vial. After centrifugation at 4000 r/min for 2 minutes, sampling was carried out using an automatic sampler. CO 2 was separated from gas mixture (high-purity helium and CO 2 ) using a gas chromatography column at 75°C and injected into the Delta V detector for analysis. Upon bombardment by high-energy electron beams, ionization occurred, forming gaseous ions with different mass-to-charge ratios such as m/z 44–46. These ions were separated in a magnetic field, converted into electrical signals by a receiver, and ultimately used to determine the carbon isotope ratio (Sharp, 2007 ). Determination of DOC and δ 13 C DOC : Transparent glass sample bottles with a volume of 40 mL were selected and washed three times with ultrapure water. Subsequently, the bottles were placed in a muffle furnace at 500°C and incinerated for 6 hours to remove organic carbon. Afterward, approximately 15 mL of filtered water sample through a 0.45 µm membrane filter was added to the prepared sample bottle. Then, high-purity concentrated hydrochloric acid solution was added dropwise to adjust the pH to 2. After ultrasonic agitation for 15 minutes, the sample was analyzed using a total organic carbon analyzer-stable isotope mass spectrometer combination system (Yu et al., 2018 ). In the experiment, the flow rate of helium gas was set to 100 mL/min, with oxidation and reduction tube temperatures set at 850°C and 600°C respectively. The adsorption and desorption temperatures of the CO 2 adsorption column were both 230°C, while the working temperature of the infrared detector was 40°C. The voltage for the stable isotope mass spectrometer ion source was set to 300 µA. 3. Results 3.1 Groundwater hydrochemical characteristics The statistical table of groundwater chemical indicators in the downstream area of Kuitun River is shown in Table 1 . The pH range of groundwater in the study area was 7.79 to 9.34, with an average value of 8.43, indicating weak alkaline to alkaline conditions. The Eh of groundwater ranged from − 101.40 to -17.20 mV, with an average value of -52.53 mV, and the Eh values of groundwater samples were all less than 0, indicating a reducing environment. The dominant cation in groundwater was Na + , followed by Ca 2+ and Mg 2+ , with K + concentration being the lowest. The dominant anion was Cl - , followed by SO 4 2- and HCO 3 - , with CO 3 2- concentration being the lowest. The coefficients of variation for both Mg 2+ and Cl - were greater than 1, which was a strong variation, indicating a wide range of variation in the concentration of these two ions in groundwater. The TDS concentration of groundwater ranged from 475.75 to 6834.64 mg/L, with an average of 2629.21 mg/L and a median of 2103.61 mg/L. Among them, there were 4, 6 and 3 groups of fresh water (TDS < 1000 mg/L), slightly saline water (1000 ≤ TDS < 3000 mg/L) and saline water (3000 ≤ TDS < 10000 mg/L), respectively. They accounted for 30.77%, 46.15% and 23.08% of the total samples, respectively. The groundwater was mainly dominated by brackish water. The classification of groundwater hydrochemical types with reference to Shukarev's classification method showed that there were eight groundwater hydrochemical types in the study area, and the main hydrochemical types were dominated by SO 4 ·Cl-Na type (23.08%), SO 4 ·Cl-Na·Ca type (23.08%), and SO 4 ·Cl-Na·Mg type (15.38%). Besides the sulfate type, there also existed a hydrochemical type dominated by bicarbonates, accounting for 15.38% of the total. Table 1 Statistical table of characteristic parameters of groundwater hydrochemistry index Index Min Max Mean Median Coefficient of variation pH 7.79 9.34 8.43 8.12 0.07 Eh (mV) -101.40 -17.20 -52.53 -31.30 -0.64 K + (mg/L) 1.01 10.24 5.27 5.10 0.64 Na + (mg/L) 78.06 1516.77 553.00 487.74 0.85 Ca 2+ (mg/L) 28.71 537.63 205.17 187.31 0.83 Mg 2+ (mg/L) 1.45 659.55 145.93 87.45 1.32 Cl − (mg/L) 38.20 3121.17 878.10 367.59 1.18 SO 4 2− (mg/L) 179.67 1323.33 666.50 651.89 0.64 HCO 3 − (mg/L) 79.43 229.94 165.00 156.82 0.24 CO 3 2− (mg/L) 3.62 18.50 10.23 9.13 0.39 TDS (mg/L) 475.75 6834.64 2629.21 2103.61 0.78 Fe (mg/L) 0.09 0.53 0.26 0.26 0.43 I − (µg/L) 51.66 552.79 177.68 134.24 0.77 According to the relevant regulations of the Chinese national standard "Delineation of Highly Iodized Water Sources and Highly Iodized Disease Areas" and the industry standard "Delineation of Iodine Deficient Areas and Iodine-Adequate Areas", the groundwater in the study area was classified into low iodine water (I - 300 µg/L) based on iodine content. There were 5 groups of low iodine water, accounting for 38.46% of the groundwater samples, while high iodine water and ultra-high iodine water accounted for 6 groups and 2 groups, respectively, comprising 46.15% and 15.39% of the groundwater samples. The range of I - concentration in groundwater was between 51.66 ~ 552.79 µg/L, with a median value of 134.24 µg/L and an average value of 177.68 µg/L. 61.54% of the groundwaters with I - concentration greater than 100 µg/L were characterized as high iodine groundwater. Based on the iodide classification and concentration limits of toxicological indicators in China's Groundwater Quality Standard (GB/T 14848 − 2017), Class I and II groundwater requires I - ≤40 µg/L, Class III water is limited to 40 µg/L < I - ≤80 µg/L, Class IV water is limited to 80 µg/L < I - ≤500 µg/L, and Class V water is limited to I - concentration of greater than 500 µg/L. In the study area, there were 2 groups of Class III groundwater, 12 groups of Class IV groundwater, and 1 group of Class V groundwater. Iodide was used as the sole indicator to classify groundwater. Consequently, the majority of groundwater in the study area was classified as Class IV, with no Class I or Class II groundwater present. 3.2 Characterisation of DIC and DOC in groundwater The relationship between DIC and DOC concentrations in groundwater of the study area is depicted in Fig. 3 . The DIC concentration in groundwater ranged from 22.97 to 100.85 mg/L, with an average of 66.04 mg/L. DIC in water bodies primarily exists in three forms: HCO 3 - , H 2 CO 3 , and CO 3 2- , with the specific form dependent on the pH of the water. In the study area, the pH of groundwater ranged from 7.79 to 9.34. Therefore, HCO 3 - was the predominant form of DIC in groundwater. The DOC concentration ranged from 2.01 to 4.22 mg/L, with an average of 2.79 mg/L. Previous study has indicated that the average concentration of DOC in natural water bodies is approximately 5 mg/L (Thurman, 2012 ). However, when domestic sewage and industrial wastewater are discharged into water bodies, the DOC concentration can exceed 5 mg/L by a significant margin. The concentration of DOC in the groundwater of the study area ranged from 2.01 to 4.22 mg/L, which indicates that the groundwater was not affected by the pollution of anthropogenic activities. In low iodine groundwater, the DIC concentration ranged from 48.34 to 74.50 mg/L, with an average of 57.16 mg/L, and the DOC concentration ranged from 2.01 to 2.70 mg/L, with an average of 2.26 mg/L. In high iodine groundwater, the DIC concentration ranged from 22.97 to 100.85 mg/L, with an average of 71.58 mg/L, and the DOC concentration ranged from 2.31 to 4.22 mg/L, with an average of 3.11 mg/L. Compared to low iodine groundwater, high iodine groundwater exhibited higher average values of both DIC and DOC. 3.3 Characteristics of stable carbon isotopes in groundwater The relationship between δ 13 C DIC and δ 13 C DOC in groundwater is illustrated in Fig. 4 . The δ 13 C DIC values in groundwater ranged from − 24.04‰ to -16.39‰, with an average of -20.00‰. Among them, the δ 13 C DIC values in low iodine groundwater ranged from − 21.74‰ to -16.39‰, with an average of -18.34‰, while those in high iodine groundwater ranged from − 24.04‰ to -17.71‰, with an average of -21.04‰. From the comparative analysis of the distribution range and mean value of δ 13 C DIC , it can be seen that the δ 13 C DIC value of high iodine groundwater was significantly more negative than that of low iodine groundwater. From Fig. 4 , the δ 13 C DOC values of groundwater in the study area ranged from − 29.58‰ to -26.79‰, with an average value of -28.51‰. Specifically, the δ 13 C DOC values of low iodine groundwater ranged from − 29.28‰ to -28.41‰, with an average of -28.99‰, while the δ 13 C DOC values of high iodine groundwater ranged from − 29.58‰ to -26.79‰, with an average of -28.20‰. The δ 13 C DOC values of high iodine groundwater exhibited a broader range, whereas those of low iodine groundwater are more concentrated and lower in comparison to high iodine groundwater. 4. Discussion 4.1 Source analysis of DIC and DOC in groundwater Groundwater DIC is mainly derived from the contribution of atmospheric CO 2 , metabolic decomposition activities of organic matter by microbial action, and weathering dissolution of carbonate and silicate minerals in the aquifer (Barth et al., 2003 ; Yang et al., 1996 ; Zhou et al., 2018 ). Different sources of DIC exhibit distinct characteristic ranges in carbon isotopes (Li et al., 2022 ). Previous studies have shown that when [HCO 3 - ]/[Ca 2+ +Mg 2+ ] < 2, there are multiple potential sources of HCO 3 - in the water (Song et al., 2020 ). In the study area, approximately 77% of groundwater samples had [HCO 3 - ]/[Ca 2+ +Mg 2+ ] values less than 2, indicating that groundwater DIC was influenced by multiple sources. The buried depth of groundwater in the study area was in the range of 90–200 m, which was confined groundwater and less affected by CO 2 in the air. DIC originating from carbonate rock dissolution typically exhibits higher δ 13 C values. When the δ 13 C DIC value of groundwater is around − 11‰, it indicates that DIC primarily originates from carbonate rock dissolution (Clark and Fritz, 2013 ; Jin et al., 2014 ; Li et al., 2022 ). However, when the δ 13 C DIC value is smaller than − 11‰, it suggests that other processes may affect the magnitude of δ 13 C DIC values. In relatively confined groundwater environments, weathering of silicate minerals produces HCO 3 - carbon isotope values δ 13 C DIC of -17‰ (Porowska, 2015 ; Zhang et al., 2015 ). The δ 13 C DIC values (-17.71‰, -16.83‰, -16.39‰) of the three samples in groundwater were about the range of δ 13 C DIC values produced by silicate mineral weathering. Combined with the previous studies on the influence of hydrogeochemical processes on iodine enrichment in the lower reaches of Kuitun River (Chao et al., 2024a ), it can be seen that the rock weathering type of groundwater in the study area was mainly affected by silicate weathering and dissolution, indicating that the weathering and dissolution of silicate minerals was part of the source of DIC in groundwater. When microbial degradation processes of organic matter in aquifers preferentially tend to utilize lighter 12 C, which enriches the products with lighter 12 C and leads to fractionation of 13 C, their reactants are enriched with larger 13 C (Chao et al., 2024b ). Consequently, microbial degradation of organic matter tends to result in smaller δ 13 C DIC values compared to weathering dissolution of silicate minerals and carbonate rocks. Previous studies have confirmed that the biological degradation of organic matter leads to negative shifts in δ 13 C DIC values (Clark and Fritz, 2013 ; Schulte et al., 2011 ). The δ 13 C values of DIC released by microbial decomposition of organic matter range from − 25‰ to -18‰ (Truesdell and Hulston, 1980 ). The relationship between δ 13 C DIC and DOC in groundwater is shown in Fig. 5 . In the study area, 76.92% of groundwater δ 13 C DIC values were within the range of δ 13 C DIC values produced by microbial degradation of organic matter. Therefore, the main source of groundwater DIC was influenced by microbial degradation of organic matter, with additional effects from the dissolution of silicate minerals. As can be seen in Fig. 5 , 80% of low iodine groundwater δ 13 C DIC values were around − 18‰, and groundwater δ 13 C DIC values in high iodine areas were in the range of -24.04‰ to -16.83‰ (mean value is -21.02‰). Compared with the low iodine groundwater, the mean δ 13 C DIC value of the high iodine groundwater was more on the small side, indicating that the microorganisms in the high iodine groundwater degraded the organic matter more. The δ 13 C values of DOC from different carbon sources exhibit distinct characteristics. The δ 13 C values in soil humus are related to the type of vegetation in the region. The δ 13 C DOC values of C3 plants (such as trees, wheat, cotton, etc.) range between − 35‰ and − 20‰, while those of C4 plants (such as maize, sorghum, and sugarcane) range from − 19‰ to -8‰ (Brookman and Ambrose, 2013 ; Clark and Fritz, 2013 ). The δ 13 C DOC values of Crassulacean Acid Metabolism (CAM) plants fall within the range of -22‰ to -10‰ (Hedges et al., 1997 ). The isotopic composition of CAM plants typically falls between the values of C3 and C4 plants' δ 13 C DOC ranges. In the study area, the δ 13 C DOC values in groundwater were distributed within the range of δ 13 C DOC values of C3 plants and were smaller compared to C4 and CAM plants. The Kuitun region has experienced many geological activities in which the vertical uplift process has led to the formation of variations in surface vegetation and has been in the center of the depositional zones in various stages during the Quaternary, forming the geological condition of a deep sedimentary layer, which is mainly dominated by mud, clayey soil and humus (Hong, 1983 ; Luo et al., 2017 ). The surface vegetation in the study area was mainly covered by cotton and trees, and the deep sedimentary layer and aquifer conditions were formed during the long period of geological movement and surface evolution; therefore, the groundwater DOC was mainly derived from C3 plants. 4.2 Indicative significance of stable carbon isotope characteristics for iodine enrichment The relationship between δ 13 C DIC and DIC in groundwater is illustrated in Fig. 6 . The δ 13 C DIC values exhibited a negative correlation with DIC ( r =-0.545, P = 0.054), which was close to the significant level. The smaller the δ 13 C DIC value was, the stronger the microbial activity was, and the higher the concentration of DIC was. The predominant form of DIC in groundwater was HCO 3 - , indicating that the organic matter degraded by microbes was one of the important sources of HCO 3 - in groundwater DIC. The δ 13 C DIC -δ 13 C DOC difference in groundwater in the study area had a highly significant positive correlation ( r = 0.959, P < 0.01) with δ 13 C DIC (Fig. 7 a), suggesting that the smaller the δ 13 C DIC in groundwater was, the greater the contribution of organic matter degradation to DIC in groundwater, the more DIC was produced, and the oxidative decomposition of DOC played a role in this process. When the value of δ 13 C DIC -δ 13 C DOC in groundwater is larger, it indicates that the dissolution of carbonate rocks and silicate is the main source of DIC; on the contrary if the value of δ 13 C DIC -δ 13 C DOC is smaller, it indicates that more inorganic carbon comes from the oxidative decomposition of organic matter, and the stronger microbial action. The difference of δ 13 C DIC -δ 13 C DOC in groundwater showed a significant negative correlation ( r =-0.591, P = 0.034) with the I - concentration (Fig. 7 b), indicating that the more DIC was produced by microbial degradation of organic matter, the smaller the δ 13 C DIC value was and the higher the I - concentration was. The δ 13 C DIC value in groundwater was significantly negatively correlated with I - concentration ( r =-0.637, P = 0.019) (Fig. 7 c), and the high iodine groundwater was mainly distributed in the lower side region of Fig. 7 c. On the whole, as δ 13 C DIC values became more negative, microbial activity strengthens, leading to higher I - concentrations. This suggested that microbial degradation of organic matter promoted the enrichment of iodide in groundwater. Numerous studies have shown that organic matter and iron oxide minerals in sediments are the main carriers of iodine (Dai et al., 2009 ; Hansen et al., 2011 ; Wang et al., 2021 ; Wang et al., 2022 ). In the study area, groundwater was in a reduced state, and anaerobic microorganisms utilized organic matter in sediments as a carbon source for degradation. During the decomposition of organic matter, iodine adsorbed on the surface of organic matter was released into the aquifer, leading to an increase in iodine concentration in groundwater, and at the same time, microbial action caused the δ 13 C DIC value of the groundwater to be constantly small (Fig. 7 c). In addition to the microbial degradation of organic matter, the reductive dissolution of iron oxide minerals was another important mechanism for the formation of high iodine groundwater. Microorganisms were able to use Fe(III) oxides/hydroxides as electron acceptors to reduce Fe(III) to Fe(II) by oxidizing aquifer organic matter, a process that resulted in the migration of iodine adsorbed on the surfaces of minerals such as iron oxides and its released into groundwater (Tao et al., 2022 ). There was a significant positive correlation between I - concentration and Fe content in high iodine groundwater ( r = 0.755, P = 0.03) (Fig. 7 d), which, combined with the fact that the groundwater in the study area was under reducing conditions, suggested that reductive dissolution of iron oxide minerals occurs, and iodine endowed on the surface of the minerals was released, resulting in the migration of solid-phase iodine to the aquifer, and causing the iodine concentration to increase in the groundwater to form high iodine groundwater. In the biogeochemical process of high iodine groundwater formation with the participation of microorganisms, organic matter and DOC in the sediments provided the main carbon and energy sources for the metabolic activities of microorganisms, and organic carbon was decomposed into inorganic carbon under the action of microorganisms, resulting in carbon transformation and fractionation. When the available carbon source to microorganisms increased, it promoted the metabolism of heterotrophic microorganisms and consumes oxygen, forming a reducing environment that was more favorable for iodine enrichment in groundwater (Chao et al., 2024b ). 4.3 Formation mechanism of high iodine groundwater Naturally occurring poor-quality groundwater is formed over time through the interaction between water and rock during the natural cycle. High iodine groundwater is the result of complex hydrological and biogeochemical processes acting over long periods and is one of the typical types of poor-quality groundwater (Wang et al., 2022 ). Based on the genesis mode of this type, due to the differences in different environments and hydrogeological conditions, the genesis mechanism of high iodine groundwater was categorized into four types, namely, burial-dissolution, evaporation-concentration, compaction-release, and leaching-enrichment (Wang et al., 2021 ). Both biotic and abiotic hydrogeochemical processes control iodine transport and enrichment. In Denmark, iodine concentrations in groundwater reached up to 500 mg/L, with brackish water being the main contributor to iodine levels (Voutchkova et al., 2017 ). In Chile, iodine concentrations in groundwater were as high as 6096 mg/L, which was attributed to the presence of nitrate-rich sedimentary formations (Alvarez et al., 2015 ). In coastal regions of Japan, groundwater contained exceptionally high concentrations of iodine, with levels reaching up to 34000 µg/L, linked to iodine-rich brines formed by local geological activity (Togo et al., 2016 ). Elevated iodine concentrations in groundwater in countries like Denmark, Chile, and Japan were associated with hydrogeological conditions of the aquifers. Microbial activity is considered an important factor contributing to iodine enrichment in groundwater (Amachi, 2008 ). In reducing groundwater environments, iron-reducing bacteria utilized lactic acid as a substance supplying electrons and receiving oxidation, and used the Fe-mineral phase in aquifer sediments as a receptor for reduction, and the adsorbed iodine in this process was released to groundwater and existed in the form of I - , which was the main mechanism for the formation of high iodine groundwater in the Datong basin, China. In this study, the groundwater was in reduction, and the iodine adsorbed on the surface of enriched organic matter and iron oxides was released and migrated to the groundwater to form high iodine groundwater through the participation of microorganisms. This mechanism was consistent with the model of high iodine groundwater genesis summarized by the genesis of poor-quality groundwater, i.e., “burial-dissolution type” (Wang et al., 2021 ). Specifically, under conditions rich in organic matter and long-term stability, microbial activity and the reductive dissolution of iron minerals were the primary processes leading to the migration and released of solid-phase iodine into groundwater. The high iodine groundwater in the study area has long been situated in a stable reducing environment and the burial depth of high iodine groundwater mainly concentrated in the range of 110–200 m, which belonged to the deep layer of confined water. The stronger the microbial activities were, the higher the I - concentration and Fe concentration were. The microbial decomposition of organic matter and the reductive dissolution of iron minerals were the main hydrobiogeochemical processes leading to the release of solid-phase iodine in the aquifer and its migration into groundwater in Kuitun River Basin. Combined with the hydrogeological conditions to which the study area belonged (Fig. 2 ), the deep sedimentary layers dominated by muddy and clayey materials were rich in organic matter, and therefore, the genesis mode of high iodine groundwater in the study area was of the burial-dissolution type. 4.4 Comparative analysis of stable carbon isotope signature and I - concentration of groundwater in different regions Different carbon sources possess certain differences in δ 13 C values, which will lead to different changes in groundwater DIC and its δ 13 C value, and the δ 13 C value change rule of groundwater DIC can be utilized to analyze the main sources of groundwater DIC and reveal the law of chemical evolution of groundwater. The characteristics of δ 13 C value and I - concentration of groundwater DIC in different regions are shown in Table 2 . Sracek and Hirata ( 2002 ) used δ 13 C DIC values in conjunction with groundwater major ion concentrations and hydrogen and oxygen isotopes to study the hydrochemical evolution of groundwater in the Guarani aquifer in the State of São Paulo, Brazil, and concluded that dissolution of calcite and cation exchange under closed-system conditions occurs in the groundwater. J. Rueedi et al. ( 2007 ) analyzed the relationship between pH and DIC and δ 13 C DIC values using groundwater carbon isotope signatures, depth to groundwater, partial pressure of CO 2 , pH, and concentrations of relevant ionic fractions, thus determining that the main sources of groundwater DIC in the three cities of the United Kingdom are soil CO 2 , dolomite dissolution, and human effluent, and they identified the evolution pathways of DIC, including calcite dissolution evolution under open system conditions, gypsum dissolution evolution under closed system conditions, and calcite isotope dissolution evolution. P. Moeller et al. ( 2008 ) analyzed the variability of groundwater δ 13 C DIC values in the northern German Basin and showed that dissolution of limestone in open and closed systems-controlled groundwater chemical evolution, primarily manifested by the dissolution of calcite and dolomite. Porowska ( 2015 ) utilized stable carbon isotopes to determine the sources of DIC in groundwater around the Otwock landfill site. The results showed that, under natural conditions, the concentration and isotopic composition of groundwater DIC mainly originated from the organic matter decomposition in aquifer sediments and the dissolution of carbonates. In contrast, groundwater contaminated by leachate depended on the degradation of organic matter in aquifer sediments and the biodegradation of organic matter stored in the landfill site. In China, Zhu et al. ( 2021 ) demonstrated the carbon isotopes of groundwater in the Datong Basin that the primary source of DIC in the recharge zone aquifer was the dissolution of carbonate rocks, while microbial activity had a significant influence on DIC sources in the runoff and excretion zones. Zhou ( 2018 ) found in their research on the carbon isotopes of groundwater in the Hetao Basin that DIC in the piedmont and transition zone was mainly influenced by carbonate rock leaching and atmospheric precipitation recharge, with some influence from microbial activity in certain transition zone groundwater, although its contribution was relatively small. However, in the plain area, microbial activity had an increasingly significant influence on groundwater DIC. Li et al. ( 2022 ) found that chemical weathering of aluminosilicate minerals in the aquifer of the North China Plain had a dominant effect on the δ 13 C DIC value of groundwater in the region. Yuan et al. ( 2020 ) found that microbial oxidation of organic matter and dissolution of carbonate rocks were the primary sources of DIC in groundwater in the Jianghan Plain. The δ 13 C DIC values of groundwater in the study area ranged from − 24.04‰ to -16.39‰. DIC was mainly influenced by microbial degradation of organic matter and partial weathering dissolution of silicate minerals. The overall δ 13 C DIC values of the groundwater were small compared with those of other areas (Table 2 ), suggesting that other areas were more enriched in 13 C. The sources of DIC in groundwater in different regions are different, and their δ 13 C DIC values have different characteristic ranges. The combined effects of various reactions in the groundwater system have an important influence on the composition of DIC and δ 13 C values in this system. Table 2 Range of δ 13 C DIC , δ 13 C DOC values, I − concentrations and depths in groundwater in different regions. Country Study area δ 13 C DIC /‰ δ 13 C DOC /‰ I − (µg/L) Depth/m References Brazil Guarani, São Paulo State -19.00~-5.20 - - - Sracek and Hirata, 2002 Britain British Midlands -20.05 ~ 2.96 - - 8.24 ~ 76.28 Rueedi et al., 2007 Germany North German Basin -22.70~-3.70 - - 86 ~ 1616 Moeller et al., 2008 Poland Suburb of Otwock -20.60 ~ 3.60 - - - Porowska, 2015 China Kuitun, Xinjiang -24.04~-16.39 -29.58~-26.79 51.66 ~ 552.79 90 ~ 200 This study Datong Basin -16.93~-7.36 - 14.40 ~ 1030.00 16 ~ 75 Zhu et al., 2021 Hetao Plain -11.80~-5.34 -22.90~-19.20 31.84 ~ 1289.57 15 ~ 80 Wang et al., 2014 ; Zhou, 2018 North China Plain -11.42~-5.95 - 4 ~ 2175 10 ~ 860 Li J., et al., 2022 Jianghan Plain -18.50~-3.28 -28.50~-19.60 2 ~ 1600 15 ~ 40 Fan et al., 2022 ; Yuan et al., 2020 In the study area, the range of δ 13 C DOC values in groundwater was − 29.58‰ to -26.79‰, which was relatively similar compared to the Hetao Plain (-22.9‰ to -19.20‰), but the average δ 13 C DOC value in the Hetao Plain was higher. The factors influencing the δ 13 C DOC value depend on the types of endogenous and exogenous organic matter in the aquifer. The range of δ 13 C DOC values in the Jianghan Plain (-28.5‰ to -19.60‰) varied widely. The Hetao Plain, Jianghan Plain, and Kuitun River Basin in Xinjiang are located in the northern Yellow River alluvial plain, middle reaches of the Yangtze River, and northwest arid inland basin, respectively. The soil organic carbon, vegetation types, and groundwater organic matter vary across different regions. Soil organic carbon is one of the exogenous carbon sources to groundwater, which can enter shallow or deep groundwater to influence groundwater DOC and δ 13 C DOC values due to rainfall and irrigation infiltration. For instance, the groundwater burial depth in the Hetao Plain and Jianghan Plain was as low as 15 m. The DOC and δ 13 C DOC values in groundwater may be influenced by inputs of soil organic carbon. Vegetation from the surface can be buried into the aquifer over prolonged geological activity, resulting in groundwater organic matter with varying DOC and δ 13 C DOC values across different vegetation types. Therefore, the δ 13 C DOC values in groundwater will vary due to differences in external organic matter (soil organic carbon) and endogenous organic matter across different regions. High iodine groundwater in the study area was mainly distributed below a burial depth of 90 m, representing deep confined aquifers, which were deeper compared to the shallow groundwater distribution in the Datong Basin, Hetao Plain, and Jianghan Plain. In reducing environments of groundwater, when microbial activity on organic matter is strong, microbes preferentially utilize more 12 C, leading to a lower δ 13 C DIC value in the resulting degradation product DIC. Comparing with the other four high iodine groundwater distribution zones in China, the δ 13 C DIC value of the groundwaters of the study area was overall smaller, and it was hypothesized that microbial activity in the groundwater of the downstream area of the Kuitun River in Xinjiang may be relatively strong. 5. Conclusions This study used stable carbon isotope technology to represent the influence of organic matter degradation process under the action of microorganisms on iodine enrichment, and deeply explored the formation mechanism of high iodine groundwater in this region. The following conclusions were drawn: 1. The concentration of I - in the groundwater in the study area ranged from 51.66 to 552.79 µg/L, with an average value of 177.68 µg/L, and the percentages of low iodine water, high iodine water and ultra-high iodine water were 38.46%, 46.15% and 15.39%, respectively. The groundwater as a whole was reductive and weakly alkaline, and the dominant anion and cation were Cl - and Na + respectively. Groundwater was dominated by brackish water, accounting for 46.15%. The hydrochemical type of groundwater was dominated by sulfuric acid type. According to the "Groundwater Quality Standards of China," groundwater was mainly classified as Class IV groundwater, with no Class I or Class II groundwater. 2. The groundwater DIC concentration in the study area ranged from 22.97 to 100.85 mg/L, while the DOC concentration ranged from 2.01 to 4.22 mg/L. The δ 13 C DIC values ranged from − 24.04‰ to -16.39‰, and the δ 13 C DOC values ranged from − 29.58‰ to -26.79‰. The range and mean of distribution of δ 13 C DIC values for high iodine groundwaters were significantly more negative than those for low iodine groundwaters. Groundwater DIC was primarily affected by the degradation of organic matter by microorganisms and the weathering and dissolution of silicate minerals, with HCO 3 - being the dominated anion. DOC was mainly derived from C3 plants. 3. In reducing environments with abundant organic matter in aquifers, the primary hydro-biogeochemical processes leading to the release of solid-phase iodine in aquifers and its migration into groundwater were microbial involvement in organic matter decomposition and the reducible dissolution of iron minerals. The genesis model of high iodine groundwater being burial-dissolution type. Declarations 6.1 Ethical Approval Not applicable. 6.2 Consent to Participate Not applicable. 6.3 Consent to Publish Consent for publications was obtained from the participants. 6.4 Authors Contributions Conceptualization, B.C. and Y.L.; methodology, B.C. and L.D.; software, B.C. and J.H.; Validation, B.C.; formal analysis, B.C., J.H. and L.D.; investigation, B.C., Y.L., L.D.; resources, Y.L.; data curation, B.C. and L.D.; writing-original draft preparation, B.C. and J.H.; writing-review and editing, B.C., J.H., Y.L., L.D., Q.Z., X.X., M.W., Z.S., X.L.; visualization, B.C. and J.H.; supervision, Y.L.; project administration, Y.L.; funding acquisition, Y.L.; All authors have read and agreed to the published version of the manuscript. 6.5 Funding The research work was financially supported by National Natural Science Foundation of China (41761097). 6.6 Competing Interests The authors have no competing interests to declare that are relevant to the content of this article.. 6.7 Availability of data and materials The data that support the findings of this study are available from the corresponding authors upon reasonable request. References Alvarez F, Reich M, Pérez-Fodich A, Snyder G, Muramatsu Y, Vargas G, Fehn U (2015) Sources, sinks and long-term cycling of iodine in the hyperarid Atacama continental margin. Geochim Cosmochim Acta 161:50–70. https://doi.org/10.1016/j.jhydrol.2022.127517 Álvarez F, Reich M, Snyder G, Pérez-Fodich A, Muramatsu Y, Daniele L, Fehn U (2016) Iodine budget in surface waters from Atacama: Natural and anthropogenic iodine sources revealed by halogen geochemistry and iodine-129 isotopes. Appl Geochem 68:53–63. https://doi.org/10.1016/j.apgeochem.2016.03.011 Amachi S (2008) Microbial contribution to global iodine cycling: volatilization, accumulation, reduction, oxidation, and sorption of iodine. Microbes Environ 23(4):269–276. https://doi.org/10.1264/jsme2.ME08548 Atekwana EA, Krishnamurthy RV (1998) Seasonal variations of dissolved inorganic carbon and δ 13 C of surface waters: application of a modified gas evolution technique. J Hydrol 205(3):265–278. https://doi.org/10.1016/S0022-1694(98)00080-8 Barth JA, Cronin AA, Dunlop J, Kalin RM (2003) Influence of carbonates on the riverine carbon cycle in an anthropogenically dominated catchment basin: evidence from major elements and stable carbon isotopes in the Lagan River (N. Ireland). Chem Geol 200(3):203–216. https://doi.org/10.1016/S0009-2541(03)00193-1 Brookman TH, Ambrose SH (2013) Kangaroo tooth enamel oxygen and carbon isotope variation on a latitudinal transect in southern Australia: implications for palaeoenvironmental reconstruction. Oecologia 171:403–416. https://doi.org/10.1007/s00442-012-2425-6 Chao B, Luo Y, Dong L, Zheng Y, Song Z, Wang M, Liu C (2024a) Characterizing the spatial distribution of iodine and analyzing its influential factors in a typical groundwater area with high arsenic content in Xinjiang. Acta Sci Circum 44(4):156–167. 10.13671/j.hjkxxb.2023.0435 (in Chinese) Chao B, Luo Y, Wang X (2024b) Stable carbon isotope signatures of high arsenic groundwater and their indicative significancein inKuitun area of Xinjiang. Environ Chem 43(3):1–10. 10.7524/j.issn.0254-6108.2022091102 (in Chinese) Clark ID, Fritz P (2013) Environmental isotopes in hydrogeology[M]. CRC, BocaRaton Dai JL, Zhang M, Hu QH, Huang YZ, Wang RQ, Zhu YG (2009) Adsorption and desorption of iodine by various Chinese soils: II. Iodide and iodate. Geoderma 153(1–2):130–135. https://doi.org/10.1016/j.geoderma.2009.07.020 Dai S (2006) Environmental Chemistry. Higher Education Press, Beijing Fan R, Deng Y, Xue J (2022) Predicting the Spatial Distribution of Geogenic Iodine-contaminatedGroundwater of Jianghan Plain Based on ExtremeGradient Boost Model. Saf Environ Eng 29(5):70–77. 10.13578/j.cnki.issn.1671-1556.20221107 (in Chinese) Guo H, Zhang Z, Cheng G, Li W, Li T, Jiao JJ (2015) Groundwater-derived land subsidence in the North China Plain. Environ Earth Sci 74:1415–1427. https://doi.org/10.1007/s12665-015-4131-2 Hansen V, Roos P, Aldahan A, Hou X, Possnert G (2011) Partition of iodine ( 129 I and 127 I) isotopes in soils and marine sediments. J Environ Radioact 102(12):1096–1104. https://doi.org/10.1016/j.jenvrad.2011.07.005 Hedges JI, Keil RG, Benner R (1997) What happens to terrestrial organic matter in the ocean? Org Geochem 27(5–6):195–212. https://doi.org/10.1016/S0146-6380(97)00066-1 Hong L (1983) A preliminary study on the disease and formation environment of high fluorine and arsenic water in Chepaizi area, north of Kuitun, Xinjiang. Xinjiang Environmental Protection. 5 (1): 22–28. DOI: CNKI:SUN:XJHB.0.1983-01-004. (in Chinese) Jin L, Ogrinc N, Yesavage T, Hasenmueller EA, Ma L, Sullivan PL, Kaye J, Duffy C, Brantley SL (2014) The CO 2 consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory. Geochim Cosmochim Acta 142:260–280. https://doi.org/10.1016/j.gca.2014.07.006 Kassim IA, Moloney G, Busili A, Nur AY, Paron P, Jooste P, Gadain H, Seal AJ (2014) Iodine intake in Somalia is excessive and associated with the source of household drinking water. J Nutr 144(3):375–381. https://doi.org/10.3945/jn.113.176693 Li J, Jiang Z, Xie X, Wang Y (2022) Mechanisms of iodine enrichment in the pore-water of fluvial/lacustrine aquifer systems: Insight from stable carbon isotopes and batch experiments. J Hydrol 613:128334. https://doi.org/10.1016/j.jhydrol.2022.128334 Li J, Qian K, Yang Y, Xie X (2017a) Iodine speciation and its potential influence on iodine enrichment in groundwater from North China Plain. Procedia Earth Planet Sci 17:312–315. https://doi.org/10.1016/j.proeps.2016.12.069 Li J, Wang Y, Guo W, Xie X, Zhang L, Liu Y, Kong S (2014) Iodine mobilization in groundwater system at Datong basin, China: evidence from hydrochemistry and fluorescence characteristics. Sci Total Environ 468:738–745. https://doi.org/10.1016/j.scitotenv.2013.08.092 Li J, Wang Y, Xie X, DePaolo DJ (2016) Effects of water-sediment interaction and irrigation practices on iodine enrichment in shallow groundwater. J Hydrol 543:293–304. https://doi.org/10.1016/j.jhydrol.2016.10.002 Li J, Zhou H, Wang Y, Xie X, Qian K (2017b) Sorption and speciation of iodine in groundwater system: The roles of organic matter and organic-mineral complexes. J Contam Hydrol 201:39–47. https://doi.org/10.1016/j.jconhyd.2017.04.008 Li Z, Cao W, Ren Y, Pan D, Wang S, Zhi C (2022) Enrichment mechanisms for the co-occurrence of arsenic-fluoride-iodine in the groundwater in different sedimentary environments of the Hetao Basin, China. Sci Total Environ 839:156184. https://doi.org/10.1016/j.scitotenv.2022.156184 Luo Y, Li J, Jiang P, Xing Y (2017) Distribution, classification and cause analysis of geogenic high-arsenicgroundwater in Kuitun,Xinjiang. Acta Sci Circum 37(8):2897–2903. 10.13671/j.hjkxxb.2017.0019 (in Chinese) Ma R, Yan M, Han P, Wang T, Li B, Zhou S, Zheng T, Hu T, Borthwick AG, Zheng C, Ni J (2022) Deficiency and excess of groundwater iodine and their health associations. Nat Commun 13(1):7354. https://doi.org/10.1038/s41467-022-35042-6 Moeller P, Weise SM, Tesmer M, Dulski P, Pekdeger A, Bayer U, Magri F (2008) Salinization of groundwater in the North German Basin: results from conjoint investigation of major, trace element and multi-isotope distribution. Int J Earth Sci 97:1057–1073. https://doi.org/10.1007/s00531-007-0211-1 Pearce EN, Andersson M, Zimmermann MB (2013) Global iodine nutrition: where do we stand in 2013? Thyroid 23(5):523–528. https://doi.org/10.1089/thy.2013.0128 Porowska D (2015) Determination of the origin of dissolved inorganic carbon in groundwater around a reclaimed landfill in Otwock using stable carbon isotopes. Waste Manag 39:216–225. https://doi.org/10.1016/j.wasman.2015.01.044 Qiao C, Wang Y, Liu Y, Li J, Zhang H, Lu J (2022) Study of Water Resources Optimal Operation Model of Multireservoir: A Case Study of Kuitun River Basin in Northwestern China. Wireless Communications and Mobile Computing. 2022, 17. https://doi.org/10.1155/2022/7715398 Rueedi J, Cronin AA, Taylor RG, Morris BL (2007) Tracing sources of carbon in urban groundwater using δ 13 C TDIC ratios. Environ Geol 52:541–557. https://doi.org/10.1007/s00254-006-0486-8 Schulte P, Van Geldern R, Freitag H, Karim A, Négrel P, Petelet-Giraud E, Probst A, Probst J, Telmer K, Veizer J, Barth JA (2011) Applications of stable water and carbon isotopes in watershed research: Weathering, carbon cycling, and water balances. Earth Sci Rev 109(1–2):20–31. https://doi.org/10.1016/j.earscirev.2011.07.003 Sharp Z (2007) Principles of stable isotope geochemistry. Pearson education. Upper Saddle River, NJ Song C, Wang G, Mao T, Huang K, Sun X, Hu Z, Chang R, Chen X, Raymond PA (2020) Spatiotemporal variability and sources of DIC in permafrost catchments of the Yangtze River source region: Insights from stable carbon isotope and water chemistry. Water Resour Res 56(1). https://doi.org/10.1029/2019WR025343 . e2019WR025343 Sracek O, Hirata R (2002) Geochemical and stable isotopic evolution of the Guarani Aquifer System in the state of São Paulo, Brazil. Hydrogeol J 10:643–655. https://doi.org/10.1007/s10040-002-0222-8 Sun D, Codling K, Chang S, Zhang S, Shen H, Su X, Chen Z, Scherpbier R, Yan J (2017) Eliminating iodine deficiency in China: achievements, challenges and global implications. Nutrients 9(4):361. https://doi.org/10.3390/nu9040361 Sun Y, Zhou J, Liang X, Zhou Y, Zeng Y, Lin L (2021) Distribution and Genesis of Shallow High-Iodine Groundwater in Southern Margin of Tarim Basin: A Case Study of Plain Area in Minfeng County. Xinjiang Earth Sci 46(8):2999–3011. https://doi.org/10.3799/dqkx.2020.260 (in Chinese) Tang Q, Xu Q, Zhang F, Huang Y, Liu J, Wang X, Yang Y, Liu X (2013) Geochemistry of iodine-rich groundwater in the Taiyuan Basin of central Shanxi Province, North China. J Geochem Explor 135:117–123. https://doi.org/10.1016/j.gexplo.2012.08.019 Tao Y, Du Y, Deng Y, Huang Y, Leng Z, Ma T, Wang Y (2022) Carbon and iron isotope approach elucidates the enrichment of geogenic phosphorus in alluvial-lacustrine sedimentary aquifers. J Hydrol 607:127517. https://doi.org/10.1016/j.jhydrol.2022.127517 Thurman EM (2012) Organic geochemistry of natural waters(Vol. 2). Springer Science & Business Media, Berlin Togo YS, Takahashi Y, Amano Y, Matsuzaki H, Suzuki Y, Terada Y, Muramatsu Y, Ito K, Iwatsuki T (2016) Age and speciation of iodine in groundwater and mudstones of the Horonobe area, Hokkaido, Japan: implications for the origin and migration of iodine during basin evolution. Geochim Cosmochim Acta 191:165–186. https://doi.org/10.1016/j.gca.2016.07.012 Truesdell A, Hulston J (1980) Chapter 5-isotopic evidence on environments of geothermal systems. Elsevier. Amsterdam Voutchkova DD, Ernstsen V, Hansen B, Sørensen BL, Zhang C, Kristiansen SM (2014a) Assessment of spatial variation in drinking water iodine and its implications for dietary intake: A new conceptual model for Denmark. Sci Total Environ 493:432–444. https://doi.org/10.1016/j.scitotenv.2014.06.008 Voutchkova DD, Ernstsen V, Kristiansen SM, Hansen B (2017) Iodine in major Danish aquifers. Environ Earth Sci 76:1–16. https://doi.org/10.1007/s12665-017-6775-6 Voutchkova DD, Kristiansen SM, Hansen B, Ernstsen V, Sørensen BL, Esbensen KH (2014b) Iodine concentrations in Danish groundwater: historical data assessment 1933–2011. Environ Geochem Health 36:1151–1164. https://doi.org/10.1007/s10653-014-9625-4 Wang LF, Wang HM, Lin FF, Sun XZ, Xu XF, Su MY, Lin J, Feng ZY, Fan XQ (1986) A preliminary investigation of endemic iodine goiter on the inclined plain in front of the Tian-Shan Mountains in Kuitun-Usum area. Chin Med J 99(8):673–678 Wang Y, Li J, Ma T, Xie X, Deng Y, Gan Y (2021) Genesis of geogenic contaminated groundwater: As, F and I. Crit Rev Environ Sci Technol 51(24):2895–2933. https://doi.org/10.1080/10643389.2020.1807452 Wang Y, Li J, Xie X (2022) Genesis and occurrence of high iodine groundwater. Earth Science Frontiers. 2022, 29(3): 1–10. 10.13745/j.esf.sf.2022.1.47 . (in Chinese) Wang Y, Ma T, Dong Y, Xu F, Yan Y, Liu L (2014) The formation of inland-high-iodine groundwater:A case study in Hangjinhou-qi, Hetao Plain. Earth Sci Front 21(4):66–73. 10.13745/j.esf.2014.04.007 (in Chinese) Wassenaar L, Aravena R, Fritz P (1989) The geochemistry and evolution of natural organic solutes in groundwater. Radiocarbon 31(3):865–876. 10.1017/s0033822200012480 Xue J, Deng Y, Luo Y, Du Y, Yang Y, Cheng Y, Xie X, Gan Y, Wang Y (2022) Unraveling the impact of iron oxides-organic matter complexes on iodine mobilization in alluvial-lacustrine aquifers from central Yangtze River Basin. Sci Total Environ 814:151930. https://doi.org/10.1016/j.scitotenv.2021.151930 Xie X, Wang Y, Ellis A, Li J, Su C, Duan M (2013) Multiple isotope (O, S and C) approach elucidates the enrichment of arsenic in the groundwater from the Datong Basin, northern China. J Hydrol 498:103–112. https://doi.org/10.1016/j.jhydrol.2013.06.024 Yang C, Telmer K, Veizer J (1996) Chemical dynamics of the St. Lawrence riverine system: δD H2O , δ 18 O H2O , δ 13 C DIC , δ 34 S sulfate , and dissolved 87 Sr/ 86 Sr. Geochim Cosmochim Acta 60(5):851–866. https://doi.org/10.1016/0016-7037(95)00445-9 Yu K, Gan Y, Zhou A, Han L, Liu Y (2015) A persulfate oxidation method for stable isotope analysis of dissolved organic carbon and the influence of inorganic ions on the results. Int J Mass Spectrom 392:63–68. https://doi.org/10.1016/j.ijms.2015.09.006 Yu K, Gan Y, Zhou A, Liu C, Duan Y, Han L, Zhang Y (2018) Organic carbon sources and controlling processes on aquifer arsenic cycling in the Jianghan Plain, central China. Chemosphere 208:773–781. https://doi.org/10.1016/j.chemosphere.2018.05.188 Yuan X, Deng Y, Du Y (2020) Characteristics of stable carbon isotopes and its implications on arsenic enrichmentin shallow groundwater of the Jianghan Plain. Bull Geol Sci Technol 39(5):156–163. 10.19509/j.cnki.dzkq.2021.0008 (in Chinese) Zhang D, Liu C, Wang F, Zhao Z (2015) Inorganic carbon cycling in subsurface environment influenced by agricultural activities. China Environ Sci 35(11):3359–3370. 10.3969/j.issn.1000-6923.2015.11.022 (in Chinese) Zhang E, Wang Y, Qian Y, Ma T, Zhang D, Zhan H, Zhang Z, Fei Y, Wang S (2013) Iodine in groundwater of the North China Plain: spatial patterns and hydrogeochemical processes of enrichment. J Geochem Explor 135:40–53. https://doi.org/10.1016/j.gexplo.2012.11.016 Zhang Y, Gan Y, Yu K, Han L (2021) Fractionation of carbon isotopes of dissolved organic matter adsorbed to goethite in the presence of arsenic to study the origin of DOM in groundwater. Environ Geochem Health 43:1225–1238. https://doi.org/10.1007/s10653-020-00644-w Zhou Y (2018) Biogeochemical processes in high arsenic groundwater in the northwestern Hetao Basin, Inner Mongolia: Evidences from hydrogeochemistry and stable isotopes (Doctoral dissertation). China University of Geosciences, Beijing Zhou Y, Guo H, Zhang Z, Lu H, Jia Y, Cao Y (2018) Characteristics and implication of stable carbon isotope in high arsenic groundwater systems in the northwest Hetao Basin, Inner Mongolia, China. J Asian Earth Sci 163:70–79. https://doi.org/10.1016/j.jseaes.2018.05.018 Zhu C, Li J, Xie X (2021) Carbon and Sulfur Isotopic Features and Its Implications for IodineMobilization in Groundwater System at Datong Basin, Northern China. Earth Sci 46(12):4480–4491. https://doi.org/10.3799/dqkx.2021.090 (in Chinese) Additional Declarations No competing interests reported. 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University","correspondingAuthor":false,"prefix":"","firstName":"Jiale","middleName":"","lastName":"He","suffix":""},{"id":414149372,"identity":"16648448-09c6-4a34-ab47-973292b4abed","order_by":2,"name":"Yanli Luo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAxUlEQVRIiWNgGAWjYBACPhDxoIKBGczjIUYLG4hIOEOylsQ2KI84LRLJxyQS5x1m55+RwPjgbRuDvDlhLWlpEonb0pglbiQwG85tYzDc2UBQS44ZUIsNM8ONBDZp3jaGBIMDRGmZI8EsfyOB/TcJWhpsmA2AtjATp4XnWbJFwrE0ZsMzD5sl55yTMNxASAs/e/LBGx9qDifLHU8++OFNmY08QVuAgEUCSCQzMDA2AGkJwuqBgPkDkLAjSukoGAWjYBSMTAAA8e43ULvsP5kAAAAASUVORK5CYII=","orcid":"","institution":"Xinjiang Agricultural University","correspondingAuthor":true,"prefix":"","firstName":"Yanli","middleName":"","lastName":"Luo","suffix":""},{"id":414149373,"identity":"737cf7e7-0929-436f-81ff-a7d65006593d","order_by":3,"name":"Lele Dong","email":"","orcid":"","institution":"Xinjiang Agricultural University","correspondingAuthor":false,"prefix":"","firstName":"Lele","middleName":"","lastName":"Dong","suffix":""},{"id":414149374,"identity":"9dd28f2c-d286-4af1-b67b-1de814dc3fc8","order_by":4,"name":"Qian Zhang","email":"","orcid":"","institution":"Xinjiang Agricultural University","correspondingAuthor":false,"prefix":"","firstName":"Qian","middleName":"","lastName":"Zhang","suffix":""},{"id":414149375,"identity":"a4b2f2b5-f480-499c-b745-b9327752a7e0","order_by":5,"name":"Xinzhe Xie","email":"","orcid":"","institution":"Xinjiang Agricultural University","correspondingAuthor":false,"prefix":"","firstName":"Xinzhe","middleName":"","lastName":"Xie","suffix":""},{"id":414149379,"identity":"f4732829-3269-4984-8687-a9fb44431843","order_by":6,"name":"Meijuan Wang","email":"","orcid":"","institution":"Xinjiang Agricultural University","correspondingAuthor":false,"prefix":"","firstName":"Meijuan","middleName":"","lastName":"Wang","suffix":""},{"id":414149380,"identity":"1432c095-1e2e-4b6b-b3ad-92458d70a38f","order_by":7,"name":"Zhen Song","email":"","orcid":"","institution":"Xinjiang Agricultural University","correspondingAuthor":false,"prefix":"","firstName":"Zhen","middleName":"","lastName":"Song","suffix":""},{"id":414149381,"identity":"d5ed7ca7-9633-4e1f-9aa2-60c3b10528e4","order_by":8,"name":"Xuan Liu","email":"","orcid":"","institution":"Xinjiang Agricultural University","correspondingAuthor":false,"prefix":"","firstName":"Xuan","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2025-02-08 06:38:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5985611/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5985611/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":76231208,"identity":"a9f97922-dd7c-4006-af65-3e4ebf80cf5f","added_by":"auto","created_at":"2025-02-13 18:22:17","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":207705,"visible":true,"origin":"","legend":"\u003cp\u003eMap of the study area and sampling point distribution in the research area\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-5985611/v1/7f2bef724a6c4b1cb31f7a54.png"},{"id":76231210,"identity":"838855ff-e45b-4536-8d77-8eaa041c1189","added_by":"auto","created_at":"2025-02-13 18:22:17","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":350835,"visible":true,"origin":"","legend":"\u003cp\u003eRegional hydrogeologic profile of the downstream area of the Kuitun River\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-5985611/v1/50a1564c8dbe3aea0dd59c3e.png"},{"id":76231209,"identity":"4f18f31b-c3ec-4d79-b550-7df42eb5c0cd","added_by":"auto","created_at":"2025-02-13 18:22:17","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":73585,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between DIC and DOC in groundwater\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-5985611/v1/145b675bf62ba0876b1cf893.png"},{"id":76231795,"identity":"946dab44-784d-4e3c-b49e-46e550258663","added_by":"auto","created_at":"2025-02-13 18:30:17","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":93382,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e in groundwater\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-5985611/v1/9500a895afc474392a3fe95a.png"},{"id":76231227,"identity":"1e0e0db1-2999-4658-9427-b97576c7377d","added_by":"auto","created_at":"2025-02-13 18:22:17","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":113092,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e and DOC in groundwater\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-5985611/v1/6aedfbe3d39c0fa056df338f.png"},{"id":76231228,"identity":"24604ed5-c9f5-4481-8e46-3d3cf20f56ef","added_by":"auto","created_at":"2025-02-13 18:22:17","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":98755,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e and DIC in groundwater\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-5985611/v1/9c7c2a5a6733d3a86d91eb85.png"},{"id":76231212,"identity":"f834bf65-6528-4613-8928-92a77832f0b0","added_by":"auto","created_at":"2025-02-13 18:22:17","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":199231,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e-δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e (a); I\u003csup\u003e-\u003c/sup\u003e concentration (b) in groundwater; Relationship between I\u003csup\u003e-\u003c/sup\u003e concentration and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e (c); Relationship between I\u003csup\u003e-\u003c/sup\u003e concentration and Fe concentration in high iodine groundwater (d)\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-5985611/v1/a5fe90d9d93dc9369be737f6.png"},{"id":79776505,"identity":"f09fcfce-3531-4551-b16b-1fac46a5914b","added_by":"auto","created_at":"2025-04-02 14:16:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2216824,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5985611/v1/dfbb46c3-5bf1-4e70-8143-3370cc5c438b.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"High iodine groundwater in the lower Kuitun River in Xinjiang: Evidences from stable carbon isotopes characteristics","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIodine (I) is a trace element that significantly impacts human health. The iodine content of groundwater has a direct influence on the intake of drinking water by residents. Adequate supply ensures the normal functioning of the human thyroid gland, while iodine deficiency or excessive intake can lead to serious metabolic disorders (Sun et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Since the 20th century, high iodine phenomena in regional groundwater have been reported in a variety of locations worldwide. This phenomenon has become increasingly prominent, leading to a public health safety issue. In response, various experts and scholars have begun to investigate the phenomenon of high iodine in water sources and conduct research on high iodine groundwater (Guo et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Kassim et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Voutchkova et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2014a\u003c/span\u003e). The Chinese government has classified the limits of high iodine groundwater according to the national standard \"Delineation of Highly Iodized Water Sources and Highly Iodized Disease Areas\" and the industry standard \"Delineation of Iodine Deficient Areas and Iodine-Adequate Areas\", which classify groundwater into low iodized groundwater (I\u003csup\u003e-\u003c/sup\u003e\u0026lt;100 \u0026micro;g/L) and highly iodized groundwater (I\u003csup\u003e-\u003c/sup\u003e\u0026ge;100 \u0026micro;g/L) according to the concentration of the iodine.\u003c/p\u003e \u003cp\u003eHigh iodine groundwater is distributed in numerous countries across the globe, including Denmark, Switzerland, Chile, Argentina, Canada, Japan, China, etc. (\u0026Aacute;lvarez et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Pearce et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Togo et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Voutchkova et al., \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2014a\u003c/span\u003e; Voutchkova et al., \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2014b\u003c/span\u003e; Voutchkova et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The extremely high concentration of iodine was observed in groundwater in coastal areas of Japan, with the highest concentration reaching 34,000 \u0026micro;g/L (Togo et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). It has been reported that high iodine content in groundwater has been identified in several provinces and cities in China, including eastern coastal regions (Hebei, Tianjin, Shandong, Fujian, Jiangsu), central regions (Henan, Shanxi, Anhui), and northwestern inland regions (Xinjiang, Shaanxi, Inner Mongolia) (Ma et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). These areas are primarily located either in arid to semi-arid inland basins (e.g., Datong Basin, Taiyuan Basin, Hetao Plain, Tarim Basin, etc.) or coastal regions (such as the North China Plain, the Huaihe River Plain) (Alvarez et al., 2016; Li et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Li, Z., et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Sun et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Tang et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Xue et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Among them, the iodine content of groundwater in the North China Plain ranged from 0.88 to 1106 \u0026micro;g/L. Notably, approximately 48.2% of sampling sites exceeding 100 \u0026micro;g/L, which was mainly distributed in the III and IV pressurized aquifers in the coastal zone (Li et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2017a\u003c/span\u003e). In the Datong Basin, the iodine content of groundwater varied from 14.4 to 2180 \u0026micro;g/L. Approximately 44.8% of groundwater samples exhibited iodine concentrations exceeding 100 \u0026micro;g/L, which was mainly distributed in the groundwater discharge area in the center of the basin (Li et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). In the downstream area of the Kuitun River in Xinjiang, the groundwater iodine content ranged from 13.96 to 574.85 \u0026micro;g/L, with 38.46% of groundwater samples classified as high iodine groundwater, and the overall groundwater I\u003csup\u003e-\u003c/sup\u003e concentration increased gradually from south to north (Chao et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e). From the eastern coastal regions of China to the central basins and plains, extending to the arid inland basins in the northwest, high iodine occurrences were observed in aquifers, and the causes of high iodine groundwater varied in different regions.\u003c/p\u003e \u003cp\u003eIn groundwater systems, in addition to the prevailing pH, redox environments and water-rock interactions, organic carbon and microorganisms are also important factors influencing the transformation, transport, and release of iodine morphology in hydric soils/sediments. All of these complex hydrologic-biogeochemical processes can affect the transport, release and enrichment of iodine in groundwater (Wang et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Microbially mediated reductive dissolution of organic matter and iron oxides is widely recognized as a crucial process in the formation of high iodine groundwater (Wang et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). During this process, dissolved organic carbon (DOC) serves as the primary carbon source and energy for microbial metabolism, thereby affecting the redox reactions and transformation of elements to some extent (Yu et al., \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Yu et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Xie et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Zhou et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The isotopic composition of DOC (δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e) can indicate the source of organic carbon in groundwater and reflect microbial metabolic activities. Dissolved inorganic carbon (DIC) is an important product of organic matter degradation by microbial activity, and its stable isotope composition (δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e) can be used to determine its source in groundwater, revealing the microbial degradation process of organic matter in groundwater (Zhang et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Carbon isotopes in groundwater systems are primarily used for distinguishing organic matter sources and indicating microbial metabolism of organic matter (Zhang et al., \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Therefore, utilizing stable carbon isotopes in groundwater to characterize the process of organic matter degradation under microbial activity and its impact on iodine enrichment holds significant indicative value.\u003c/p\u003e \u003cp\u003eResearch on high iodine groundwater have primarily focused on the North China Plain and the Datong Basin in central China, with relatively fewer studies conducted in the arid inland basins of northwest China. The downstream area of the Kuitun River in Xinjiang constituted the primary distribution area of high iodine groundwater in the arid inland basin of Northwest China. This groundwater was largely distributed in the deep confined aquifers, which was different from the distribution in other regions, where the high iodine groundwater was buried in the phreatic aquifers or the shallow confined aquifers. Previous studies have indicated that 73% of the high iodine groundwater in the downstream area of the Kuitun River was buried between 170 and 200 m (in deep confined aquifers). The reductive dissolution of iron oxides in the aquifer and competitive adsorption between HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and I\u003csup\u003e-\u003c/sup\u003e were the main factors affecting the iodine enrichment in the groundwater, and additionally, the weakly alkaline reducing environment, deep sediment layers, and sluggish flow conditions in the groundwater provided favorable conditions for iodine enrichment (Chao et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e). Organic matter in groundwater served as nutrients and electron donors for microbial activity, exhibiting strong adsorption capacity for iodine. Under reducing conditions, the metabolic activity of anaerobic microorganisms was enhanced, and iodine complexed with organic matter will be released into groundwater with the degradation of organic matter (Li et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2017b\u003c/span\u003e). Therefore, the microbial-mediated degradation of organic matter holds significant importance in studying the enrichment of iodine in groundwater. However, the role of organic matter biodegradation in iodine enrichment in groundwater remains unclear in the study area. Consequently, this study took the deep confined groundwater in the downstream area of the Kuitun River as its research object. It was based on the hydrochemical characteristics of groundwater and stable carbon isotope analysis to identify the degradation process of organic matter under the influence of microorganisms on iodine enrichment in the high iodine groundwater. This will further enhance understanding of the genesis mechanism of high iodine groundwater in Kuitun, Xinjiang, providing theoretical guidance for the protection and effective utilization of groundwater.\u003c/p\u003e"},{"header":"2. Material and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Study area\u003c/h2\u003e \u003cp\u003eThe study area is located in the Kuitun River Basin, situated in the middle section of the Tianshan Mountains and the southwestern part of the Jungar Basin in Xinjiang, China (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), and this basin segment lies deep within the interior of the Eurasian continent and experiences a temperate continental arid desert climate. The long-term average temperature is relatively low at 7.3\u0026deg;C, with an annual average precipitation of 165 mm, and whereas, the evaporation rate is high, reaching up to 2080 mm annually, significantly surpassing the precipitation (Qiao et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe downstream area of the Kuitun River is relatively sluggish, flat and low-lying topography, with stagnant groundwater runoff and strong evaporation. This results in the area acting as a groundwater discharge area. The geomorphology of the area is mainly alluvial fine-soil plains and alluvial lake plains. The aquifer structure in the study area comprises a multi-layered confined aquifer system, with an upper phreatic aquifer and lower confined aquifers. The depth of the phreatic aquifer generally is less than 10 m, with aquifer thicknesses typically falling within the range of 10 to 30 m. The buried depth of confined aquifers was greater than 30 m, and the buried depth of the confined aquifer water level gradually increases from south to north. In the downstream plain area, groundwater receives lateral recharge from the Jotun Ailisheng Desert, the alluvial plains of the Beishan Mountains, and the impacted fine-soil plain of the Kuitun River. Vertical recharge comes from agricultural irrigation water and rainfall infiltration. The lateral runoff recharge is the most important source of pressurized water in the middle and deep parts of the downstream plains area on the horizontal side, and the vertical infiltration recharge is mainly for the phreatic aquifers. During the Quaternary period, the Kuitun region has consistently remained at the center of sedimentary zone, resulting in the formation of thick sedimentary layers primarily composed of mud and clay. The Quaternary formations are widely distributed across the plain area, with outcrops in the hinterland consisting of alluvial, aeolian, lacustrine, or composite origins, underlain by buried lacustrine and glacial-lacustrine deposits. The lithology of the aquifer in the downstream area comprises a multi-layered structure, from top to bottom consisting of sub-sandy loam, gravelly loam, fine sand, and sandy gravel, with localized occurrences of sub-clay weakly permeable layers (Chao et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Sample collection and pretreatment\u003c/h2\u003e \u003cp\u003ePrevious studies have shown that high iodine groundwater in the Kuitun area was mainly concentrated in the downstream region of the Kuitun River (Chao et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e1986\u003c/span\u003e). Therefore, this study was conducted in July 2023 to investigate and collect samples from groundwater wells already in use in the downstream area of the Kuitun River. A total of 13 groups of groundwater samples were collected, with well depths ranging from 90 to 200 m. Among them, 12 groups were identified well depths greater than or equal to 100 m, which were primarily used for agricultural irrigation. One group had a depth less than 100 m, serving as household wells for domestic water supply and yard irrigation. According to the structure of the aquifer downstream of the Kuitun River, all the groundwater collected in this study is deep confined aquifers. The well probe was cleaned before collecting water samples, and then the sampling bottles were rinsed with clear groundwater for three times before collecting groundwater samples, which were sealed and labeled for classification. Cation analyses (major elements and trace elements) were acidified to pH\u0026thinsp;\u0026lt;\u0026thinsp;2 with appropriate amounts of concentrated nitric acid of superior purity and stored away from light; anion analysis and isotopic determination were performed on filtered water samples directly after collection. All water samples were collected without bubbles in the bottle and stored at 4\u0026deg;C. Additionally, the depth, latitude, and longitude of each groundwater sampling point were recorded on-site, and pH and Eh were measured using a multiparameter portable instrument (HI 8424, HANNA).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Chemical analysis of groundwater\u003c/h2\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.3.1 Conventional water chemical indicators\u003c/h2\u003e \u003cp\u003eGroundwater cation analysis followed the Chinese National Standard for groundwater quality analysis method (DZ/T 0064-2021) using flame atomic absorption spectrophotometry, and the detection limits for Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Ca\u003csup\u003e2+\u003c/sup\u003e, and Mg\u003csup\u003e2+\u003c/sup\u003e were all 0.1 mg/L. HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e were determined using the dual-indicator-neutralization titration method, while Cl\u003csup\u003e-\u003c/sup\u003e was determined using the silver nitrate titration method, both with detection limits of 1 mg/L. SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e was determined using the barium chloride titration method with a detection limit of 5 mg/L. The Total dissolved solids (TDS) value was obtained by the summation of the concentrations of eight ions, including Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Ca\u003csup\u003e2+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e, HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e, Cl\u003csup\u003e-\u003c/sup\u003e, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e (Dai, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Fe was determined using a TAS-990 atomic absorption spectrophotometer. I\u003csup\u003e-\u003c/sup\u003e in groundwater was determined using the T6 New Century UV-Visible spectrophotometer according to the groundwater quality analysis method (DZ/T 0064-2021). The iodide starch spectrophotometric method was employed, and the detection limit was set at 10 \u0026micro;g/L. The measurement range was between 10 and 500 \u0026micro;g/L. Groundwater samples exceeding this measurement range needed to be diluted before analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.3.2 Stable carbon isotope\u003c/h2\u003e \u003cp\u003eThe determination of DIC, δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e, DOC, and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e in groundwater was performed using the ISOPRIME100 isotope ratio mass spectrometer from Elementar and the total organic carbon analyzer ISO TOC CUBE. The analysis was conducted by Chengdu Baihui Biotechnology Co., Ltd. in China. Determination of DIC and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e: 8 drops of anhydrous phosphoric acid were added to a 12 ml headspace vial, which was then sealed with a cap. Under helium gas conditions with a flow rate of 100 mL/min and purity\u0026thinsp;\u0026gt;\u0026thinsp;99.999%, an automatic sample introduction needle was used to evacuate each sealed sample vial for 300 s. This process aimed to eliminate the influence of residual air in the sample vial on the measurement results of the carbon isotope ratio of the sample (Atekwana and Krishnamurthy, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Wassenaar et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). After the evacuation process, 0.2 mL of groundwater sample was added to the vial. After centrifugation at 4000 r/min for 2 minutes, sampling was carried out using an automatic sampler. CO\u003csub\u003e2\u003c/sub\u003e was separated from gas mixture (high-purity helium and CO\u003csub\u003e2\u003c/sub\u003e) using a gas chromatography column at 75\u0026deg;C and injected into the Delta V detector for analysis. Upon bombardment by high-energy electron beams, ionization occurred, forming gaseous ions with different mass-to-charge ratios such as m/z 44\u0026ndash;46. These ions were separated in a magnetic field, converted into electrical signals by a receiver, and ultimately used to determine the carbon isotope ratio (Sharp, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2007\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDetermination of DOC and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e: Transparent glass sample bottles with a volume of 40 mL were selected and washed three times with ultrapure water. Subsequently, the bottles were placed in a muffle furnace at 500\u0026deg;C and incinerated for 6 hours to remove organic carbon. Afterward, approximately 15 mL of filtered water sample through a 0.45 \u0026micro;m membrane filter was added to the prepared sample bottle. Then, high-purity concentrated hydrochloric acid solution was added dropwise to adjust the pH to 2. After ultrasonic agitation for 15 minutes, the sample was analyzed using a total organic carbon analyzer-stable isotope mass spectrometer combination system (Yu et al., \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). In the experiment, the flow rate of helium gas was set to 100 mL/min, with oxidation and reduction tube temperatures set at 850\u0026deg;C and 600\u0026deg;C respectively. The adsorption and desorption temperatures of the CO\u003csub\u003e2\u003c/sub\u003e adsorption column were both 230\u0026deg;C, while the working temperature of the infrared detector was 40\u0026deg;C. The voltage for the stable isotope mass spectrometer ion source was set to 300 \u0026micro;A.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Groundwater hydrochemical characteristics\u003c/h2\u003e \u003cp\u003eThe statistical table of groundwater chemical indicators in the downstream area of Kuitun River is shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The pH range of groundwater in the study area was 7.79 to 9.34, with an average value of 8.43, indicating weak alkaline to alkaline conditions. The Eh of groundwater ranged from \u0026minus;\u0026thinsp;101.40 to -17.20 mV, with an average value of -52.53 mV, and the Eh values of groundwater samples were all less than 0, indicating a reducing environment. The dominant cation in groundwater was Na\u003csup\u003e+\u003c/sup\u003e, followed by Ca\u003csup\u003e2+\u003c/sup\u003e and Mg\u003csup\u003e2+\u003c/sup\u003e, with K\u003csup\u003e+\u003c/sup\u003e concentration being the lowest. The dominant anion was Cl\u003csup\u003e-\u003c/sup\u003e, followed by SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e and HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, with CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e concentration being the lowest. The coefficients of variation for both Mg\u003csup\u003e2+\u003c/sup\u003e and Cl\u003csup\u003e-\u003c/sup\u003e were greater than 1, which was a strong variation, indicating a wide range of variation in the concentration of these two ions in groundwater. The TDS concentration of groundwater ranged from 475.75 to 6834.64 mg/L, with an average of 2629.21 mg/L and a median of 2103.61 mg/L. Among them, there were 4, 6 and 3 groups of fresh water (TDS\u0026thinsp;\u0026lt;\u0026thinsp;1000 mg/L), slightly saline water (1000\u0026thinsp;\u0026le;\u0026thinsp;TDS\u0026thinsp;\u0026lt;\u0026thinsp;3000 mg/L) and saline water (3000\u0026thinsp;\u0026le;\u0026thinsp;TDS\u0026thinsp;\u0026lt;\u0026thinsp;10000 mg/L), respectively. They accounted for 30.77%, 46.15% and 23.08% of the total samples, respectively. The groundwater was mainly dominated by brackish water. The classification of groundwater hydrochemical types with reference to Shukarev's classification method showed that there were eight groundwater hydrochemical types in the study area, and the main hydrochemical types were dominated by SO\u003csub\u003e4\u003c/sub\u003e\u0026middot;Cl-Na type (23.08%), SO\u003csub\u003e4\u003c/sub\u003e\u0026middot;Cl-Na\u0026middot;Ca type (23.08%), and SO\u003csub\u003e4\u003c/sub\u003e\u0026middot;Cl-Na\u0026middot;Mg type (15.38%). Besides the sulfate type, there also existed a hydrochemical type dominated by bicarbonates, accounting for 15.38% of the total.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eStatistical table of characteristic parameters of groundwater hydrochemistry index\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eIndex\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMin\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMax\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMean\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMedian\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCoefficient\u003c/p\u003e \u003cp\u003eof variation\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003epH\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e7.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e8.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e8.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEh (mV)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e-101.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-17.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-52.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-31.30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e-0.64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK\u003csup\u003e+\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e10.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003csup\u003e+\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e78.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1516.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e553.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e487.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.85\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCa\u003csup\u003e2+\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e28.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e537.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e205.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e187.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.83\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMg\u003csup\u003e2+\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e659.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e145.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e87.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCl\u003csup\u003e\u0026minus;\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e38.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3121.17\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e878.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e367.59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e179.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1323.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e666.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e651.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e79.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e229.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e165.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e156.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.24\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e18.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e10.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e9.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.39\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTDS (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e475.75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6834.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2629.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2103.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.78\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe (mg/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.43\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eI\u003csup\u003e\u0026minus;\u003c/sup\u003e (\u0026micro;g/L)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e51.66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e552.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e177.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e134.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.77\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAccording to the relevant regulations of the Chinese national standard \"Delineation of Highly Iodized Water Sources and Highly Iodized Disease Areas\" and the industry standard \"Delineation of Iodine Deficient Areas and Iodine-Adequate Areas\", the groundwater in the study area was classified into low iodine water (I\u003csup\u003e-\u003c/sup\u003e \u0026lt;100 \u0026micro;g/L), high iodine water (100\u0026thinsp;\u0026le;\u0026thinsp;I\u003csup\u003e-\u003c/sup\u003e concentration\u0026thinsp;\u0026le;\u0026thinsp;300 \u0026micro;g/L), and ultra-high iodine water (I\u003csup\u003e-\u003c/sup\u003e \u0026gt;300 \u0026micro;g/L) based on iodine content. There were 5 groups of low iodine water, accounting for 38.46% of the groundwater samples, while high iodine water and ultra-high iodine water accounted for 6 groups and 2 groups, respectively, comprising 46.15% and 15.39% of the groundwater samples. The range of I\u003csup\u003e-\u003c/sup\u003e concentration in groundwater was between 51.66\u0026thinsp;~\u0026thinsp;552.79 \u0026micro;g/L, with a median value of 134.24 \u0026micro;g/L and an average value of 177.68 \u0026micro;g/L. 61.54% of the groundwaters with I\u003csup\u003e-\u003c/sup\u003e concentration greater than 100 \u0026micro;g/L were characterized as high iodine groundwater. Based on the iodide classification and concentration limits of toxicological indicators in China's Groundwater Quality Standard (GB/T 14848\u0026thinsp;\u0026minus;\u0026thinsp;2017), Class I and II groundwater requires I\u003csup\u003e-\u003c/sup\u003e \u0026le;40 \u0026micro;g/L, Class III water is limited to 40 \u0026micro;g/L\u0026thinsp;\u0026lt;\u0026thinsp;I\u003csup\u003e-\u003c/sup\u003e \u0026le;80 \u0026micro;g/L, Class IV water is limited to 80 \u0026micro;g/L\u0026thinsp;\u0026lt;\u0026thinsp;I\u003csup\u003e-\u003c/sup\u003e \u0026le;500 \u0026micro;g/L, and Class V water is limited to I\u003csup\u003e-\u003c/sup\u003e concentration of greater than 500 \u0026micro;g/L. In the study area, there were 2 groups of Class III groundwater, 12 groups of Class IV groundwater, and 1 group of Class V groundwater. Iodide was used as the sole indicator to classify groundwater. Consequently, the majority of groundwater in the study area was classified as Class IV, with no Class I or Class II groundwater present.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Characterisation of DIC and DOC in groundwater\u003c/h2\u003e \u003cp\u003eThe relationship between DIC and DOC concentrations in groundwater of the study area is depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The DIC concentration in groundwater ranged from 22.97 to 100.85 mg/L, with an average of 66.04 mg/L. DIC in water bodies primarily exists in three forms: HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, H\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, and CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e, with the specific form dependent on the pH of the water. In the study area, the pH of groundwater ranged from 7.79 to 9.34. Therefore, HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e was the predominant form of DIC in groundwater. The DOC concentration ranged from 2.01 to 4.22 mg/L, with an average of 2.79 mg/L. Previous study has indicated that the average concentration of DOC in natural water bodies is approximately 5 mg/L (Thurman, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). However, when domestic sewage and industrial wastewater are discharged into water bodies, the DOC concentration can exceed 5 mg/L by a significant margin. The concentration of DOC in the groundwater of the study area ranged from 2.01 to 4.22 mg/L, which indicates that the groundwater was not affected by the pollution of anthropogenic activities. In low iodine groundwater, the DIC concentration ranged from 48.34 to 74.50 mg/L, with an average of 57.16 mg/L, and the DOC concentration ranged from 2.01 to 2.70 mg/L, with an average of 2.26 mg/L. In high iodine groundwater, the DIC concentration ranged from 22.97 to 100.85 mg/L, with an average of 71.58 mg/L, and the DOC concentration ranged from 2.31 to 4.22 mg/L, with an average of 3.11 mg/L. Compared to low iodine groundwater, high iodine groundwater exhibited higher average values of both DIC and DOC.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Characteristics of stable carbon isotopes in groundwater\u003c/h2\u003e \u003cp\u003eThe relationship between δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e in groundwater is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values in groundwater ranged from \u0026minus;\u0026thinsp;24.04\u0026permil; to -16.39\u0026permil;, with an average of -20.00\u0026permil;. Among them, the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values in low iodine groundwater ranged from \u0026minus;\u0026thinsp;21.74\u0026permil; to -16.39\u0026permil;, with an average of -18.34\u0026permil;, while those in high iodine groundwater ranged from \u0026minus;\u0026thinsp;24.04\u0026permil; to -17.71\u0026permil;, with an average of -21.04\u0026permil;. From the comparative analysis of the distribution range and mean value of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e, it can be seen that the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value of high iodine groundwater was significantly more negative than that of low iodine groundwater. From Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values of groundwater in the study area ranged from \u0026minus;\u0026thinsp;29.58\u0026permil; to -26.79\u0026permil;, with an average value of -28.51\u0026permil;. Specifically, the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values of low iodine groundwater ranged from \u0026minus;\u0026thinsp;29.28\u0026permil; to -28.41\u0026permil;, with an average of -28.99\u0026permil;, while the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values of high iodine groundwater ranged from \u0026minus;\u0026thinsp;29.58\u0026permil; to -26.79\u0026permil;, with an average of -28.20\u0026permil;. The δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values of high iodine groundwater exhibited a broader range, whereas those of low iodine groundwater are more concentrated and lower in comparison to high iodine groundwater.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Source analysis of DIC and DOC in groundwater\u003c/h2\u003e \u003cp\u003eGroundwater DIC is mainly derived from the contribution of atmospheric CO\u003csub\u003e2\u003c/sub\u003e, metabolic decomposition activities of organic matter by microbial action, and weathering dissolution of carbonate and silicate minerals in the aquifer (Barth et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Yang et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Zhou et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Different sources of DIC exhibit distinct characteristic ranges in carbon isotopes (Li et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Previous studies have shown that when [HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e]/[Ca\u003csup\u003e2+\u003c/sup\u003e+Mg\u003csup\u003e2+\u003c/sup\u003e]\u0026thinsp;\u0026lt;\u0026thinsp;2, there are multiple potential sources of HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e in the water (Song et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In the study area, approximately 77% of groundwater samples had [HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e]/[Ca\u003csup\u003e2+\u003c/sup\u003e+Mg\u003csup\u003e2+\u003c/sup\u003e] values less than 2, indicating that groundwater DIC was influenced by multiple sources. The buried depth of groundwater in the study area was in the range of 90\u0026ndash;200 m, which was confined groundwater and less affected by CO\u003csub\u003e2\u003c/sub\u003e in the air. DIC originating from carbonate rock dissolution typically exhibits higher δ\u003csup\u003e13\u003c/sup\u003eC values. When the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value of groundwater is around \u0026minus;\u0026thinsp;11\u0026permil;, it indicates that DIC primarily originates from carbonate rock dissolution (Clark and Fritz, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Jin et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Li et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, when the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value is smaller than \u0026minus;\u0026thinsp;11\u0026permil;, it suggests that other processes may affect the magnitude of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values. In relatively confined groundwater environments, weathering of silicate minerals produces HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e carbon isotope values δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e of -17\u0026permil; (Porowska, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values (-17.71\u0026permil;, -16.83\u0026permil;, -16.39\u0026permil;) of the three samples in groundwater were about the range of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values produced by silicate mineral weathering. Combined with the previous studies on the influence of hydrogeochemical processes on iodine enrichment in the lower reaches of Kuitun River (Chao et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2024a\u003c/span\u003e), it can be seen that the rock weathering type of groundwater in the study area was mainly affected by silicate weathering and dissolution, indicating that the weathering and dissolution of silicate minerals was part of the source of DIC in groundwater. When microbial degradation processes of organic matter in aquifers preferentially tend to utilize lighter \u003csup\u003e12\u003c/sup\u003eC, which enriches the products with lighter \u003csup\u003e12\u003c/sup\u003eC and leads to fractionation of \u003csup\u003e13\u003c/sup\u003eC, their reactants are enriched with larger \u003csup\u003e13\u003c/sup\u003eC (Chao et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2024b\u003c/span\u003e). Consequently, microbial degradation of organic matter tends to result in smaller δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values compared to weathering dissolution of silicate minerals and carbonate rocks. Previous studies have confirmed that the biological degradation of organic matter leads to negative shifts in δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values (Clark and Fritz, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Schulte et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The δ\u003csup\u003e13\u003c/sup\u003eC values of DIC released by microbial decomposition of organic matter range from \u0026minus;\u0026thinsp;25\u0026permil; to -18\u0026permil; (Truesdell and Hulston, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1980\u003c/span\u003e). The relationship between δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e and DOC in groundwater is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. In the study area, 76.92% of groundwater δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values were within the range of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values produced by microbial degradation of organic matter. Therefore, the main source of groundwater DIC was influenced by microbial degradation of organic matter, with additional effects from the dissolution of silicate minerals. As can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, 80% of low iodine groundwater δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values were around \u0026minus;\u0026thinsp;18\u0026permil;, and groundwater δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values in high iodine areas were in the range of -24.04\u0026permil; to -16.83\u0026permil; (mean value is -21.02\u0026permil;). Compared with the low iodine groundwater, the mean δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value of the high iodine groundwater was more on the small side, indicating that the microorganisms in the high iodine groundwater degraded the organic matter more.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe δ\u003csup\u003e13\u003c/sup\u003eC values of DOC from different carbon sources exhibit distinct characteristics. The δ\u003csup\u003e13\u003c/sup\u003eC values in soil humus are related to the type of vegetation in the region. The δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values of C3 plants (such as trees, wheat, cotton, etc.) range between \u0026minus;\u0026thinsp;35\u0026permil; and \u0026minus;\u0026thinsp;20\u0026permil;, while those of C4 plants (such as maize, sorghum, and sugarcane) range from \u0026minus;\u0026thinsp;19\u0026permil; to -8\u0026permil; (Brookman and Ambrose, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Clark and Fritz, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). The δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values of Crassulacean Acid Metabolism (CAM) plants fall within the range of -22\u0026permil; to -10\u0026permil; (Hedges et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1997\u003c/span\u003e). The isotopic composition of CAM plants typically falls between the values of C3 and C4 plants' δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e ranges. In the study area, the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values in groundwater were distributed within the range of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values of C3 plants and were smaller compared to C4 and CAM plants. The Kuitun region has experienced many geological activities in which the vertical uplift process has led to the formation of variations in surface vegetation and has been in the center of the depositional zones in various stages during the Quaternary, forming the geological condition of a deep sedimentary layer, which is mainly dominated by mud, clayey soil and humus (Hong, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Luo et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The surface vegetation in the study area was mainly covered by cotton and trees, and the deep sedimentary layer and aquifer conditions were formed during the long period of geological movement and surface evolution; therefore, the groundwater DOC was mainly derived from C3 plants.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Indicative significance of stable carbon isotope characteristics for iodine enrichment\u003c/h2\u003e \u003cp\u003eThe relationship between δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e and DIC in groundwater is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values exhibited a negative correlation with DIC (\u003cem\u003er\u003c/em\u003e=-0.545, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.054), which was close to the significant level. The smaller the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value was, the stronger the microbial activity was, and the higher the concentration of DIC was. The predominant form of DIC in groundwater was HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, indicating that the organic matter degraded by microbes was one of the important sources of HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e in groundwater DIC.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e-δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e difference in groundwater in the study area had a highly significant positive correlation (\u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.959, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01) with δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea), suggesting that the smaller the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e in groundwater was, the greater the contribution of organic matter degradation to DIC in groundwater, the more DIC was produced, and the oxidative decomposition of DOC played a role in this process. When the value of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e-δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e in groundwater is larger, it indicates that the dissolution of carbonate rocks and silicate is the main source of DIC; on the contrary if the value of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e-δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e is smaller, it indicates that more inorganic carbon comes from the oxidative decomposition of organic matter, and the stronger microbial action. The difference of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e-δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e in groundwater showed a significant negative correlation (\u003cem\u003er\u003c/em\u003e=-0.591, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.034) with the I\u003csup\u003e-\u003c/sup\u003e concentration (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb), indicating that the more DIC was produced by microbial degradation of organic matter, the smaller the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value was and the higher the I\u003csup\u003e-\u003c/sup\u003e concentration was. The δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value in groundwater was significantly negatively correlated with I\u003csup\u003e-\u003c/sup\u003e concentration (\u003cem\u003er\u003c/em\u003e=-0.637, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.019) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec), and the high iodine groundwater was mainly distributed in the lower side region of Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec. On the whole, as δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values became more negative, microbial activity strengthens, leading to higher I\u003csup\u003e-\u003c/sup\u003e concentrations. This suggested that microbial degradation of organic matter promoted the enrichment of iodide in groundwater. Numerous studies have shown that organic matter and iron oxide minerals in sediments are the main carriers of iodine (Dai et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Hansen et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In the study area, groundwater was in a reduced state, and anaerobic microorganisms utilized organic matter in sediments as a carbon source for degradation. During the decomposition of organic matter, iodine adsorbed on the surface of organic matter was released into the aquifer, leading to an increase in iodine concentration in groundwater, and at the same time, microbial action caused the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value of the groundwater to be constantly small (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec). In addition to the microbial degradation of organic matter, the reductive dissolution of iron oxide minerals was another important mechanism for the formation of high iodine groundwater. Microorganisms were able to use Fe(III) oxides/hydroxides as electron acceptors to reduce Fe(III) to Fe(II) by oxidizing aquifer organic matter, a process that resulted in the migration of iodine adsorbed on the surfaces of minerals such as iron oxides and its released into groundwater (Tao et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). There was a significant positive correlation between I\u003csup\u003e-\u003c/sup\u003e concentration and Fe content in high iodine groundwater (\u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.755, \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.03) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed), which, combined with the fact that the groundwater in the study area was under reducing conditions, suggested that reductive dissolution of iron oxide minerals occurs, and iodine endowed on the surface of the minerals was released, resulting in the migration of solid-phase iodine to the aquifer, and causing the iodine concentration to increase in the groundwater to form high iodine groundwater. In the biogeochemical process of high iodine groundwater formation with the participation of microorganisms, organic matter and DOC in the sediments provided the main carbon and energy sources for the metabolic activities of microorganisms, and organic carbon was decomposed into inorganic carbon under the action of microorganisms, resulting in carbon transformation and fractionation. When the available carbon source to microorganisms increased, it promoted the metabolism of heterotrophic microorganisms and consumes oxygen, forming a reducing environment that was more favorable for iodine enrichment in groundwater (Chao et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2024b\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e4.3 Formation mechanism of high iodine groundwater\u003c/h2\u003e \u003cp\u003eNaturally occurring poor-quality groundwater is formed over time through the interaction between water and rock during the natural cycle. High iodine groundwater is the result of complex hydrological and biogeochemical processes acting over long periods and is one of the typical types of poor-quality groundwater (Wang et al., \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Based on the genesis mode of this type, due to the differences in different environments and hydrogeological conditions, the genesis mechanism of high iodine groundwater was categorized into four types, namely, burial-dissolution, evaporation-concentration, compaction-release, and leaching-enrichment (Wang et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Both biotic and abiotic hydrogeochemical processes control iodine transport and enrichment. In Denmark, iodine concentrations in groundwater reached up to 500 mg/L, with brackish water being the main contributor to iodine levels (Voutchkova et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). In Chile, iodine concentrations in groundwater were as high as 6096 mg/L, which was attributed to the presence of nitrate-rich sedimentary formations (Alvarez et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). In coastal regions of Japan, groundwater contained exceptionally high concentrations of iodine, with levels reaching up to 34000 \u0026micro;g/L, linked to iodine-rich brines formed by local geological activity (Togo et al., \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Elevated iodine concentrations in groundwater in countries like Denmark, Chile, and Japan were associated with hydrogeological conditions of the aquifers. Microbial activity is considered an important factor contributing to iodine enrichment in groundwater (Amachi, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). In reducing groundwater environments, iron-reducing bacteria utilized lactic acid as a substance supplying electrons and receiving oxidation, and used the Fe-mineral phase in aquifer sediments as a receptor for reduction, and the adsorbed iodine in this process was released to groundwater and existed in the form of I\u003csup\u003e-\u003c/sup\u003e, which was the main mechanism for the formation of high iodine groundwater in the Datong basin, China. In this study, the groundwater was in reduction, and the iodine adsorbed on the surface of enriched organic matter and iron oxides was released and migrated to the groundwater to form high iodine groundwater through the participation of microorganisms. This mechanism was consistent with the model of high iodine groundwater genesis summarized by the genesis of poor-quality groundwater, i.e., \u0026ldquo;burial-dissolution type\u0026rdquo; (Wang et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Specifically, under conditions rich in organic matter and long-term stability, microbial activity and the reductive dissolution of iron minerals were the primary processes leading to the migration and released of solid-phase iodine into groundwater. The high iodine groundwater in the study area has long been situated in a stable reducing environment and the burial depth of high iodine groundwater mainly concentrated in the range of 110\u0026ndash;200 m, which belonged to the deep layer of confined water. The stronger the microbial activities were, the higher the I\u003csup\u003e-\u003c/sup\u003e concentration and Fe concentration were. The microbial decomposition of organic matter and the reductive dissolution of iron minerals were the main hydrobiogeochemical processes leading to the release of solid-phase iodine in the aquifer and its migration into groundwater in Kuitun River Basin. Combined with the hydrogeological conditions to which the study area belonged (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), the deep sedimentary layers dominated by muddy and clayey materials were rich in organic matter, and therefore, the genesis mode of high iodine groundwater in the study area was of the burial-dissolution type.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e4.4 Comparative analysis of stable carbon isotope signature and I\u003csup\u003e-\u003c/sup\u003e concentration of groundwater in different regions\u003c/h2\u003e \u003cp\u003eDifferent carbon sources possess certain differences in δ\u003csup\u003e13\u003c/sup\u003eC values, which will lead to different changes in groundwater DIC and its δ\u003csup\u003e13\u003c/sup\u003eC value, and the δ\u003csup\u003e13\u003c/sup\u003eC value change rule of groundwater DIC can be utilized to analyze the main sources of groundwater DIC and reveal the law of chemical evolution of groundwater. The characteristics of δ\u003csup\u003e13\u003c/sup\u003eC value and I\u003csup\u003e-\u003c/sup\u003e concentration of groundwater DIC in different regions are shown in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Sracek and Hirata (\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) used δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values in conjunction with groundwater major ion concentrations and hydrogen and oxygen isotopes to study the hydrochemical evolution of groundwater in the Guarani aquifer in the State of S\u0026atilde;o Paulo, Brazil, and concluded that dissolution of calcite and cation exchange under closed-system conditions occurs in the groundwater. J. Rueedi et al. (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) analyzed the relationship between pH and DIC and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values using groundwater carbon isotope signatures, depth to groundwater, partial pressure of CO\u003csub\u003e2\u003c/sub\u003e, pH, and concentrations of relevant ionic fractions, thus determining that the main sources of groundwater DIC in the three cities of the United Kingdom are soil CO\u003csub\u003e2\u003c/sub\u003e, dolomite dissolution, and human effluent, and they identified the evolution pathways of DIC, including calcite dissolution evolution under open system conditions, gypsum dissolution evolution under closed system conditions, and calcite isotope dissolution evolution. P. Moeller et al. (\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) analyzed the variability of groundwater δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values in the northern German Basin and showed that dissolution of limestone in open and closed systems-controlled groundwater chemical evolution, primarily manifested by the dissolution of calcite and dolomite. Porowska (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) utilized stable carbon isotopes to determine the sources of DIC in groundwater around the Otwock landfill site. The results showed that, under natural conditions, the concentration and isotopic composition of groundwater DIC mainly originated from the organic matter decomposition in aquifer sediments and the dissolution of carbonates. In contrast, groundwater contaminated by leachate depended on the degradation of organic matter in aquifer sediments and the biodegradation of organic matter stored in the landfill site. In China, Zhu et al. (\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) demonstrated the carbon isotopes of groundwater in the Datong Basin that the primary source of DIC in the recharge zone aquifer was the dissolution of carbonate rocks, while microbial activity had a significant influence on DIC sources in the runoff and excretion zones. Zhou (\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) found in their research on the carbon isotopes of groundwater in the Hetao Basin that DIC in the piedmont and transition zone was mainly influenced by carbonate rock leaching and atmospheric precipitation recharge, with some influence from microbial activity in certain transition zone groundwater, although its contribution was relatively small. However, in the plain area, microbial activity had an increasingly significant influence on groundwater DIC. Li et al. (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) found that chemical weathering of aluminosilicate minerals in the aquifer of the North China Plain had a dominant effect on the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value of groundwater in the region. Yuan et al. (\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) found that microbial oxidation of organic matter and dissolution of carbonate rocks were the primary sources of DIC in groundwater in the Jianghan Plain. The δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values of groundwater in the study area ranged from \u0026minus;\u0026thinsp;24.04\u0026permil; to -16.39\u0026permil;. DIC was mainly influenced by microbial degradation of organic matter and partial weathering dissolution of silicate minerals. The overall δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values of the groundwater were small compared with those of other areas (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), suggesting that other areas were more enriched in \u003csup\u003e13\u003c/sup\u003eC. The sources of DIC in groundwater in different regions are different, and their δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values have different characteristic ranges. The combined effects of various reactions in the groundwater system have an important influence on the composition of DIC and δ\u003csup\u003e13\u003c/sup\u003eC values in this system.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eRange of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e, δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values, I\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations and depths in groundwater in different regions.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026minus;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCountry\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eStudy area\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eδ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e/\u0026permil;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eδ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e/\u0026permil;\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eI\u003csup\u003e\u0026minus;\u003c/sup\u003e(\u0026micro;g/L)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eDepth/m\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eReferences\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBrazil\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eGuarani, S\u0026atilde;o Paulo State\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c3\"\u003e \u003cp\u003e-19.00~-5.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eSracek and Hirata, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2002\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBritain\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBritish Midlands\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-20.05\u0026thinsp;~\u0026thinsp;2.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e8.24\u0026thinsp;~\u0026thinsp;76.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eRueedi et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2007\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGermany\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNorth German Basin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c3\"\u003e \u003cp\u003e-22.70~-3.70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e86\u0026thinsp;~\u0026thinsp;1616\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eMoeller et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2008\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePoland\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSuburb of Otwock\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-20.60\u0026thinsp;~\u0026thinsp;3.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003ePorowska, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2015\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"4\" rowspan=\"5\"\u003e \u003cp\u003eChina\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKuitun, Xinjiang\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c3\"\u003e \u003cp\u003e-24.04~-16.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-29.58~-26.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e51.66\u0026thinsp;~\u0026thinsp;552.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e90\u0026thinsp;~\u0026thinsp;200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eThis study\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDatong Basin\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c3\"\u003e \u003cp\u003e-16.93~-7.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e14.40\u0026thinsp;~\u0026thinsp;1030.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e16\u0026thinsp;~\u0026thinsp;75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eZhu et al., \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2021\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHetao Plain\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c3\"\u003e \u003cp\u003e-11.80~-5.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-22.90~-19.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e31.84\u0026thinsp;~\u0026thinsp;1289.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e15\u0026thinsp;~\u0026thinsp;80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eWang et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2014\u003c/span\u003e;\u003c/p\u003e \u003cp\u003eZhou, \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2018\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNorth China Plain\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c3\"\u003e \u003cp\u003e-11.42~-5.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e4\u0026thinsp;~\u0026thinsp;2175\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e10\u0026thinsp;~\u0026thinsp;860\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eLi J., et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2022\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eJianghan Plain\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026minus;\" colname=\"c3\"\u003e \u003cp\u003e-18.50~-3.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-28.50~-19.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2\u0026thinsp;~\u0026thinsp;1600\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e15\u0026thinsp;~\u0026thinsp;40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eFan et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2022\u003c/span\u003e;\u003c/p\u003e \u003cp\u003eYuan et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2020\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eIn the study area, the range of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values in groundwater was \u0026minus;\u0026thinsp;29.58\u0026permil; to -26.79\u0026permil;, which was relatively similar compared to the Hetao Plain (-22.9\u0026permil; to -19.20\u0026permil;), but the average δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e value in the Hetao Plain was higher. The factors influencing the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e value depend on the types of endogenous and exogenous organic matter in the aquifer. The range of δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values in the Jianghan Plain (-28.5\u0026permil; to -19.60\u0026permil;) varied widely. The Hetao Plain, Jianghan Plain, and Kuitun River Basin in Xinjiang are located in the northern Yellow River alluvial plain, middle reaches of the Yangtze River, and northwest arid inland basin, respectively. The soil organic carbon, vegetation types, and groundwater organic matter vary across different regions. Soil organic carbon is one of the exogenous carbon sources to groundwater, which can enter shallow or deep groundwater to influence groundwater DOC and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values due to rainfall and irrigation infiltration. For instance, the groundwater burial depth in the Hetao Plain and Jianghan Plain was as low as 15 m. The DOC and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values in groundwater may be influenced by inputs of soil organic carbon. Vegetation from the surface can be buried into the aquifer over prolonged geological activity, resulting in groundwater organic matter with varying DOC and δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values across different vegetation types. Therefore, the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values in groundwater will vary due to differences in external organic matter (soil organic carbon) and endogenous organic matter across different regions. High iodine groundwater in the study area was mainly distributed below a burial depth of 90 m, representing deep confined aquifers, which were deeper compared to the shallow groundwater distribution in the Datong Basin, Hetao Plain, and Jianghan Plain. In reducing environments of groundwater, when microbial activity on organic matter is strong, microbes preferentially utilize more \u003csup\u003e12\u003c/sup\u003eC, leading to a lower δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value in the resulting degradation product DIC. Comparing with the other four high iodine groundwater distribution zones in China, the δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e value of the groundwaters of the study area was overall smaller, and it was hypothesized that microbial activity in the groundwater of the downstream area of the Kuitun River in Xinjiang may be relatively strong.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThis study used stable carbon isotope technology to represent the influence of organic matter degradation process under the action of microorganisms on iodine enrichment, and deeply explored the formation mechanism of high iodine groundwater in this region. The following conclusions were drawn:\u003c/p\u003e\n\u003cp\u003e\u003cspan\u003e\u003c/span\u003e\u003c/p\u003e\n\u003cp\u003e1. The concentration of I\u003csup\u003e-\u003c/sup\u003e in the groundwater in the study area ranged from 51.66 to 552.79 \u0026micro;g/L, with an average value of 177.68 \u0026micro;g/L, and the percentages of low iodine water, high iodine water and ultra-high iodine water were 38.46%, 46.15% and 15.39%, respectively. The groundwater as a whole was reductive and weakly alkaline, and the dominant anion and cation were Cl\u003csup\u003e-\u003c/sup\u003e and Na\u003csup\u003e+\u003c/sup\u003e respectively. Groundwater was dominated by brackish water, accounting for 46.15%. The hydrochemical type of groundwater was dominated by sulfuric acid type. According to the \u0026quot;Groundwater Quality Standards of China,\u0026quot; groundwater was mainly classified as Class IV groundwater, with no Class I or Class II groundwater.\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003e2. The groundwater DIC concentration in the study area ranged from 22.97 to 100.85 mg/L, while the DOC concentration ranged from 2.01 to 4.22 mg/L. The \u0026delta;\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values ranged from \u0026minus;\u0026thinsp;24.04\u0026permil; to -16.39\u0026permil;, and the \u0026delta;\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDOC\u003c/sub\u003e values ranged from \u0026minus;\u0026thinsp;29.58\u0026permil; to -26.79\u0026permil;. The range and mean of distribution of \u0026delta;\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e values for high iodine groundwaters were significantly more negative than those for low iodine groundwaters. Groundwater DIC was primarily affected by the degradation of organic matter by microorganisms and the weathering and dissolution of silicate minerals, with HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e being the dominated anion. DOC was mainly derived from C3 plants.\u003c/p\u003e\n\u003c/span\u003e\u003cspan\u003e\n \u003cp\u003e3. In reducing environments with abundant organic matter in aquifers, the primary hydro-biogeochemical processes leading to the release of solid-phase iodine in aquifers and its migration into groundwater were microbial involvement in organic matter decomposition and the reducible dissolution of iron minerals. The genesis model of high iodine groundwater being burial-dissolution type.\u003c/p\u003e\n\u003c/span\u003e\n\u003cp\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cem\u003e6.1 Ethical Approval\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e6.2 Consent to Participate\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e6.3 Consent to Publish\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eConsent for publications was obtained from the participants.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e6.4 Authors Contributions\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization, B.C. and Y.L.; methodology, B.C. and L.D.; software, B.C. and J.H.; Validation, B.C.; formal analysis, B.C., J.H. and L.D.; investigation, B.C., Y.L., L.D.; resources, Y.L.; data curation, B.C. and L.D.; writing-original draft preparation, B.C. and J.H.; writing-review and editing, B.C., J.H., Y.L., L.D., Q.Z., X.X., M.W., Z.S., X.L.; visualization, B.C. and J.H.; supervision, Y.L.; project administration, Y.L.; funding acquisition, Y.L.; All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e6.5 Funding\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe research work was financially supported by National Natural Science Foundation of China (41761097).\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e6.6 Competing Interests\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no competing interests to declare that are relevant to the content of this article..\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e6.7 Availability of data and materials\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding authors upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAlvarez F, Reich M, P\u0026eacute;rez-Fodich A, Snyder G, Muramatsu Y, Vargas G, Fehn U (2015) Sources, sinks and long-term cycling of iodine in the hyperarid Atacama continental margin. Geochim Cosmochim Acta 161:50\u0026ndash;70. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhydrol.2022.127517\u003c/span\u003e\u003cspan address=\"10.1016/j.jhydrol.2022.127517\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003e\u0026Aacute;lvarez F, Reich M, Snyder G, P\u0026eacute;rez-Fodich A, Muramatsu Y, Daniele L, Fehn U (2016) Iodine budget in surface waters from Atacama: Natural and anthropogenic iodine sources revealed by halogen geochemistry and iodine-129 isotopes. Appl Geochem 68:53\u0026ndash;63. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.apgeochem.2016.03.011\u003c/span\u003e\u003cspan address=\"10.1016/j.apgeochem.2016.03.011\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAmachi S (2008) Microbial contribution to global iodine cycling: volatilization, accumulation, reduction, oxidation, and sorption of iodine. Microbes Environ 23(4):269\u0026ndash;276. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1264/jsme2.ME08548\u003c/span\u003e\u003cspan address=\"10.1264/jsme2.ME08548\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAtekwana EA, Krishnamurthy RV (1998) Seasonal variations of dissolved inorganic carbon and δ\u003csup\u003e13\u003c/sup\u003eC of surface waters: application of a modified gas evolution technique. J Hydrol 205(3):265\u0026ndash;278. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/S0022-1694(98)00080-8\u003c/span\u003e\u003cspan address=\"10.1016/S0022-1694(98)00080-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarth JA, Cronin AA, Dunlop J, Kalin RM (2003) Influence of carbonates on the riverine carbon cycle in an anthropogenically dominated catchment basin: evidence from major elements and stable carbon isotopes in the Lagan River (N. Ireland). Chem Geol 200(3):203\u0026ndash;216. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/S0009-2541(03)00193-1\u003c/span\u003e\u003cspan address=\"10.1016/S0009-2541(03)00193-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBrookman TH, Ambrose SH (2013) Kangaroo tooth enamel oxygen and carbon isotope variation on a latitudinal transect in southern Australia: implications for palaeoenvironmental reconstruction. Oecologia 171:403\u0026ndash;416. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00442-012-2425-6\u003c/span\u003e\u003cspan address=\"10.1007/s00442-012-2425-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChao B, Luo Y, Dong L, Zheng Y, Song Z, Wang M, Liu C (2024a) Characterizing the spatial distribution of iodine and analyzing its influential factors in a typical groundwater area with high arsenic content in Xinjiang. Acta Sci Circum 44(4):156\u0026ndash;167. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.13671/j.hjkxxb.2023.0435\u003c/span\u003e\u003cspan address=\"10.13671/j.hjkxxb.2023.0435\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e(in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChao B, Luo Y, Wang X (2024b) Stable carbon isotope signatures of high arsenic groundwater and their indicative significancein inKuitun area of Xinjiang. Environ Chem 43(3):1\u0026ndash;10. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.7524/j.issn.0254-6108.2022091102\u003c/span\u003e\u003cspan address=\"10.7524/j.issn.0254-6108.2022091102\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e(in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eClark ID, Fritz P (2013) Environmental isotopes in hydrogeology[M]. CRC, BocaRaton\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDai JL, Zhang M, Hu QH, Huang YZ, Wang RQ, Zhu YG (2009) Adsorption and desorption of iodine by various Chinese soils: II. Iodide and iodate. Geoderma 153(1\u0026ndash;2):130\u0026ndash;135. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.geoderma.2009.07.020\u003c/span\u003e\u003cspan address=\"10.1016/j.geoderma.2009.07.020\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDai S (2006) Environmental Chemistry. Higher Education Press, Beijing\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFan R, Deng Y, Xue J (2022) Predicting the Spatial Distribution of Geogenic Iodine-contaminatedGroundwater of Jianghan Plain Based on ExtremeGradient Boost Model. Saf Environ Eng 29(5):70\u0026ndash;77. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.13578/j.cnki.issn.1671-1556.20221107\u003c/span\u003e\u003cspan address=\"10.13578/j.cnki.issn.1671-1556.20221107\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e(in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGuo H, Zhang Z, Cheng G, Li W, Li T, Jiao JJ (2015) Groundwater-derived land subsidence in the North China Plain. Environ Earth Sci 74:1415\u0026ndash;1427. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s12665-015-4131-2\u003c/span\u003e\u003cspan address=\"10.1007/s12665-015-4131-2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHansen V, Roos P, Aldahan A, Hou X, Possnert G (2011) Partition of iodine (\u003csup\u003e129\u003c/sup\u003eI and \u003csup\u003e127\u003c/sup\u003eI) isotopes in soils and marine sediments. J Environ Radioact 102(12):1096\u0026ndash;1104. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jenvrad.2011.07.005\u003c/span\u003e\u003cspan address=\"10.1016/j.jenvrad.2011.07.005\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHedges JI, Keil RG, Benner R (1997) What happens to terrestrial organic matter in the ocean? Org Geochem 27(5\u0026ndash;6):195\u0026ndash;212. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/S0146-6380(97)00066-1\u003c/span\u003e\u003cspan address=\"10.1016/S0146-6380(97)00066-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHong L (1983) A preliminary study on the disease and formation environment of high fluorine and arsenic water in Chepaizi area, north of Kuitun, Xinjiang. Xinjiang Environmental Protection. 5 (1): 22\u0026ndash;28. DOI: CNKI:SUN:XJHB.0.1983-01-004. (in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJin L, Ogrinc N, Yesavage T, Hasenmueller EA, Ma L, Sullivan PL, Kaye J, Duffy C, Brantley SL (2014) The CO\u003csub\u003e2\u003c/sub\u003e consumption potential during gray shale weathering: Insights from the evolution of carbon isotopes in the Susquehanna Shale Hills critical zone observatory. Geochim Cosmochim Acta 142:260\u0026ndash;280. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.gca.2014.07.006\u003c/span\u003e\u003cspan address=\"10.1016/j.gca.2014.07.006\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKassim IA, Moloney G, Busili A, Nur AY, Paron P, Jooste P, Gadain H, Seal AJ (2014) Iodine intake in Somalia is excessive and associated with the source of household drinking water. J Nutr 144(3):375\u0026ndash;381. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3945/jn.113.176693\u003c/span\u003e\u003cspan address=\"10.3945/jn.113.176693\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi J, Jiang Z, Xie X, Wang Y (2022) Mechanisms of iodine enrichment in the pore-water of fluvial/lacustrine aquifer systems: Insight from stable carbon isotopes and batch experiments. J Hydrol 613:128334. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhydrol.2022.128334\u003c/span\u003e\u003cspan address=\"10.1016/j.jhydrol.2022.128334\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi J, Qian K, Yang Y, Xie X (2017a) Iodine speciation and its potential influence on iodine enrichment in groundwater from North China Plain. Procedia Earth Planet Sci 17:312\u0026ndash;315. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.proeps.2016.12.069\u003c/span\u003e\u003cspan address=\"10.1016/j.proeps.2016.12.069\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi J, Wang Y, Guo W, Xie X, Zhang L, Liu Y, Kong S (2014) Iodine mobilization in groundwater system at Datong basin, China: evidence from hydrochemistry and fluorescence characteristics. Sci Total Environ 468:738\u0026ndash;745. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.scitotenv.2013.08.092\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2013.08.092\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi J, Wang Y, Xie X, DePaolo DJ (2016) Effects of water-sediment interaction and irrigation practices on iodine enrichment in shallow groundwater. J Hydrol 543:293\u0026ndash;304. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhydrol.2016.10.002\u003c/span\u003e\u003cspan address=\"10.1016/j.jhydrol.2016.10.002\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi J, Zhou H, Wang Y, Xie X, Qian K (2017b) Sorption and speciation of iodine in groundwater system: The roles of organic matter and organic-mineral complexes. J Contam Hydrol 201:39\u0026ndash;47. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jconhyd.2017.04.008\u003c/span\u003e\u003cspan address=\"10.1016/j.jconhyd.2017.04.008\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi Z, Cao W, Ren Y, Pan D, Wang S, Zhi C (2022) Enrichment mechanisms for the co-occurrence of arsenic-fluoride-iodine in the groundwater in different sedimentary environments of the Hetao Basin, China. Sci Total Environ 839:156184. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.scitotenv.2022.156184\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2022.156184\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLuo Y, Li J, Jiang P, Xing Y (2017) Distribution, classification and cause analysis of geogenic high-arsenicgroundwater in Kuitun,Xinjiang. Acta Sci Circum 37(8):2897\u0026ndash;2903. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.13671/j.hjkxxb.2017.0019\u003c/span\u003e\u003cspan address=\"10.13671/j.hjkxxb.2017.0019\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e(in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMa R, Yan M, Han P, Wang T, Li B, Zhou S, Zheng T, Hu T, Borthwick AG, Zheng C, Ni J (2022) Deficiency and excess of groundwater iodine and their health associations. Nat Commun 13(1):7354. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1038/s41467-022-35042-6\u003c/span\u003e\u003cspan address=\"10.1038/s41467-022-35042-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMoeller P, Weise SM, Tesmer M, Dulski P, Pekdeger A, Bayer U, Magri F (2008) Salinization of groundwater in the North German Basin: results from conjoint investigation of major, trace element and multi-isotope distribution. Int J Earth Sci 97:1057\u0026ndash;1073. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00531-007-0211-1\u003c/span\u003e\u003cspan address=\"10.1007/s00531-007-0211-1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePearce EN, Andersson M, Zimmermann MB (2013) Global iodine nutrition: where do we stand in 2013? Thyroid 23(5):523\u0026ndash;528. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1089/thy.2013.0128\u003c/span\u003e\u003cspan address=\"10.1089/thy.2013.0128\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePorowska D (2015) Determination of the origin of dissolved inorganic carbon in groundwater around a reclaimed landfill in Otwock using stable carbon isotopes. Waste Manag 39:216\u0026ndash;225. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.wasman.2015.01.044\u003c/span\u003e\u003cspan address=\"10.1016/j.wasman.2015.01.044\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eQiao C, Wang Y, Liu Y, Li J, Zhang H, Lu J (2022) Study of Water Resources Optimal Operation Model of Multireservoir: A Case Study of Kuitun River Basin in Northwestern China. Wireless Communications and Mobile Computing. 2022, 17. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1155/2022/7715398\u003c/span\u003e\u003cspan address=\"10.1155/2022/7715398\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRueedi J, Cronin AA, Taylor RG, Morris BL (2007) Tracing sources of carbon in urban groundwater using δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eTDIC\u003c/sub\u003e ratios. Environ Geol 52:541\u0026ndash;557. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s00254-006-0486-8\u003c/span\u003e\u003cspan address=\"10.1007/s00254-006-0486-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSchulte P, Van Geldern R, Freitag H, Karim A, N\u0026eacute;grel P, Petelet-Giraud E, Probst A, Probst J, Telmer K, Veizer J, Barth JA (2011) Applications of stable water and carbon isotopes in watershed research: Weathering, carbon cycling, and water balances. Earth Sci Rev 109(1\u0026ndash;2):20\u0026ndash;31. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.earscirev.2011.07.003\u003c/span\u003e\u003cspan address=\"10.1016/j.earscirev.2011.07.003\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSharp Z (2007) Principles of stable isotope geochemistry. Pearson education. Upper Saddle River, NJ\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSong C, Wang G, Mao T, Huang K, Sun X, Hu Z, Chang R, Chen X, Raymond PA (2020) Spatiotemporal variability and sources of DIC in permafrost catchments of the Yangtze River source region: Insights from stable carbon isotope and water chemistry. Water Resour Res 56(1). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1029/2019WR025343\u003c/span\u003e\u003cspan address=\"10.1029/2019WR025343\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. e2019WR025343\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSracek O, Hirata R (2002) Geochemical and stable isotopic evolution of the Guarani Aquifer System in the state of S\u0026atilde;o Paulo, Brazil. Hydrogeol J 10:643\u0026ndash;655. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s10040-002-0222-8\u003c/span\u003e\u003cspan address=\"10.1007/s10040-002-0222-8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSun D, Codling K, Chang S, Zhang S, Shen H, Su X, Chen Z, Scherpbier R, Yan J (2017) Eliminating iodine deficiency in China: achievements, challenges and global implications. Nutrients 9(4):361. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/nu9040361\u003c/span\u003e\u003cspan address=\"10.3390/nu9040361\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSun Y, Zhou J, Liang X, Zhou Y, Zeng Y, Lin L (2021) Distribution and Genesis of Shallow High-Iodine Groundwater in Southern Margin of Tarim Basin: A Case Study of Plain Area in Minfeng County. Xinjiang Earth Sci 46(8):2999\u0026ndash;3011. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3799/dqkx.2020.260\u003c/span\u003e\u003cspan address=\"10.3799/dqkx.2020.260\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e(in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTang Q, Xu Q, Zhang F, Huang Y, Liu J, Wang X, Yang Y, Liu X (2013) Geochemistry of iodine-rich groundwater in the Taiyuan Basin of central Shanxi Province, North China. J Geochem Explor 135:117\u0026ndash;123. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.gexplo.2012.08.019\u003c/span\u003e\u003cspan address=\"10.1016/j.gexplo.2012.08.019\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTao Y, Du Y, Deng Y, Huang Y, Leng Z, Ma T, Wang Y (2022) Carbon and iron isotope approach elucidates the enrichment of geogenic phosphorus in alluvial-lacustrine sedimentary aquifers. J Hydrol 607:127517. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhydrol.2022.127517\u003c/span\u003e\u003cspan address=\"10.1016/j.jhydrol.2022.127517\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eThurman EM (2012) Organic geochemistry of natural waters(Vol. 2). Springer Science \u0026amp; Business Media, Berlin\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTogo YS, Takahashi Y, Amano Y, Matsuzaki H, Suzuki Y, Terada Y, Muramatsu Y, Ito K, Iwatsuki T (2016) Age and speciation of iodine in groundwater and mudstones of the Horonobe area, Hokkaido, Japan: implications for the origin and migration of iodine during basin evolution. Geochim Cosmochim Acta 191:165\u0026ndash;186. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.gca.2016.07.012\u003c/span\u003e\u003cspan address=\"10.1016/j.gca.2016.07.012\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTruesdell A, Hulston J (1980) Chapter 5-isotopic evidence on environments of geothermal systems. Elsevier. Amsterdam\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVoutchkova DD, Ernstsen V, Hansen B, S\u0026oslash;rensen BL, Zhang C, Kristiansen SM (2014a) Assessment of spatial variation in drinking water iodine and its implications for dietary intake: A new conceptual model for Denmark. Sci Total Environ 493:432\u0026ndash;444. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.scitotenv.2014.06.008\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2014.06.008\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVoutchkova DD, Ernstsen V, Kristiansen SM, Hansen B (2017) Iodine in major Danish aquifers. Environ Earth Sci 76:1\u0026ndash;16. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s12665-017-6775-6\u003c/span\u003e\u003cspan address=\"10.1007/s12665-017-6775-6\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVoutchkova DD, Kristiansen SM, Hansen B, Ernstsen V, S\u0026oslash;rensen BL, Esbensen KH (2014b) Iodine concentrations in Danish groundwater: historical data assessment 1933\u0026ndash;2011. Environ Geochem Health 36:1151\u0026ndash;1164. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s10653-014-9625-4\u003c/span\u003e\u003cspan address=\"10.1007/s10653-014-9625-4\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang LF, Wang HM, Lin FF, Sun XZ, Xu XF, Su MY, Lin J, Feng ZY, Fan XQ (1986) A preliminary investigation of endemic iodine goiter on the inclined plain in front of the Tian-Shan Mountains in Kuitun-Usum area. Chin Med J 99(8):673\u0026ndash;678\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang Y, Li J, Ma T, Xie X, Deng Y, Gan Y (2021) Genesis of geogenic contaminated groundwater: As, F and I. Crit Rev Environ Sci Technol 51(24):2895\u0026ndash;2933. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1080/10643389.2020.1807452\u003c/span\u003e\u003cspan address=\"10.1080/10643389.2020.1807452\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang Y, Li J, Xie X (2022) Genesis and occurrence of high iodine groundwater. Earth Science Frontiers. 2022, 29(3): 1\u0026ndash;10. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.13745/j.esf.sf.2022.1.47\u003c/span\u003e\u003cspan address=\"10.13745/j.esf.sf.2022.1.47\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. (in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang Y, Ma T, Dong Y, Xu F, Yan Y, Liu L (2014) The formation of inland-high-iodine groundwater:A case study in Hangjinhou-qi, Hetao Plain. Earth Sci Front 21(4):66\u0026ndash;73. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.13745/j.esf.2014.04.007\u003c/span\u003e\u003cspan address=\"10.13745/j.esf.2014.04.007\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e(in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWassenaar L, Aravena R, Fritz P (1989) The geochemistry and evolution of natural organic solutes in groundwater. Radiocarbon 31(3):865\u0026ndash;876. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1017/s0033822200012480\u003c/span\u003e\u003cspan address=\"10.1017/s0033822200012480\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXue J, Deng Y, Luo Y, Du Y, Yang Y, Cheng Y, Xie X, Gan Y, Wang Y (2022) Unraveling the impact of iron oxides-organic matter complexes on iodine mobilization in alluvial-lacustrine aquifers from central Yangtze River Basin. Sci Total Environ 814:151930. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.scitotenv.2021.151930\u003c/span\u003e\u003cspan address=\"10.1016/j.scitotenv.2021.151930\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXie X, Wang Y, Ellis A, Li J, Su C, Duan M (2013) Multiple isotope (O, S and C) approach elucidates the enrichment of arsenic in the groundwater from the Datong Basin, northern China. J Hydrol 498:103\u0026ndash;112. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jhydrol.2013.06.024\u003c/span\u003e\u003cspan address=\"10.1016/j.jhydrol.2013.06.024\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYang C, Telmer K, Veizer J (1996) Chemical dynamics of the St. Lawrence riverine system: δD\u003csub\u003eH2O\u003c/sub\u003e, δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003eH2O\u003c/sub\u003e, δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003eDIC\u003c/sub\u003e, δ\u003csup\u003e34\u003c/sup\u003eS\u003csub\u003esulfate\u003c/sub\u003e, and dissolved \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr. Geochim Cosmochim Acta 60(5):851\u0026ndash;866. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/0016-7037(95)00445-9\u003c/span\u003e\u003cspan address=\"10.1016/0016-7037(95)00445-9\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYu K, Gan Y, Zhou A, Han L, Liu Y (2015) A persulfate oxidation method for stable isotope analysis of dissolved organic carbon and the influence of inorganic ions on the results. Int J Mass Spectrom 392:63\u0026ndash;68. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ijms.2015.09.006\u003c/span\u003e\u003cspan address=\"10.1016/j.ijms.2015.09.006\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYu K, Gan Y, Zhou A, Liu C, Duan Y, Han L, Zhang Y (2018) Organic carbon sources and controlling processes on aquifer arsenic cycling in the Jianghan Plain, central China. Chemosphere 208:773\u0026ndash;781. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.chemosphere.2018.05.188\u003c/span\u003e\u003cspan address=\"10.1016/j.chemosphere.2018.05.188\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYuan X, Deng Y, Du Y (2020) Characteristics of stable carbon isotopes and its implications on arsenic enrichmentin shallow groundwater of the Jianghan Plain. Bull Geol Sci Technol 39(5):156\u0026ndash;163. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.19509/j.cnki.dzkq.2021.0008\u003c/span\u003e\u003cspan address=\"10.19509/j.cnki.dzkq.2021.0008\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e(in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang D, Liu C, Wang F, Zhao Z (2015) Inorganic carbon cycling in subsurface environment influenced by agricultural activities. China Environ Sci 35(11):3359\u0026ndash;3370. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3969/j.issn.1000-6923.2015.11.022\u003c/span\u003e\u003cspan address=\"10.3969/j.issn.1000-6923.2015.11.022\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e(in Chinese)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang E, Wang Y, Qian Y, Ma T, Zhang D, Zhan H, Zhang Z, Fei Y, Wang S (2013) Iodine in groundwater of the North China Plain: spatial patterns and hydrogeochemical processes of enrichment. J Geochem Explor 135:40\u0026ndash;53. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.gexplo.2012.11.016\u003c/span\u003e\u003cspan address=\"10.1016/j.gexplo.2012.11.016\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang Y, Gan Y, Yu K, Han L (2021) Fractionation of carbon isotopes of dissolved organic matter adsorbed to goethite in the presence of arsenic to study the origin of DOM in groundwater. Environ Geochem Health 43:1225\u0026ndash;1238. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s10653-020-00644-w\u003c/span\u003e\u003cspan address=\"10.1007/s10653-020-00644-w\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhou Y (2018) Biogeochemical processes in high arsenic groundwater in the northwestern Hetao Basin, Inner Mongolia: Evidences from hydrogeochemistry and stable isotopes (Doctoral dissertation). China University of Geosciences, Beijing\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhou Y, Guo H, Zhang Z, Lu H, Jia Y, Cao Y (2018) Characteristics and implication of stable carbon isotope in high arsenic groundwater systems in the northwest Hetao Basin, Inner Mongolia, China. J Asian Earth Sci 163:70\u0026ndash;79. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jseaes.2018.05.018\u003c/span\u003e\u003cspan address=\"10.1016/j.jseaes.2018.05.018\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhu C, Li J, Xie X (2021) Carbon and Sulfur Isotopic Features and Its Implications for IodineMobilization in Groundwater System at Datong Basin, Northern China. Earth Sci 46(12):4480\u0026ndash;4491. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3799/dqkx.2021.090\u003c/span\u003e\u003cspan address=\"10.3799/dqkx.2021.090\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e(in Chinese)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Iodine, Groundwater, Stable carbon isotope, Dissolved organic carbon, Dissolved inorganic carbon","lastPublishedDoi":"10.21203/rs.3.rs-5985611/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5985611/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe ubiquitous presence of high-iodine groundwater poses a risk to human health. Degradation of organic matter through microbial activities is an important process of iodine enrichment in groundwater systems. The stable carbon isotope ratios of groundwater have certain indicative significance for understanding the formation process of high-iodine groundwater. This study aimed to explore the role of microbiological processes in enriching iodine in high-iodine groundwater downstream of the Kuitun River in China and employed stable carbon isotopes to assess the influence of organic matter biodegradation on groundwater iodine enrichment. The results showed that all groundwater in our study area exhibited reducing conditions and was weakly alkaline, primarily consisting of slightly saline water with dominant anions and cations being Cl\u003csup\u003e-\u003c/sup\u003e and Na\u003csup\u003e+\u003c/sup\u003e, respectively. The concentration of I\u003csup\u003e-\u003c/sup\u003e in groundwater ranged from 51.66 to 552.79 \u0026micro;g/L, with an average of 177.68 \u0026micro;g/L. Approximately 61.54% of the groundwater was highly enriched in iodine. Dissolved inorganic carbon (DIC) concentration in groundwater ranged from 22.97 to 100.85 mg/L, primarily due to microbial degradation of organic matter and weathering dissolution of silicate minerals, primarily consisting of HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. DOC concentration ranged from 2.01 to 4.22 mg/L, mainly originating from C3 plants. In reducing environments with abundant organic matter in aquifers, microbial involvement in organic matter decomposition and reducible dissolution of iron minerals were the primary hydro-biogeochemical processes leading to the release of solid-phase iodine in aquifers and its migration into groundwater. The model for the origin of high-iodine groundwater in the study area was of the burial-dissolution type.\u003c/p\u003e","manuscriptTitle":"High iodine groundwater in the lower Kuitun River in Xinjiang: Evidences from stable carbon isotopes characteristics","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-13 18:22:12","doi":"10.21203/rs.3.rs-5985611/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"d0b9c850-6cf7-48a9-9083-b2fdbbe8494c","owner":[],"postedDate":"February 13th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-04-02T14:08:23+00:00","versionOfRecord":[],"versionCreatedAt":"2025-02-13 18:22:12","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5985611","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5985611","identity":"rs-5985611","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}