Distribution characteristics, speciation and risk assessment of mercury in surface sediments of urban lakes in Nanchang city, China

Distribution characteristics, speciation and risk assessment of mercury in surface sediments of urban lakes in Nanchang city, China | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Distribution characteristics, speciation and risk assessment of mercury in surface sediments of urban lakes in Nanchang city, China xiaozhen liu, Ying Tong, Jia Fei, Chen Yang, Dengbiao Jiang, Luqiang Zhou, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7733844/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 6 You are reading this latest preprint version Abstract Mercury is a toxic and harmful heavy metal pollutant that is prone to migration and accumulation. The increase of mercury emission and the aggravation of mercury pollution have attracted the attention of many scholars. The study focused on six urban lakes in Nanchang city, China as the research area. By determining the content of total mercury (THg) and mercury speciation in the surface sediments, Pearson correlation analysis, Tessier continuous extraction method and multiple ecological risk assessment methods were employed to investigate the distribution characteristics, migration and transformation, and ecological risks of mercury in the surface sediments of urban lakes in Nanchang. The results showed that the total mercury concentrations ranged from 0.109 ~ 0.377 ng/g, with an average of 0.197 ± 0.103 ng/g. The mercury pollution was relatively severe, and the closer to the city center, the more serious the mercury pollution. Mercury primarily existed in the form of residue, accounting for approximately 48.3% to 63.6% of the total mercury. The proportions of bioavailable mercury was also relatively high, ranging from 36.4% to 51.7%. Among the six urban lakes, the content of bioavailable mercury in the sediments of Qingshan Lake was the highest, which was significantly affected by human activities and easy to be released back into water. The overall mercury pollution in the sediment of six urban lakes in Nanchang was moderate pollution with high risk, while the biological toxicity and release possibility of mercury were moderate risk. Mercury Mercury speciation Sediment Risk assessment Urban lakes Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 High lights 1. The overall mercury pollution in the surface sediments of six urban lakes in Nanchang was moderate pollution with high risk, and it generally showed that the closer to the city center, the more serious the mercury pollution. 2. Mercury primarily existed in the form of residue. The biological toxicity and release possibility of mercury were moderate risk. 3. Among the six urban lakes, the content of bioavailable mercury in the sediments of Qingshan Lake was the highest, which was significantly affected by human activities and easy to be released back into water. 1. Introduction Mercury is a heavy metal with potent neurotoxicity, and it is persistent, accumulative and migratory(Gworek, Dmuchowski and Baczewska-Dabrowska 2020 ), posing a significant threat to ecosystems and human health(Hsu-Kim, Kucharzyk, Zhang and Deshusses 2013 ; Xie, Wang, Li, Zhang, Tian, Cheng, Zhang and Wang 2021). Due to the persistence of mercury in the atmosphere, it can be transported over long distances and return to the Earth's surface through dry and wet deposition(Aksentov and Kalinchuk 2012 ), and enter water bodies through surface runoff and underground runoff, eventually accumulating in sediments of lakes(Acquavita, Floreani and Covelli 2021 ; Rodrigues, Ferrari, dos Santos and Conte Junior 2019 ). Lake sediments are important "sources" and "sinks" of mercury and other trace heavy metals in water bodies(Liu, Hu, Lin, Li and Guo 2017 ), and also key microbial methylation sites, playing an indispensable role in the biogeochemical cycle of mercury(Drott, Lambertsson, Bjorn and Skyllberg 2008 ). Dissolved mercury in water migrates and transforms into sediments through adsorption and gravitational settling, and then accumulates in the sediments(Amos, Jacob, Kocman, Horowitz, Zhang, Dutkiewicz, Horvat, Corbitt, Krabbenhoft and Sunderland 2014). Mercury in the surface sediments can also be released back into the water through diffusion and resuspension when environmental conditions change, affecting its distribution and accumulation in the water(Ibanga, Nkwoji, Usese, Onyema and Chukwu 2019 ). The total mercury content in the surface sediments of lakes can reflect the degree of mercury pollution in the water environment, however it cannot reflect its potential hazards. The bioavailability, migration and transformation of mercury in the sediments are also closely related to its occurrence form(Bing, Zhou, Wu, Wang, Sun and Li 2016 ; Vieira, Bordalo, Figueroa, Soares, Morgado, Abreu and Rendon-von Osten 2021). Heavy metals undergo precipitation-dissolution, oxidation-reduction, adsorption-desorption, complexation and other processes in the sediments. Under different environmental conditions, these heavy metals alter their physicochemical properties through migration and transformation, ultimately forming various occurrence forms, and showing different degrees of hazard, biological toxicity and migration characteristics(Bing, Zhou, Wu, Wang, Sun and Li 2016 ; Choppala, Bolan, Lamb and Kunhikrishnan 2013 ). Therefore, the systematic assessment of mercury pollution should include not only measuring the total mercury content but also various occurrence forms of mercury to determine its toxicity and ecological risk to organisms and the environment. In recent decades, with the intensification of human activities, mercury emissions have increased rapidly and made the mercury pollution more and more serious, which have become a major environmental issue of global concern(Li, Yu, Li, Deng, Xu, Ding, Gao, Hong and Wong 2013). Studies have shown that rivers flowing through human settlements are particularly susceptible to direct impacts from human activities, resulting in more complex mercury sources. Moreover, during this process, the rate and scale of mercury transfer into aquatic ecosystems may also be affected, and all these changes are recorded in sediments(Yang, Turner and Rose 2016 ). Lake sediments have been proven to be reliable natural archives of historical mercury accumulation in aquatic ecosystems(Landis and Keeler 2002 ; Li, Yu, Li, Deng, Xu, Ding, Gao, Hong and Wong 2013), and are widely used for studying regional pollution status and history. Most of studies on mercury pollution in sediments come from relatively remote large watershed lakes and bays(Cossa, Dang, Knoery, Patel-Sorrentino, Tessier, Démoulin and Garnier 2024; Cugler de Pontes, Vicente, Kasper, Machado and Wasserman 2023 ; Ma, Perrot, Baeyens, Li, Lievens, Ngo, Nguyen, Leermakers and Gao 2024; Monteiro, Vieira, Bernardi, Moraes, Rodrigues, de Souza, de Souza, Bastos, Passos and Dórea 2023; Samaniego, Gibaga, Tanciongco, Quierrez, Reyes and Gervasio 2024 ; Zhang, Zhang, Zhao, Shi, Sun, Lu, Liu, Li, Zhao and Cui 2025; Zhao, Zhao, Shi, Lu, Cui, Zhang, Zhang, Zhang and Han 2024). However, there is little research on the mercury pollution in the sediments of lakes directly affected by human activities, such as urban lakes, and most of them focus on the total mercury content, without paying attention to its occurrence forms. Therefore, this paper selected six urban lakes in Nanchang as the research areas, determined the total mercury content in the sediments, and further studied its occurrence forms to analyze the distribution characteristics, migration, transformation and risk assessment of mercury pollution in the surface sediments of urban lakes in Nanchang. 2. Methods and materials 2.1. Study area and Sample collection Nanchang City, located in the central-northern part of Jiangxi Province, China, is the capital of Jiangxi Province with a mild and humid climate. It spans 121 kilometers north to south and 108 kilometers east to west, with a total area of approximately 7,195 km². By the end of 2024, the permanent resident population of the city is about 6.67 million, and the annual regional GDP of the city reaches 720.35 billion yuan. The city has a dense water network and numerous lakes. The water area within the city is 2204.37 km 2 , accounting for 29.78% of the total area. The six urban lakes, namely Dong Lake (DL), Xianshi Lake (XSL), Qingshan Lake (QSL), Xiang Lake (XL), Huangjia Lake (HJL) and Qian Lake (QL), were selected as the research area, shown in Fig. 1 . The surrounding environment of each lake was shown in Table 1 . Table 1 The environmental characteristics around the lake. Abbreviation Name Environment description DL Dong Lake Located in the city center, it has been built into an urban park, with a large commuting population and good greenery XSL Xianshi Lake Close to the city center, it has been built into an urban park, with dense population around QSL Qingshan Lake Close to the city center, it has been developed into a scenic spot, surrounded by residential areas and some industries XL Xiang Lake Located on the outskirts of the city, it has been developed into a wetland park with beautiful scenery and clear water quality HJL Huangjia Lake Located in the new urban district, it is undeveloped and surrounded by many new residential areas QL Qian Lake Located on the outskirts of the city, it has been built into a scenic park surrounded by several colleges and universities A booster column soil sampler (Ø = 10 cm) was used to take samples three times within a range of 3 m. The undisturbed upper part was mixed and packed into a polyethylene sealed bag and transported back to the laboratory. The sediment samples were freeze-dried, ground and screened 100 mesh for the determination of total mercury and mercury speciation. 2.2. Determination of total mercury The determination of total mercury content in the sediment followed the "Determination of mercury, arsenic, selenium, bismuth and antimony in the soil and sediment"(HJ 680–2013). The 0.5 g air-dried sample was weighed and placed into a dissolution cup. A small quantity of water was added to ensure adequate moistening, subsequently followed by 6 ml of hydrochloric acid and the gradual addition of 2 ml of nitric acid. Following the reaction completion, the sample was transferred to a digestion tank for microwave digestion according to the programmed heating protocol. After cooling to room temperature, the solution was filtered and transferred to a 50 ml volumetric flask. The solution was then filled to the mark and analyzed using atomic fluorescence spectrometry for determination. 2.3. Determination of forms of mercury The Tessier(Tessier, Campbell and Bisson 1979 ) continuous extraction method was used to gradually extract the exchangeable (F1), carbonate-bound (F2), iron-manganese oxide-bound (F3), organic-bound (F4) and residual (F5) mercury in sediments. The extraction procedure was performed as follows. Step 1: The 1 g of air-dried sample and 8 ml of 1 mol/L magnesium chloride were added to a 50 ml plastic centrifuge tube. The exchangeable mercury was extracted by oscillating at room temperature at 200 rpm for 1 hour. After extraction, the sample was centrifuged at 4000 rpm for 10 min and the supernatant was the solution to be determining. Step 2: The sediment left from the first step was oscillated with 8 ml of 1 mol/L sodium acetate at 200 rpm for 8 h at room temperature to extract the carbonate-bound mercury. The subsequence was the same as step 1. Step 3: The sediment left from the second step was oscillated with 20 ml of 0.04 mol/L hydroxylamine hydrochloride at 200 rpm for 8 h at 96 ± 3℃ to extract the iron-manganese oxide-bound mercury. The subsequence was the same as step 1. Step 4: The sediment left from the third step was mixed with 3 ml of 0.02 mol/L nitric acid and 5 ml of 30% hydrogen peroxide for 2 h at 85 ± 2℃. Then another 5 ml of 30% hydrogen peroxide was added and mixed for 3 h at 85 ± 2℃. After cooling, 5 ml of 3.2 mol/L ammonium acetate was added and mixed for 0.5 h at room temperature to extract the organic-bound mercury. The subsequence was the same as step 1. Step 5: The residual fraction was extracted through wet acid-based digestion with a 5:1 mixture of hydrofluoric and perchloric acids. After digesting, the residue was dissolved in 12 N HCl and diluted to 25 ml for determining. 2.4. Ecological risk assessment of mercury pollution The Index of Geoaccumulation, proposed by a German scientist Muller(Müller, Bhattacharyya and Pfefferkorn 1979 ) in the early 1960s, has been widely used to study the contamination degree of heavy metals in the sediment. The index considers the impact of human activity on the environment. The calculation formula is as follows, where I geo is the geoaccumulation index, C n is the measured heavy metal concentration, B n is the geochemical background value, and k is the correction index used to characterize sedimentary characteristics, rock geology and other effects, usually taking 1.5. The evaluation criteria of the Geoaccumulation index are as follows: I geo ≤0, unpollution; 0 < I geo ≤1, mild pollution; 1 < I geo ≤2, mild to moderate pollution; 2 < I geo ≤3, moderate pollution; 3 < I geo ≤4, moderate to heavy pollution; 4 5, extreme pollution. $$\:{I}_{geo}\:=\:{{log}}_{2}\left(\frac{{C}_{n}}{k{B}_{n}}\right)$$ 1 The potential ecological risk index was proposed by a Swedish scientist Hakanson(Hakanson 1980 ) in the 1980s. It is usually used to classify the pollution level of heavy metals in the sediment and the degree of potential ecological risk. It is a relatively quick, simple and standard method. The calculation formula is as follows, where E r is the potential ecological risk index, T i is the toxicity correlation coefficient of heavy metal elements (the toxicity coefficient of mercury is 40), C n is the measured heavy metal concentration, B n is the geochemical background value. In this study, the background value of mercury in the soil of Nanchang City was 42ng/g(Cheng, Li, Li, Yang, Liu and Cheng 2014 ). The evaluation criteria of the potential ecological risk index are as follows: E r <40, low risk; 40 < E r ≤80, Moderate risk; 80 < E r ≤160, High risk; 160 < E r ≤320, Very high risk; E r ≥320, Extremely high risk. $$\:{E}_{r}={T}_{i}\times\:\frac{{C}_{n}}{{B}_{n}}$$ 2 The Risk Assessment Code is a risk assessment method based on the occurrence forms of heavy metals in the sediment. It evaluates the potential migration, transformation and bioavailability of heavy metals in sediments by calculating the ratio of easily bioavailable forms to total metal content(Liu, Li, Yin and Shan 2008 ; Sundaray, Nayak, Lin and Bhatta 2011 ). The calculation formula is as follows, where RAC is the risk assessment code, C j is the sum of exchangeable and carbonate-bound heavy metal, C n is the measured heavy metal concentration. The evaluation criteria of the risk assessment code are as follows: RAC < 1, no risk; 1 ≤ RAC < 10, Low risk; 10 ≤ RAC < 30, Moderate risk; 30 ≤ RAC < 50, High risk; RAC ≥ 50, Very high risk. $$\:RAC=\frac{{C}_{j}}{{C}_{n}}\times\:100$$ 3 The Ratio of Secondary Phase and Primary Phase evaluates the potential ecological hazards of heavy metals to the environment by calculating the ratio of secondary phase to primary phase(Yan, Liu, Xie, Gao, Han, Wang and Li 2016). The primary phase refers to the residual state of heavy metals, while the secondary phase consists of four other forms, namely, the exchangeable state, the carbonate-bound state, the iron-manganese oxide-bound state and the organic bound state. The calculation formula is as follows, where M sec is the content of secondary phase, M prim is the content of primary phase. The evaluation criteria of the ratio of secondary phase and primary phase are as follows: RSP < 1, unpollution; 1 ≤ RSP < 2, mild pollution; 2 ≤ RSP < 3, moderate pollution; RSP ≥ 3, heavy pollution. $$\:RSP=\frac{{M}_{sec}}{{M}_{prim}}$$ 4 3. Results and discussion 3.1. Total mercury in sediments The concentrations of total mercury in surface sediments of six urban lakes in Nanchang were present in Fig. 2 . The concentrations of total mercury ranged from 109 to 377 ng/g, with an average value of 197 ± 103 ng/g, which was 4.7 times higher than the soil geochemical background value of 42 ng/g in Nanchang. The total mercury content at all sites was significantly higher than the soil background value. The order of total mercury concentrations in six urban lakes was as follows: Xianshi Lake (377 ng/g) > Qingshan Lake (253 ng/g) > Huangjia Lake (180 ng/g) > Dong Lake (141 ng/g) > Xiang Lake (120 ng/g) > Qian Lake (109 ng/g). Among them, Xianshi Lake, Qingshan Lake, Huangjia Lake and Dong Lake are located in residential areas, Xiang Lake is located in scenic areas, and Qian Lake is located on the outskirts of the city. It can be found that the overall variation trend of total mercury concentration in sediments was that the closer to the city center, the higher the concentration of total mercury, indicating that the mercury content in sediments was greatly affected by human activities. Among the 6 urban lakes, Xianshi Lake is the smallest, while Qingshan Lake is the largest. However, both had relatively high total mercury concentration, indicating that lake size had little impact on mercury concentration in the sediment. From the perspective of geographical location and surrounding environment of the six lakes, Xianshi Lake and Qingshan Lake are both close to the city center of Nanchang, with dense residential populations, significant domestic sewage discharge, and heavy traffic leading to substantial vehicle exhaust emissions. The concentrations of mercury in the sediments of both lakes were relatively higher and both were higher than the average concentration of mercury in lake sediments in China (220 ng/g)(Li, Dai, Zhang, Wan and Xu 2023 ). Huangjia Lake is located near the new urban district, surrounded by residential areas with a large floating population, thus showing relatively higher mercury concentration in the sediment. Dong Lake is located in the city center with a large commuting population, but the concentration of mercury in the sediment was not high, because the Nanchang municipal government has carried out comprehensive treatment of Dong Lake, such as pipeline network renovation to intercept pollution, dredging to suppress endogenous pollution, planting submerged plants and so on. The other two lakes, Qian Lake and Xiang Lake, are located in the suburban area and have been developed into a scenic park and tourist attraction, with clean surroundings and minimal pollution, thus having lower mercury concentration in the sediment. The internationally recognized sediment quality guidelines (SQGs)(MacDonald, Ingersoll and Berger 2000 ), which assess the potential biological toxicity of heavy metals in sediments from lakes, rivers, and other water bodies, were used to evaluate the potential environmental and biological hazards of total mercury in the sediments of Nanchang urban lakes. In the SQGs method, TEL stands for the threshold effect concentration, and PEL stands for the probable effect concentration. If the heavy metal content is below the TEL, biological toxicity is unlikely to occur. If the heavy metal content is between the TEL and PEL, biological toxicity may occasionally occur. If the heavy metal content exceeds the PEL, biological toxicity is highly likely to occur. The TEL and PEL values for Hg were 174 ng/g and 486 ng/g, respectively. The total mercury content in the sediments of Xianshi Lake, Qingshan Lake, and Huangjia Lake were greater than the TEL value but less than the PEL value, indicating that mercury pollution in the surface sediments can cause significant ecological toxicity and pose a considerable threat to the water environment. In contrast, the total mercury content in the sediments of Dong Lake, Xiang Lake, and Qian Lake were below the TEL value, meaning that mercury pollution in the surface sediments had almost no ecological toxicity effect. Compared to other representative lakes, the average total mercury content in the surface sediments of Nanchang urban lakes was significantly lower than that of Ya-Er Lake (547 ng/g)(Chen, Zhang, Cao, Pan, Xiao, Wang, Liang, Liu and Cai 2021) in a region with a thriving aquaculture industry, and Vembanad Lake (407 ng/g)(Mohan, Chandran and Ramasamy 2021 ) in the southwest coast of India. It was also higher than the Nansi Lake (38 ng/g)(Yang, Zhang, Ren, Cao, Chen, Zhang and Shang 2020) in China, Zapotlán Lake (96 ng/g)(Malczyk and Branfireun 2015 ) in Mexico, and the salt marsh estuary (23 ng/g)(Wang and Obrist 2022 ) in Massachusetts of New England. Compared to other natural water bodies like Daya Bay (46 ng/g)(Liu, Kuang, Xu, Chen, Sun, Lin and Lin 2022) and Jiaozhou Bay(53 ng/g)(Mao, Liu, Wang, Lin, Xin, Zhang, Wu, He and Ouyang 2020), the total mercury content in the surface sediments of Nanchang urban lakes was relatively higher, indicating that mercury pollution in lakes affected by human activities is relatively higher. 3.2. Mercury speciation in sediments Mercury in different occurrence forms has different bioavailability and toxicity. The total mercury content in lake sediments cannot truly reflect its hazards. The distribution of mercury speciation can directly affect the migration, transformation and bioavailability of mercury. Compared with the total mercury concentration in the sediment, studying the proportion and distribution of different forms of mercury can more accurately assess the potential hazards caused by mercury pollution when environmental conditions change(Ferrans, Jani, Burlakovs, Klavins and Hogland 2021 ). The spatial distributions and speciation proportions of mercury in surface sediments of six urban lakes in Nanchang were presented in Fig. 3 , 4 . The contents of exchangeable, carbonate-bound, iron-manganese oxide-bound, organic-bound, and residual fractions of mercury were 20 ± 17 ng/g, 2 ± 2 ng/g, 17 ± 7 ng/g, 45 ± 26 ng/g, and 101 ± 31 ng/g, respectively. The proportions of each form of mercury in total mercury were ranked as follows: residual state (54.7%) > organic bound state (24.1%) > exchangeable state (11.0%) > iron-manganese oxide-bound state (9.0%) > carbonate-bound state (2.0%). The results showed that mercury in the surface sediments of urban lakes in Nanchang mainly exists in the form of residue. The residual mercury was tightly bound to the mineral lattice, making it difficult to release and resulting in low bioavailability. However, the exchangeable and carbonate-bound mercury are more susceptible to environmental factors such as pH and ionic strength, exhibiting higher mobility and bioavailability. The iron-manganese oxide-bound and organic bound mercury are relatively stable but can also be released under specific environmental conditions, such as when the redox state changes. Moreover, the total mercury content in sediments far exceeded the environmental background value of Nanchang. When the external environment changes, the amount of mercury that can be released and utilized by organisms was relatively high, posing a certain ecological risk. Among the five different occurrence forms of mercury, the exchangeable fraction, carbonate-bound fraction, iron-manganese oxide-bound fraction, and organic-bound fraction can transform between different forms or release into the aqueous phase when external environmental conditions change, posing significant potential hazards to aquatic ecosystems. Residual state mercury is chemically stable and resistant to changes in the environment, generally considered as non-biodegradable and lacking biological availability. In this study, the content of bioavailable mercury was calculated as the sum of exchangeable fraction, carbonate-bound fraction, iron-manganese oxide-bound fraction and organic-bound fraction. The proportion of bioavailable fraction to the total mercury content was regarded as bioavailability. The proportions of bioavailable mercury in total mercury ranged from 36.4% to 51.7%, with an average of 45.3%. The proportions of bioavailable mercury in total mercury were ranked as follows: Qingshan Lake (51.7%) > Xianshi Lake (49.0%) > Huangjia Lake (43.0%) > Dong Lake (41.2%) > Qian Lake (37.7%) > Xiang Lake (36.4%). Qingshan Lake and Xianshi Lake were the two lakes with the highest proportion of bioavailable mercury in the surface sediments of the six lakes, which were 51.7% and 49.0% respectively. It indicated that the activity of mercury in the sediments of these two lakes were relatively higher and easy to be released with greater potential hazards. Both lakes were relatively close to the city center, with a dense residential population and there was industrial development around Qingshan Lake. It indicated that the bioavailable mercury in the urban lake sediments of Nanchang was significantly affected by human activities, and the bioavailability of mercury in the lake sediments contaminated by industry was relatively high. 3.3. Correlation analysis between mercury and Mercury speciation In order to understand the migration and transformation mechanisms among different occurrence forms of mercury, Pearson correlation analysis was conducted on the total mercury and different occurrence forms of mercury using Origin 2024. The analysis results were shown in Fig. 5 . The total mercury was significantly positively correlated with all five occurrence forms of mercury. The p value of total mercury combined with the carbonate state was less than 0.05, and less than 0.01 (extremely significant correlation) combined with the other four forms, indicating that all forms of mercury were constantly transformed during the continuous increase of the total mercury content. The correlation coefficients of total mercury with the residual fraction and the bioavailable fraction were 0.992 and 0.995 respectively, indicating that the transformation amount of the bioavailable fraction was slightly greater than that of the residual fraction. The bioavailability of mercury would gradually increase over time, and the potential hazard would become greater. In addition, there was also a significantly positive correlation among mercury in different occurrence forms, indicating that each form of mercury promoted the transformation of each other. 3.4. Ecological risk assessment of mercury pollution The mercury pollution status in the sediments of six urban lakes in Nanchang was evaluated by the geoaccumulation index and the potential ecological risk index based on the total content of heavy metals. The bio-toxicity and release possibility of mercury in the sediments were evaluated by the Risk Assessment Code and the Ratio of Secondary Phase and Primary Phase based on the occurrence forms of heavy metals. The evaluation results are shown in Fig. 6 . The I geo of the six urban lakes ranged from 0.79 to 2.58, with an average value of 1.50, and the overall mercury pollution presented as mild to moderate pollution. The I geo of Xianshi Lake and Qingshan Lake were both more than 2, indicating moderate pollution; Dong Lake and Huangjia Lake were between 1 and 2, indicating mild to moderate pollution; Xiang Lake and Qian Lake were between 0 and 1, indicating mild pollution. The I geo of the six lakes was ranked as follows: Xianshi Lake > Qingshan Lake > Huangjia Lake > Dong Lake > Xiang Lake > Qian Lake. The E r of the six urban lakes ranged from 103.8 to 359.0, with an average value of 187.3, and the overall potential ecological risk degree was very high. The E r of Xianshi Lake was greater than 320, presenting an extremely high risk; Qingshan Lake and Huangjia Lake were between 160 and 320, presenting a very high risk; Dong Lake, Xiang Lake and Qian Lake were between 80 and 160, presenting a high potential risk. The E r of the six lakes was ranked as follows: Xianshi Lake > Qingshan Lake > Huangjia Lake > Dong Lake > Xiang Lake > Qian Lake. The RAC of the six urban lakes ranged from 5.8 to 15.8, with an average value of 10.1, and the overall risk assessment degree was moderate. The RAC of Xianshi Lake, Qingshan Lake and Huangjia Lake were between 10 and 30, showing a moderate risk, relatively high migration ability and biological activity. Dong Lake, Qian Lake and Xiang Lake were between 1 and 10, showing a low risk. The RSP of the six urban lakes ranged from 0.57 to 1.07, with an average of 0.78, indicating no pollution and weak mercury migration in the sediment. Among the six urban lakes, only the RSP of Qingshan Lake was greater than 1 showing mild pollution, while the other five lakes were all less than 1 showing no pollution, indicating that the mercury in the sediments of the Qingshan Lake was relatively easy to release and had a higher potential hazard. 4. Conclusion The surface sediments of six urban lakes in Nanchang all exhibited mercury pollution, which was severely influenced by human activities. And it generally showed that the closer to the city center, the more severe the mercury pollution in the surface sediments. The total mercury concentrations in the surface sediments of these six urban lakes ranged from 0.109 ~ 0.377 ng/g, with an average of 0.197 ± 0.103 ng/g, all exceeding the soil background values of Nanchang. Among them, the total mercury concentrations in Qingshan Lake and Xianshi Lake were the highest. Mercury in the sediments mainly existed in the form of residue. The surrounding area of Qingshan Lake had a dense population and industrial development, and the content of bioavailable mercury in its sediment was the highest, which was easy to be released from the sediment and had a relatively high potential hazard. Overall, the mercury pollution in the sediments of these six urban lakes was moderately polluted with a high ecological risk, while the biological toxicity and potential migration characteristics of mercury were considered to be of moderate risk. Among them, the degree of mercury pollution in the sediment of Xianshi Lake was the highest, and the potential mobility of mercury in the sediment of Qingshan Lake was the strongest. Declarations Acknowledgements The authors would like to appreciate the staff who participated in data collection and analysis of this study. Funding This study was kindly supported by Natural Science Foundation of Jiangxi Province (No. 20192BAB203023) . Authors’ Contributions Xiaozhen Liu and Jia Fei designed the study; Chen Yang, Dengbiao Jiang, Luqiang Zhou and Zhiyu Ding contributed to the study investigation; Jia Fei conducted the overall experiment and measurements; Xiaozhen Liu supervised the experiment and measurements; Ying Tong analyzed the data and visualized with the assistance from Xiaozhen Liu. Ying Tong wrote the first draft and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Ethical Approval This is not applicable Consent to Participate This is not applicable Consent to Publish This is not applicable Competing Interests All authors of the manuscript declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data Availability Statement Data will be made available on request. 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Environmental Pollution 363:125183. https://doi.org/https://doi.org/10.1016/j.envpol.2024.125183 Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major Revision 15 Apr, 2026 Reviewers agreed at journal 19 Nov, 2025 Reviewers invited by journal 19 Nov, 2025 Editor invited by journal 04 Nov, 2025 Editor assigned by journal 29 Oct, 2025 First submitted to journal 25 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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2","display":"","copyAsset":false,"role":"figure","size":2713451,"visible":true,"origin":"","legend":"\u003cp\u003eConcentrations of the total mercury in the surface sediments of six urban lakes in Nanchang. 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(a) Proportions of mercury in five different forms. (b) Proportions of bioavailable mercury and residual mercury.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7733844/v1/6bd0c770381bf71c215741e9.png"},{"id":96919393,"identity":"8d591d74-961d-495c-b725-4956310854d8","added_by":"auto","created_at":"2025-11-27 14:13:48","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":3684697,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelation analysis of the total mercury and different forms of mercury.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7733844/v1/ea3cd5176f340a9cf0d0c5d9.png"},{"id":96881303,"identity":"0d77f7ca-f009-4aef-b086-acf062273985","added_by":"auto","created_at":"2025-11-27 07:17:21","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1411053,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of the evaluation results of the four ecological risk assessment methods. (a) The index of geoaccumulation. (b) The potential ecological risk index. (c) The Risk Assessment Code. (d) The Ratio of Secondary Phase and Primary Phase.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7733844/v1/3bb8b5e2348e8a1c60ba03e5.png"},{"id":97248559,"identity":"779ec039-4e0e-4720-a3a8-9a04481df2c0","added_by":"auto","created_at":"2025-12-02 13:03:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":15103884,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7733844/v1/bb9e88f2-d237-4eec-96ce-732ec4fab338.pdf"}],"financialInterests":"","formattedTitle":"Distribution characteristics, speciation and risk assessment of mercury in surface sediments of urban lakes in Nanchang city, China","fulltext":[{"header":"High lights","content":"\u003cp\u003e1. The overall mercury pollution in the surface sediments of six urban lakes in Nanchang was moderate pollution with high risk, and it generally showed that the closer to the city center, the more serious the mercury pollution.\u003c/p\u003e\u003cp\u003e2. Mercury primarily existed in the form of residue. The biological toxicity and release possibility of mercury were moderate risk.\u003c/p\u003e\u003cp\u003e3. Among the six urban lakes, the content of bioavailable mercury in the sediments of Qingshan Lake was the highest, which was significantly affected by human activities and easy to be released back into water.\u003c/p\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eMercury is a heavy metal with potent neurotoxicity, and it is persistent, accumulative and migratory(Gworek, Dmuchowski and Baczewska-Dabrowska \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), posing a significant threat to ecosystems and human health(Hsu-Kim, Kucharzyk, Zhang and Deshusses \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Xie, Wang, Li, Zhang, Tian, Cheng, Zhang and Wang 2021). Due to the persistence of mercury in the atmosphere, it can be transported over long distances and return to the Earth's surface through dry and wet deposition(Aksentov and Kalinchuk \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), and enter water bodies through surface runoff and underground runoff, eventually accumulating in sediments of lakes(Acquavita, Floreani and Covelli \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Rodrigues, Ferrari, dos Santos and Conte Junior \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Lake sediments are important \"sources\" and \"sinks\" of mercury and other trace heavy metals in water bodies(Liu, Hu, Lin, Li and Guo \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), and also key microbial methylation sites, playing an indispensable role in the biogeochemical cycle of mercury(Drott, Lambertsson, Bjorn and Skyllberg \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Dissolved mercury in water migrates and transforms into sediments through adsorption and gravitational settling, and then accumulates in the sediments(Amos, Jacob, Kocman, Horowitz, Zhang, Dutkiewicz, Horvat, Corbitt, Krabbenhoft and Sunderland 2014). Mercury in the surface sediments can also be released back into the water through diffusion and resuspension when environmental conditions change, affecting its distribution and accumulation in the water(Ibanga, Nkwoji, Usese, Onyema and Chukwu \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The total mercury content in the surface sediments of lakes can reflect the degree of mercury pollution in the water environment, however it cannot reflect its potential hazards. The bioavailability, migration and transformation of mercury in the sediments are also closely related to its occurrence form(Bing, Zhou, Wu, Wang, Sun and Li \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Vieira, Bordalo, Figueroa, Soares, Morgado, Abreu and Rendon-von Osten 2021). Heavy metals undergo precipitation-dissolution, oxidation-reduction, adsorption-desorption, complexation and other processes in the sediments. Under different environmental conditions, these heavy metals alter their physicochemical properties through migration and transformation, ultimately forming various occurrence forms, and showing different degrees of hazard, biological toxicity and migration characteristics(Bing, Zhou, Wu, Wang, Sun and Li \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Choppala, Bolan, Lamb and Kunhikrishnan \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Therefore, the systematic assessment of mercury pollution should include not only measuring the total mercury content but also various occurrence forms of mercury to determine its toxicity and ecological risk to organisms and the environment.\u003c/p\u003e\u003cp\u003eIn recent decades, with the intensification of human activities, mercury emissions have increased rapidly and made the mercury pollution more and more serious, which have become a major environmental issue of global concern(Li, Yu, Li, Deng, Xu, Ding, Gao, Hong and Wong 2013). Studies have shown that rivers flowing through human settlements are particularly susceptible to direct impacts from human activities, resulting in more complex mercury sources. Moreover, during this process, the rate and scale of mercury transfer into aquatic ecosystems may also be affected, and all these changes are recorded in sediments(Yang, Turner and Rose \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Lake sediments have been proven to be reliable natural archives of historical mercury accumulation in aquatic ecosystems(Landis and Keeler \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Li, Yu, Li, Deng, Xu, Ding, Gao, Hong and Wong 2013), and are widely used for studying regional pollution status and history. Most of studies on mercury pollution in sediments come from relatively remote large watershed lakes and bays(Cossa, Dang, Knoery, Patel-Sorrentino, Tessier, D\u0026eacute;moulin and Garnier 2024; Cugler de Pontes, Vicente, Kasper, Machado and Wasserman \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Ma, Perrot, Baeyens, Li, Lievens, Ngo, Nguyen, Leermakers and Gao 2024; Monteiro, Vieira, Bernardi, Moraes, Rodrigues, de Souza, de Souza, Bastos, Passos and D\u0026oacute;rea 2023; Samaniego, Gibaga, Tanciongco, Quierrez, Reyes and Gervasio \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Zhang, Zhang, Zhao, Shi, Sun, Lu, Liu, Li, Zhao and Cui 2025; Zhao, Zhao, Shi, Lu, Cui, Zhang, Zhang, Zhang and Han 2024). However, there is little research on the mercury pollution in the sediments of lakes directly affected by human activities, such as urban lakes, and most of them focus on the total mercury content, without paying attention to its occurrence forms. Therefore, this paper selected six urban lakes in Nanchang as the research areas, determined the total mercury content in the sediments, and further studied its occurrence forms to analyze the distribution characteristics, migration, transformation and risk assessment of mercury pollution in the surface sediments of urban lakes in Nanchang.\u003c/p\u003e"},{"header":"2. Methods and materials","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Study area and Sample collection\u003c/h2\u003e\u003cp\u003eNanchang City, located in the central-northern part of Jiangxi Province, China, is the capital of Jiangxi Province with a mild and humid climate. It spans 121 kilometers north to south and 108 kilometers east to west, with a total area of approximately 7,195 km\u0026sup2;. By the end of 2024, the permanent resident population of the city is about 6.67\u0026nbsp;million, and the annual regional GDP of the city reaches 720.35\u0026nbsp;billion yuan. The city has a dense water network and numerous lakes. The water area within the city is 2204.37 km\u003csup\u003e2\u003c/sup\u003e, accounting for 29.78% of the total area. The six urban lakes, namely Dong Lake (DL), Xianshi Lake (XSL), Qingshan Lake (QSL), Xiang Lake (XL), Huangjia Lake (HJL) and Qian Lake (QL), were selected as the research area, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The surrounding environment of each lake was shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\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\u003eThe environmental characteristics around the lake.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\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=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAbbreviation\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eName\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEnvironment description\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDong Lake\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLocated in the city center, it has been built into an urban park, with a large commuting population and good greenery\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eXSL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eXianshi Lake\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eClose to the city center, it has been built into an urban park, with dense population around\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eQSL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eQingshan Lake\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eClose to the city center, it has been developed into a scenic spot, surrounded by residential areas and some industries\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eXL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eXiang Lake\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLocated on the outskirts of the city, it has been developed into a wetland park with beautiful scenery and clear water quality\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHJL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eHuangjia Lake\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLocated in the new urban district, it is undeveloped and surrounded by many new residential areas\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eQL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eQian Lake\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eLocated on the outskirts of the city, it has been built into a scenic park surrounded by several colleges and universities\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\u003eA booster column soil sampler (\u0026Oslash; = 10 cm) was used to take samples three times within a range of 3 m. The undisturbed upper part was mixed and packed into a polyethylene sealed bag and transported back to the laboratory. The sediment samples were freeze-dried, ground and screened 100 mesh for the determination of total mercury and mercury speciation.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Determination of total mercury\u003c/h2\u003e\u003cp\u003eThe determination of total mercury content in the sediment followed the \"Determination of mercury, arsenic, selenium, bismuth and antimony in the soil and sediment\"(HJ 680\u0026ndash;2013). The 0.5 g air-dried sample was weighed and placed into a dissolution cup. A small quantity of water was added to ensure adequate moistening, subsequently followed by 6 ml of hydrochloric acid and the gradual addition of 2 ml of nitric acid. Following the reaction completion, the sample was transferred to a digestion tank for microwave digestion according to the programmed heating protocol. After cooling to room temperature, the solution was filtered and transferred to a 50 ml volumetric flask. The solution was then filled to the mark and analyzed using atomic fluorescence spectrometry for determination.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Determination of forms of mercury\u003c/h2\u003e\u003cp\u003eThe Tessier(Tessier, Campbell and Bisson \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e1979\u003c/span\u003e) continuous extraction method was used to gradually extract the exchangeable (F1), carbonate-bound (F2), iron-manganese oxide-bound (F3), organic-bound (F4) and residual (F5) mercury in sediments. The extraction procedure was performed as follows.\u003c/p\u003e\u003cp\u003eStep 1: The 1 g of air-dried sample and 8 ml of 1 mol/L magnesium chloride were added to a 50 ml plastic centrifuge tube. The exchangeable mercury was extracted by oscillating at room temperature at 200 rpm for 1 hour. After extraction, the sample was centrifuged at 4000 rpm for 10 min and the supernatant was the solution to be determining.\u003c/p\u003e\u003cp\u003eStep 2: The sediment left from the first step was oscillated with 8 ml of 1 mol/L sodium acetate at 200 rpm for 8 h at room temperature to extract the carbonate-bound mercury. The subsequence was the same as step 1.\u003c/p\u003e\u003cp\u003eStep 3: The sediment left from the second step was oscillated with 20 ml of 0.04 mol/L hydroxylamine hydrochloride at 200 rpm for 8 h at 96\u0026thinsp;\u0026plusmn;\u0026thinsp;3℃ to extract the iron-manganese oxide-bound mercury. The subsequence was the same as step 1.\u003c/p\u003e\u003cp\u003eStep 4: The sediment left from the third step was mixed with 3 ml of 0.02 mol/L nitric acid and 5 ml of 30% hydrogen peroxide for 2 h at 85\u0026thinsp;\u0026plusmn;\u0026thinsp;2℃. Then another 5 ml of 30% hydrogen peroxide was added and mixed for 3 h at 85\u0026thinsp;\u0026plusmn;\u0026thinsp;2℃. After cooling, 5 ml of 3.2 mol/L ammonium acetate was added and mixed for 0.5 h at room temperature to extract the organic-bound mercury. The subsequence was the same as step 1.\u003c/p\u003e\u003cp\u003eStep 5: The residual fraction was extracted through wet acid-based digestion with a 5:1 mixture of hydrofluoric and perchloric acids. After digesting, the residue was dissolved in 12 N HCl and diluted to 25 ml for determining.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. Ecological risk assessment of mercury pollution\u003c/h2\u003e\u003cp\u003eThe Index of Geoaccumulation, proposed by a German scientist Muller(M\u0026uuml;ller, Bhattacharyya and Pfefferkorn \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1979\u003c/span\u003e) in the early 1960s, has been widely used to study the contamination degree of heavy metals in the sediment. The index considers the impact of human activity on the environment. The calculation formula is as follows, where \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e is the geoaccumulation index, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e is the measured heavy metal concentration, \u003cem\u003eB\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e is the geochemical background value, and \u003cem\u003ek\u003c/em\u003e is the correction index used to characterize sedimentary characteristics, rock geology and other effects, usually taking 1.5. The evaluation criteria of the Geoaccumulation index are as follows: \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e \u0026le;0, unpollution; 0\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e \u0026le;1, mild pollution; 1\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e \u0026le;2, mild to moderate pollution; 2\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e \u0026le;3, moderate pollution; 3\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e \u0026le;4, moderate to heavy pollution; 4\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e \u0026le;5, heavy pollution; \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e \u0026gt;5, extreme pollution.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{I}_{geo}\\:=\\:{{log}}_{2}\\left(\\frac{{C}_{n}}{k{B}_{n}}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe potential ecological risk index was proposed by a Swedish scientist Hakanson(Hakanson \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1980\u003c/span\u003e) in the 1980s. It is usually used to classify the pollution level of heavy metals in the sediment and the degree of potential ecological risk. It is a relatively quick, simple and standard method. The calculation formula is as follows, where \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e is the potential ecological risk index, \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e is the toxicity correlation coefficient of heavy metal elements (the toxicity coefficient of mercury is 40), \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e is the measured heavy metal concentration, \u003cem\u003eB\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e is the geochemical background value. In this study, the background value of mercury in the soil of Nanchang City was 42ng/g(Cheng, Li, Li, Yang, Liu and Cheng \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The evaluation criteria of the potential ecological risk index are as follows: \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e \u0026lt;40, low risk; 40\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e \u0026le;80, Moderate risk; 80\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e \u0026le;160, High risk; 160\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e \u0026le;320, Very high risk; \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e \u0026ge;320, Extremely high risk.\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{E}_{r}={T}_{i}\\times\\:\\frac{{C}_{n}}{{B}_{n}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe Risk Assessment Code is a risk assessment method based on the occurrence forms of heavy metals in the sediment. It evaluates the potential migration, transformation and bioavailability of heavy metals in sediments by calculating the ratio of easily bioavailable forms to total metal content(Liu, Li, Yin and Shan \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Sundaray, Nayak, Lin and Bhatta \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). The calculation formula is as follows, where \u003cem\u003eRAC\u003c/em\u003e is the risk assessment code, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003ej\u003c/em\u003e\u003c/sub\u003e is the sum of exchangeable and carbonate-bound heavy metal, \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e is the measured heavy metal concentration. The evaluation criteria of the risk assessment code are as follows: \u003cem\u003eRAC\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;1, no risk; 1\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003eRAC\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;10, Low risk; 10\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003eRAC\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;30, Moderate risk; 30\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003eRAC\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;50, High risk; \u003cem\u003eRAC\u003c/em\u003e\u0026thinsp;\u0026ge;\u0026thinsp;50, Very high risk.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:RAC=\\frac{{C}_{j}}{{C}_{n}}\\times\\:100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe Ratio of Secondary Phase and Primary Phase evaluates the potential ecological hazards of heavy metals to the environment by calculating the ratio of secondary phase to primary phase(Yan, Liu, Xie, Gao, Han, Wang and Li 2016). The primary phase refers to the residual state of heavy metals, while the secondary phase consists of four other forms, namely, the exchangeable state, the carbonate-bound state, the iron-manganese oxide-bound state and the organic bound state. The calculation formula is as follows, where \u003cem\u003eM\u003c/em\u003e\u003csub\u003e\u003cem\u003esec\u003c/em\u003e\u003c/sub\u003e is the content of secondary phase, \u003cem\u003eM\u003c/em\u003e\u003csub\u003e\u003cem\u003eprim\u003c/em\u003e\u003c/sub\u003e is the content of primary phase. The evaluation criteria of the ratio of secondary phase and primary phase are as follows: \u003cem\u003eRSP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;1, unpollution; 1\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003eRSP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;2, mild pollution; 2\u0026thinsp;\u0026le;\u0026thinsp;\u003cem\u003eRSP\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;3, moderate pollution; \u003cem\u003eRSP\u003c/em\u003e\u0026thinsp;\u0026ge;\u0026thinsp;3, heavy pollution.\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:RSP=\\frac{{M}_{sec}}{{M}_{prim}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Total mercury in sediments\u003c/h2\u003e\u003cp\u003eThe concentrations of total mercury in surface sediments of six urban lakes in Nanchang were present in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The concentrations of total mercury ranged from 109 to 377 ng/g, with an average value of 197\u0026thinsp;\u0026plusmn;\u0026thinsp;103 ng/g, which was 4.7 times higher than the soil geochemical background value of 42 ng/g in Nanchang. The total mercury content at all sites was significantly higher than the soil background value. The order of total mercury concentrations in six urban lakes was as follows: Xianshi Lake (377 ng/g)\u0026thinsp;\u0026gt;\u0026thinsp;Qingshan Lake (253 ng/g)\u0026thinsp;\u0026gt;\u0026thinsp;Huangjia Lake (180 ng/g)\u0026thinsp;\u0026gt;\u0026thinsp;Dong Lake (141 ng/g)\u0026thinsp;\u0026gt;\u0026thinsp;Xiang Lake (120 ng/g)\u0026thinsp;\u0026gt;\u0026thinsp;Qian Lake (109 ng/g). Among them, Xianshi Lake, Qingshan Lake, Huangjia Lake and Dong Lake are located in residential areas, Xiang Lake is located in scenic areas, and Qian Lake is located on the outskirts of the city. It can be found that the overall variation trend of total mercury concentration in sediments was that the closer to the city center, the higher the concentration of total mercury, indicating that the mercury content in sediments was greatly affected by human activities.\u003c/p\u003e\u003cp\u003eAmong the 6 urban lakes, Xianshi Lake is the smallest, while Qingshan Lake is the largest. However, both had relatively high total mercury concentration, indicating that lake size had little impact on mercury concentration in the sediment. From the perspective of geographical location and surrounding environment of the six lakes, Xianshi Lake and Qingshan Lake are both close to the city center of Nanchang, with dense residential populations, significant domestic sewage discharge, and heavy traffic leading to substantial vehicle exhaust emissions. The concentrations of mercury in the sediments of both lakes were relatively higher and both were higher than the average concentration of mercury in lake sediments in China (220 ng/g)(Li, Dai, Zhang, Wan and Xu \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Huangjia Lake is located near the new urban district, surrounded by residential areas with a large floating population, thus showing relatively higher mercury concentration in the sediment. Dong Lake is located in the city center with a large commuting population, but the concentration of mercury in the sediment was not high, because the Nanchang municipal government has carried out comprehensive treatment of Dong Lake, such as pipeline network renovation to intercept pollution, dredging to suppress endogenous pollution, planting submerged plants and so on. The other two lakes, Qian Lake and Xiang Lake, are located in the suburban area and have been developed into a scenic park and tourist attraction, with clean surroundings and minimal pollution, thus having lower mercury concentration in the sediment.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe internationally recognized sediment quality guidelines (SQGs)(MacDonald, Ingersoll and Berger \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2000\u003c/span\u003e), which assess the potential biological toxicity of heavy metals in sediments from lakes, rivers, and other water bodies, were used to evaluate the potential environmental and biological hazards of total mercury in the sediments of Nanchang urban lakes. In the SQGs method, TEL stands for the threshold effect concentration, and PEL stands for the probable effect concentration. If the heavy metal content is below the TEL, biological toxicity is unlikely to occur. If the heavy metal content is between the TEL and PEL, biological toxicity may occasionally occur. If the heavy metal content exceeds the PEL, biological toxicity is highly likely to occur. The TEL and PEL values for Hg were 174 ng/g and 486 ng/g, respectively. The total mercury content in the sediments of Xianshi Lake, Qingshan Lake, and Huangjia Lake were greater than the TEL value but less than the PEL value, indicating that mercury pollution in the surface sediments can cause significant ecological toxicity and pose a considerable threat to the water environment. In contrast, the total mercury content in the sediments of Dong Lake, Xiang Lake, and Qian Lake were below the TEL value, meaning that mercury pollution in the surface sediments had almost no ecological toxicity effect.\u003c/p\u003e\u003cp\u003eCompared to other representative lakes, the average total mercury content in the surface sediments of Nanchang urban lakes was significantly lower than that of Ya-Er Lake (547 ng/g)(Chen, Zhang, Cao, Pan, Xiao, Wang, Liang, Liu and Cai 2021) in a region with a thriving aquaculture industry, and Vembanad Lake (407 ng/g)(Mohan, Chandran and Ramasamy \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) in the southwest coast of India. It was also higher than the Nansi Lake (38 ng/g)(Yang, Zhang, Ren, Cao, Chen, Zhang and Shang 2020) in China, Zapotl\u0026aacute;n Lake (96 ng/g)(Malczyk and Branfireun \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) in Mexico, and the salt marsh estuary (23 ng/g)(Wang and Obrist \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) in Massachusetts of New England. Compared to other natural water bodies like Daya Bay (46 ng/g)(Liu, Kuang, Xu, Chen, Sun, Lin and Lin 2022) and Jiaozhou Bay(53 ng/g)(Mao, Liu, Wang, Lin, Xin, Zhang, Wu, He and Ouyang 2020), the total mercury content in the surface sediments of Nanchang urban lakes was relatively higher, indicating that mercury pollution in lakes affected by human activities is relatively higher.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.2. Mercury speciation in sediments\u003c/h2\u003e\u003cp\u003eMercury in different occurrence forms has different bioavailability and toxicity. The total mercury content in lake sediments cannot truly reflect its hazards. The distribution of mercury speciation can directly affect the migration, transformation and bioavailability of mercury. Compared with the total mercury concentration in the sediment, studying the proportion and distribution of different forms of mercury can more accurately assess the potential hazards caused by mercury pollution when environmental conditions change(Ferrans, Jani, Burlakovs, Klavins and Hogland \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe spatial distributions and speciation proportions of mercury in surface sediments of six urban lakes in Nanchang were presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The contents of exchangeable, carbonate-bound, iron-manganese oxide-bound, organic-bound, and residual fractions of mercury were 20\u0026thinsp;\u0026plusmn;\u0026thinsp;17 ng/g, 2\u0026thinsp;\u0026plusmn;\u0026thinsp;2 ng/g, 17\u0026thinsp;\u0026plusmn;\u0026thinsp;7 ng/g, 45\u0026thinsp;\u0026plusmn;\u0026thinsp;26 ng/g, and 101\u0026thinsp;\u0026plusmn;\u0026thinsp;31 ng/g, respectively. The proportions of each form of mercury in total mercury were ranked as follows: residual state (54.7%)\u0026thinsp;\u0026gt;\u0026thinsp;organic bound state (24.1%)\u0026thinsp;\u0026gt;\u0026thinsp;exchangeable state (11.0%)\u0026thinsp;\u0026gt;\u0026thinsp;iron-manganese oxide-bound state (9.0%)\u0026thinsp;\u0026gt;\u0026thinsp;carbonate-bound state (2.0%). The results showed that mercury in the surface sediments of urban lakes in Nanchang mainly exists in the form of residue. The residual mercury was tightly bound to the mineral lattice, making it difficult to release and resulting in low bioavailability. However, the exchangeable and carbonate-bound mercury are more susceptible to environmental factors such as pH and ionic strength, exhibiting higher mobility and bioavailability. The iron-manganese oxide-bound and organic bound mercury are relatively stable but can also be released under specific environmental conditions, such as when the redox state changes. Moreover, the total mercury content in sediments far exceeded the environmental background value of Nanchang. When the external environment changes, the amount of mercury that can be released and utilized by organisms was relatively high, posing a certain ecological risk.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAmong the five different occurrence forms of mercury, the exchangeable fraction, carbonate-bound fraction, iron-manganese oxide-bound fraction, and organic-bound fraction can transform between different forms or release into the aqueous phase when external environmental conditions change, posing significant potential hazards to aquatic ecosystems. Residual state mercury is chemically stable and resistant to changes in the environment, generally considered as non-biodegradable and lacking biological availability. In this study, the content of bioavailable mercury was calculated as the sum of exchangeable fraction, carbonate-bound fraction, iron-manganese oxide-bound fraction and organic-bound fraction. The proportion of bioavailable fraction to the total mercury content was regarded as bioavailability. The proportions of bioavailable mercury in total mercury ranged from 36.4% to 51.7%, with an average of 45.3%. The proportions of bioavailable mercury in total mercury were ranked as follows: Qingshan Lake (51.7%)\u0026thinsp;\u0026gt;\u0026thinsp;Xianshi Lake (49.0%)\u0026thinsp;\u0026gt;\u0026thinsp;Huangjia Lake (43.0%)\u0026thinsp;\u0026gt;\u0026thinsp;Dong Lake (41.2%)\u0026thinsp;\u0026gt;\u0026thinsp;Qian Lake (37.7%)\u0026thinsp;\u0026gt;\u0026thinsp;Xiang Lake (36.4%). Qingshan Lake and Xianshi Lake were the two lakes with the highest proportion of bioavailable mercury in the surface sediments of the six lakes, which were 51.7% and 49.0% respectively. It indicated that the activity of mercury in the sediments of these two lakes were relatively higher and easy to be released with greater potential hazards. Both lakes were relatively close to the city center, with a dense residential population and there was industrial development around Qingshan Lake. It indicated that the bioavailable mercury in the urban lake sediments of Nanchang was significantly affected by human activities, and the bioavailability of mercury in the lake sediments contaminated by industry was relatively high.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Correlation analysis between mercury and Mercury speciation\u003c/h2\u003e\u003cp\u003eIn order to understand the migration and transformation mechanisms among different occurrence forms of mercury, Pearson correlation analysis was conducted on the total mercury and different occurrence forms of mercury using Origin 2024. The analysis results were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The total mercury was significantly positively correlated with all five occurrence forms of mercury. The \u003cem\u003ep\u003c/em\u003e value of total mercury combined with the carbonate state was less than 0.05, and less than 0.01 (extremely significant correlation) combined with the other four forms, indicating that all forms of mercury were constantly transformed during the continuous increase of the total mercury content. The correlation coefficients of total mercury with the residual fraction and the bioavailable fraction were 0.992 and 0.995 respectively, indicating that the transformation amount of the bioavailable fraction was slightly greater than that of the residual fraction. The bioavailability of mercury would gradually increase over time, and the potential hazard would become greater. In addition, there was also a significantly positive correlation among mercury in different occurrence forms, indicating that each form of mercury promoted the transformation of each other.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.4. Ecological risk assessment of mercury pollution\u003c/h2\u003e\u003cp\u003eThe mercury pollution status in the sediments of six urban lakes in Nanchang was evaluated by the geoaccumulation index and the potential ecological risk index based on the total content of heavy metals. The bio-toxicity and release possibility of mercury in the sediments were evaluated by the Risk Assessment Code and the Ratio of Secondary Phase and Primary Phase based on the occurrence forms of heavy metals. The evaluation results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e of the six urban lakes ranged from 0.79 to 2.58, with an average value of 1.50, and the overall mercury pollution presented as mild to moderate pollution. The \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e of Xianshi Lake and Qingshan Lake were both more than 2, indicating moderate pollution; Dong Lake and Huangjia Lake were between 1 and 2, indicating mild to moderate pollution; Xiang Lake and Qian Lake were between 0 and 1, indicating mild pollution. The \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003egeo\u003c/em\u003e\u003c/sub\u003e of the six lakes was ranked as follows: Xianshi Lake\u0026thinsp;\u0026gt;\u0026thinsp;Qingshan Lake\u0026thinsp;\u0026gt;\u0026thinsp;Huangjia Lake\u0026thinsp;\u0026gt;\u0026thinsp;Dong Lake\u0026thinsp;\u0026gt;\u0026thinsp;Xiang Lake\u0026thinsp;\u0026gt;\u0026thinsp;Qian Lake.\u003c/p\u003e\u003cp\u003eThe \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e of the six urban lakes ranged from 103.8 to 359.0, with an average value of 187.3, and the overall potential ecological risk degree was very high. The \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e of Xianshi Lake was greater than 320, presenting an extremely high risk; Qingshan Lake and Huangjia Lake were between 160 and 320, presenting a very high risk; Dong Lake, Xiang Lake and Qian Lake were between 80 and 160, presenting a high potential risk. The \u003cem\u003eE\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e of the six lakes was ranked as follows: Xianshi Lake\u0026thinsp;\u0026gt;\u0026thinsp;Qingshan Lake\u0026thinsp;\u0026gt;\u0026thinsp;Huangjia Lake\u0026thinsp;\u0026gt;\u0026thinsp;Dong Lake\u0026thinsp;\u0026gt;\u0026thinsp;Xiang Lake\u0026thinsp;\u0026gt;\u0026thinsp;Qian Lake.\u003c/p\u003e\u003cp\u003eThe \u003cem\u003eRAC\u003c/em\u003e of the six urban lakes ranged from 5.8 to 15.8, with an average value of 10.1, and the overall risk assessment degree was moderate. The \u003cem\u003eRAC\u003c/em\u003e of Xianshi Lake, Qingshan Lake and Huangjia Lake were between 10 and 30, showing a moderate risk, relatively high migration ability and biological activity. Dong Lake, Qian Lake and Xiang Lake were between 1 and 10, showing a low risk.\u003c/p\u003e\u003cp\u003eThe \u003cem\u003eRSP\u003c/em\u003e of the six urban lakes ranged from 0.57 to 1.07, with an average of 0.78, indicating no pollution and weak mercury migration in the sediment. Among the six urban lakes, only the \u003cem\u003eRSP\u003c/em\u003e of Qingshan Lake was greater than 1 showing mild pollution, while the other five lakes were all less than 1 showing no pollution, indicating that the mercury in the sediments of the Qingshan Lake was relatively easy to release and had a higher potential hazard.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThe surface sediments of six urban lakes in Nanchang all exhibited mercury pollution, which was severely influenced by human activities. And it generally showed that the closer to the city center, the more severe the mercury pollution in the surface sediments. The total mercury concentrations in the surface sediments of these six urban lakes ranged from 0.109\u0026thinsp;~\u0026thinsp;0.377 ng/g, with an average of 0.197\u0026thinsp;\u0026plusmn;\u0026thinsp;0.103 ng/g, all exceeding the soil background values of Nanchang. Among them, the total mercury concentrations in Qingshan Lake and Xianshi Lake were the highest. Mercury in the sediments mainly existed in the form of residue. The surrounding area of Qingshan Lake had a dense population and industrial development, and the content of bioavailable mercury in its sediment was the highest, which was easy to be released from the sediment and had a relatively high potential hazard. Overall, the mercury pollution in the sediments of these six urban lakes was moderately polluted with a high ecological risk, while the biological toxicity and potential migration characteristics of mercury were considered to be of moderate risk. Among them, the degree of mercury pollution in the sediment of Xianshi Lake was the highest, and the potential mobility of mercury in the sediment of Qingshan Lake was the strongest.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eAcknowledgements\u003c/p\u003e\n\u003cp\u003eThe authors would like to appreciate the staff who participated in data collection and analysis of this study.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eThis study was kindly supported by Natural Science Foundation of Jiangxi Province (No. 20192BAB203023) .\u003c/p\u003e\n\u003cp\u003eAuthors\u0026rsquo; Contributions\u003c/p\u003e\n\u003cp\u003eXiaozhen Liu and Jia Fei designed the study; Chen Yang, Dengbiao Jiang, Luqiang Zhou and Zhiyu Ding contributed to the study investigation; Jia Fei conducted the overall experiment and measurements; Xiaozhen Liu supervised the experiment and measurements; Ying Tong analyzed the data and visualized with the assistance from Xiaozhen Liu. Ying Tong wrote the first draft and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003eEthical Approval\u003c/p\u003e\n\u003cp\u003eThis is not applicable\u003c/p\u003e\n\u003cp\u003eConsent to Participate\u003c/p\u003e\n\u003cp\u003eThis is not applicable\u003c/p\u003e\n\u003cp\u003eConsent to Publish\u003c/p\u003e\n\u003cp\u003eThis is not applicable\u003c/p\u003e\n\u003cp\u003eCompeting Interests\u003c/p\u003e\n\u003cp\u003eAll authors of the manuscript declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003eData Availability Statement\u003c/p\u003e\n\u003cp\u003eData will be made available on request.\u003c/p\u003e"},{"header":"References","content":"\u003cp\u003eAcquavita A, Floreani F, Covelli S (2021) Occurrence and speciation of arsenic and mercury in alluvial and coastal sediments. 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The increase of mercury emission and the aggravation of mercury pollution have attracted the attention of many scholars. The study focused on six urban lakes in Nanchang city, China as the research area. By determining the content of total mercury (THg) and mercury speciation in the surface sediments, Pearson correlation analysis, Tessier continuous extraction method and multiple ecological risk assessment methods were employed to investigate the distribution characteristics, migration and transformation, and ecological risks of mercury in the surface sediments of urban lakes in Nanchang. The results showed that the total mercury concentrations ranged from 0.109\u0026thinsp;~\u0026thinsp;0.377 ng/g, with an average of 0.197\u0026thinsp;\u0026plusmn;\u0026thinsp;0.103 ng/g. The mercury pollution was relatively severe, and the closer to the city center, the more serious the mercury pollution. Mercury primarily existed in the form of residue, accounting for approximately 48.3% to 63.6% of the total mercury. The proportions of bioavailable mercury was also relatively high, ranging from 36.4% to 51.7%. Among the six urban lakes, the content of bioavailable mercury in the sediments of Qingshan Lake was the highest, which was significantly affected by human activities and easy to be released back into water. The overall mercury pollution in the sediment of six urban lakes in Nanchang was moderate pollution with high risk, while the biological toxicity and release possibility of mercury were moderate risk.\u003c/p\u003e","manuscriptTitle":"Distribution characteristics, speciation and risk assessment of mercury in surface sediments of urban lakes in Nanchang city, China","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-27 07:17:17","doi":"10.21203/rs.3.rs-7733844/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2026-04-15T15:48:17+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-11-19T14:24:40+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-19T14:12:51+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Environmental Science and Pollution Research","date":"2025-11-04T12:04:54+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-29T05:13:02+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2025-10-25T19:37:40+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"b8cccb06-f542-4bf8-95ca-3db85ab64f5c","owner":[],"postedDate":"November 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-17T19:44:00+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-27 07:17:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7733844","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7733844","identity":"rs-7733844","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}