Source, transport and fate of nitrate in shallow groundwater in the eastern Niger Delta

Source, transport and fate of nitrate in shallow groundwater in the eastern Niger Delta | 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 Source, transport and fate of nitrate in shallow groundwater in the eastern Niger Delta Dogo Lawrence Aleku, Kirsten Dähnke, Thomas Pichler This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4390029/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 20 Nov, 2024 Read the published version in Environmental Science and Pollution Research → Version 1 posted 6 You are reading this latest preprint version Abstract The eastern Niger Delta region in Nigeria is a hotspot for reactive nitrogen pollution due to extensive animal husbandry, pit latrine usage and agricultural practices. Despite the high level of human activity, the sources and processes affecting nitrogen in groundwater remain understudied. Groundwater nitrate (NO 3 − ) concentrations are highly variable, with some areas recording values well above the safe drinking water threshold of 50 mg/L. This is particularly true near municipal sewage systems. Elevated nitrite (NO 2 − ) and ammonium (NH 4 + ) concentrations were also detected in the study area. Sewage analysis revealed NO 3 − concentrations ranging from 1 to 145 mg/L, NO 2 − from 0.2 to 2 mg/L, and notably high NH 4 + concentrations. A comparison of major ions indicated that 71%, 90%, 87%, and 92% of groundwater samples surpassed reference site levels for calcium (Ca 2+ ), sodium (Na + ), potassium (K + ), and chloride (Cl − ), respectively, pointing to sewage as a likely source of contamination. The NO 3 − /Cl − ratios at several sites suggested that most groundwater NO 3 − originates from human waste. Stable isotope analysis of NO 3 − showed a general enrichment in 15 N and, in some cases, a depletion in 18 O, indicating that the NO 3 − originates from sewage-derived NH 4 + nitrification. Although denitrification, a process that reduces NO 3 − , is present, the high dissolved oxygen (DO) and NO 3 − levels in the groundwater suggest that denitrification is insufficient to fully mitigate NO 3 − pollution. Consequently, there is a risk of NO 3 − leaching from shallow aquifers into the Niger Delta’s surface waters and ultimately into the coastal ocean. Nitrate nitrite groundwater source sewage isotopic hydrochemical contamination anthropogenic. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 1. Introduction Globally, excess nitrate (NO 3 - ) in groundwater is an environmental problem threatening human health, either directly due to its adverse health effects or by inducing the release of toxic metals, such as cadmium (Cd) from the aquifer matrix (e.g., Kubier et al. 2020, Kubier et al. 2019, Ward et al. 2018). Migration of NO 3 - from groundwater to surface waters and subsequently into the coastal ocean has also become a cause of concern worldwide (e.g., Guo et al. 2021, Harris et al. 2022). As a result, various global organizations have implemented measures to reduce NO 3 - levels in groundwater. For instance, the European Union (EU) established a range of measures to reduce NO 3 - contributions from agricultural and non-agricultural sources in the EU (Stark &Richards 2008). To this effect, the EU selected a concentration of 50 mg/L as the guideline value for NO 3 - in groundwater. Similarly, several countries, including Nigeria (NSDWQ 2015), and organizations like the World Health Organization (WHO 2017) have set 50 mg/L as the guideline for NO 3 - in drinking water. Various authors investigated the groundwater quality and geochemistry in Nigeria's urban and rural areas (Abanyie et al. 2023, Eludoyin &Fajiwe 2023, Obrike et al. 2022, Raimi et al. 2023). Some studies included NO 3 - and concentrations reported were up to 2.1 mg/L in Nnewi, 4.2 mg/L in Awka (Ayejoto &Egbueri 2023), 21.1 mg/L in Umunya (Egbueri et al. 2023), 36 mg/L in Ogbaru (Unigwe et al. 2022), 157 mg/L in Gboko (Omonona &Okogbue 2021), and up to 770 mg/L in Maiduguri (Goni et al. 2019). NO 3 - sources were not identified in those studies, although agricultural activities, pit latrines and animal waste were hypothesized as possible sources. Similar investigations in other parts of the world suggested that, most commonly, anthropogenic sources such as nitrogen fertilizer, manure, municipal and domestic sewage discharge, pit latrines, soil organic nitrogen and atmospheric deposition contribute to NO 3 - loading of groundwaters (Biddau et al. 2023, Kendall et al. 2007). To effectively reduce the excess levels of NO 3 - in groundwater, improved groundwater management practices that minimize the release of nitrogen compounds into the environment are required. Determining NO 3 - sources and variability is essential to improve nitrogen management practices. However, identifying a given source and its contribution can be complicated when multiple nitrogen sources exist. Such uncertainty, for instance, is typical in urban areas where intensive agricultural activities involving nitrogen fertilizers are common (Minet et al. 2017). Hence, accurately identifying the NO 3 - source(s) in groundwater, evaluating the ongoing biogeochemical process in the aquifer and calculating NO 3 - contributions from different potential sources are necessary for effective management measures to reduce NO 3 - levels in groundwater. Stable oxygen and nitrogen isotopic signatures of NO 3 - have been effectively applied to identify NO 3 - sources while also detecting nitrification, denitrification or dilution in groundwater (e.g., Anornu et al. 2017, Carrey et al. 2021, Guo et al. 2020, Harris et al. 2022). However, uncertainties remain during data interpretation, which include (1) significant overlaps resulting from multiple nitrogen sources during the early leaching process within unsaturated zones or as nitrification proceeds and (2) a mixing process between the multiple nitrogen sources, subsequent NO 3 - removal due to denitrification (Kendall et al. 2007), and the concurrent productions of NO 3 - during anaerobic ammonium oxidation under limited oxygen conditions (Granger &Wankel 2016). This complicates nitrogen source identification in groundwaters (Kendall et al. 2007), hence the need for an approach that combines major ion and isotope data to reduce such uncertainties (Minet et al. 2017). This is possible because, for instance, municipal sewage and animal wastes are typically enriched with chloride (Cl - ), potassium (K + ), and sodium (Na + ), amongst many other contaminants, which are all released by decomposing organic matter (e.g., Ranjbar &Jalali 2012). With this in mind, we combined stable NO 3 - isotope data with hydrochemical markers (i.e., Ca 2+ , Na + , K + , and Cl − ) for nitrogen source identification. It is important to note that there are no available studies on groundwater NO 3 - in the eastern Niger Delta, despite the widespread nitrogen-related anthropogenic activities. Hence, this study presents a unique opportunity to investigate NO 3 - and NO 2 - source, transport, and fate across the eastern Niger Delta groundwater systems to improve management and remediation efforts. 2. Materials and methods 2.1. Site description, geology, and hydrogeology The study site is in the eastern Niger Delta Region of Nigeria (Latitude 4°44ˈ57ˈˈN to 4°47ˈ42ˈˈN and Longitude 7°05ˈ26ˈˈE to 7°09ˈ54ˈˈE) and comprises the following communities: Alesa, Ogale, Ebubu, Alode and Okochiri. The sampling locations are shown in Fig. 1. Alesa, Ogale, and Ebubu are in the northern part of the study area, commonly characterized by the presence of (1) municipal and domestic sewage in the drainage systems, and (2) unlined pit latrine toilets for human excrement. In these communities, the sewage flow was hindered by blockage resulting from indiscriminate solid waste disposal and the gentle nature of the topography. The municipal sewage was more commonly observed in Alesa than in Ogale and Ebubu. In contrast, sewage was not observed in the Alode and Okochiri drainage systems. The steep nature of the topography appears to play an essential role in aiding the free flow and eventual absence of municipal sewage. Three major lithostratigraphic units have been identified within the Niger Delta Basin: the Benin Formation, Agbada Formation and Akata Formation (Obaje 2009) . The Oligocene to Recent Benin Formation is about 2 km thick and predominantly consists of clay units, coarse-grained, sub-angular to well-rounded, poorly sorted coastal plain sand and alluvial deposits of about 95 % to 99 % quartz grains at shallow depths (Nwajide 2013) . The Formation serves as a groundwater reservoir for the region (Adelana et al. 2008) . The aquifer is recharged mainly by direct precipitation at 2,532 mm/year and exfiltration from major regional rivers (Abam &Nwankwoala 2020) . The sandy and permeable nature of the aquifer further facilitates rapid infiltration into the upper units of the formation (Abam &Nwankwoala 2020) . However, the anthropogenic activities in the region have left the shallow groundwater vulnerable to pollution (Adeniran et al. 2023) . 2.2. Groundwater sampling The groundwater samples for this study were collected from shallow wells (1 to 30 m) in the Benin Formation in April 2022 and April 2023. Groundwater and sewage samples were collected from communities with municipal and domestic sewage and areas considered relatively unaffected by municipal wastewater (i.e., reference sites 1 to 5). The r eference samples were collected from Alode (Ref 1 and 2), Okochiri (Ref 3), Okrika Island (Ref 4) and Ogale (Ref 5) within the same geological unit in relatively new residential areas without municipal wastewater or other potential anthropogenic contamination sources. The groundwater samples were collected either (1) manually, using a water bailer made of polyvinyl chloride , or (2) with an electric submersible pump in cases where those were installed in the wells. First, groundwater was pumped into the overhead storage tank to purge the wells for 30 minutes before sampling directly from the wellhead . The bailer was rinsed three times with the groundwater before sampling. Sampling was conducted during the early hours (between 6:00 and 8:00 a.m.) when the wells were actively used to ensure that fresh samples were collected. In total, 180 samples (105 in 2022 and 75 in 2023) were collected from private supply wells (PSW) and community supply wells (CSW) next to municipal or domestic sewage drainages. In private residences, most wells were next to pit latrine toilets, usually between 2 and 9 m apart. Immediately after collection, the samples were filtered through 0.45 μm cellulose acetate (CA) membranes and separated into aliquots for the different chemical analyses (isotopes, major ions, and dissolved organic carbon). The samples were stored in 25 mL glass vials for DOC, 30 mL brown HDPE vials for major cations and 20 mL clear HDPE vials for anions and isotopes. The sub-samples for DOC and major cations were preserved with 2 % concentrated nitric acid (HNO 3 ). All samples were stored at 4 °C until laboratory analyses. The pH, conductivity (EC), total dissolved solids (TDS), temperature, dissolved oxygen (DO), salinity, redox potential (ORP), and resistivity were determined immediately in situ using aHanna instrument HI98494 multiparameter. In the field, the total alkalinity (CaCO 3 ) was determined by colorimetric titration with 0.16 N H 2 SO 4 in combination with a bromcresol green-methyl red indicator. The bromcresol green-methyl red indicator powder was added to 100 mL of the groundwater sample and titrated using a Hach digital titrator to a light pink color. The total alkalinity was reported as mg/L CaCO 3 . Additionally, 8 samples were collected from municipal and domestic sewage in Alesa, Ogale, and Ebubu.The samples were filtered through 0.45 μm cellulose acetate (CA) membrane filters and collected into 20 mL clear HDPE vials. 2.3. Analytical procedures 2.3.1. Cation, anion and DOC measurements Major cations and trace elements were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) using a Perkin Elmer Optima 7300 DV instrument. The precision of the measurement was checked using EnviroMAT Groundwater Low (ES-L-2) and High (ES-H-2) certified water from SCP Science, Canada, showing errors of < 3 % for all analytes. Major anions (including NO 3 - and NO 2 - )were determined using a Metrohm 883 Basic IC plus instrument with a 5 μL injection loop and a Metrosep A Supp5 (150 × 4.0 mm; 5 μm) column. An internal standard was used to check the accuracy and precision of the measurement, and errors of less than 10 % were recorded. Dissolved organic carbon (DOC), the fraction of organic carbon that can pass through a 0.45 μm pore size, was determined using a Shimadzu TOC analyzer TOC-V CPN (Shimadzu Corporation). A certified Total Organic Carbon Standard of 50 mg/L (Aqua Solutions) was used for quality control, and the measurement error was determined to be less than 6 %. The ammonium (NH 4 + ) was determined photometrically at 655 nm with salicylate following standard procedures (DIN 38406 1983). 2.3.2. Determination of NO 3 - isotopes ( d 15 N-NO 3 - and d 18 O-NO 3 - ) A subset of 20 groundwater and 2 municipal wastewater samples were analyzed for stable isotopes, specifically from wells where their owners had granted permission to collect samples. The 15 N/ 14 N and 18 O/ 16 O ratios in dissolved NO 3 - were measured and expressed as d 15 N-NO 3 - and d 18 O-NO 3 - . Isotope ratios were determined following the denitrifier method (Casciotti et al., 2002; Sigman et al., 2001). NO 3 - and NO 2 - are quantitatively converted to nitrous oxide (N 2 O) by the denitrifying bacteria ( Pseudomonas aureofaciens , ATCC#13985) that lack N 2 O reductase. The sample volume for isotope determination was adjusted to achieve 10 nmol of N 2 O. N 2 O was extracted from the sample vials by purging with helium and measured with a GasBench II (Thermo, Germany), coupled to an isotope ratio mass spectrometer (Delta Plus XP, Thermo, Germany). For quality assurance, two external standards (USGS34: δ 15 N: -1.8 ‰, δ 18 O: -27.9 ‰; IAEA-NO 3 - : d 15 N: +4.7 ‰, d 18 O: +25.6 ‰) and one internal standard were measured with each sample batch. The standard deviation of samples and standards was < 0.2 ‰ for δ 15 N-NO 3 - ( n = 4) and < 0.5 ‰ for δ 18 O-NO 3 - ( n = 4). Note that this method yields combined isotope values for NO 3 - + NO 2 - . In two samples, NO 2 - concentration exceeded 5 % of the nitrate concentration. These samples were excluded from the isotopic analysis. 3. Results 3.1. Field measurements and chemical data The supplemental information (SI) Tables 1S and 2S present data for all samples, including minimum, maximum, median and average. The pH ranged from 3.5 to 6.9, temperature from 25 to 34 °C, EC from 16 to 852 µS/cm, TDS from 8 to 427 mg/L, DO from 0.7 to 8.9 mg/L and salinity from 0.01 to 0.4 PSU. The EC ranged from 17 to 69 µS/cm at the reference site , and the TDS ranged from 9 to 32 mg/L. Most groundwater quality parameters at the contaminated and reference sites were in accordance with WHO (2017) guidelines for drinking water. Nevertheless, the parameters associated with contamination from NO 3 - fertilizer and animal/human waste effluents (Cl - and K + ) or animal/human wastes (Na + ) (Minet et al. 2017) showed higher concentrations at the contaminated sites than those at the reference sites. The concentrations of Na + in groundwater ranged from 1 to 56 mg/L, and 57 % of the samples exceeded the measured reference value range of 1 to 2 mg/L. The concentrations of K + ranged from 0.1 to 59 mg/L, and 33 % of the samples exceeded the 0.3 to 0.6 mg/L range at the reference sites. The concentrations of Cl - ranged from 1 to 66 mg/L, with 25 % of the samples exceeding the 2 to 5 mg/L range at the reference site. Ca 2+ ranged from 0.2 to 51 mg/L, and 71 % of the samples exceeded the 1 mg/L range at the reference sites. In the sewage, Na levels ranged from 5 to 363 mg/L, K + concentration from 1 to 74 mg/L, Cl - concentrations from 28 to 242 mg/L and Ca 2+ concentrations from 22 to 65 mg/L. The concentration of dissolved NO 3 - in the water samples ranged from less than 0.01 up to 211 mg/L. Out of the 180 samples collected, 24 had concentrations that exceeded the maximum guideline value of 50 mg/L for NO 3 - in drinking water. Elevated concentrations were observed in 2022 and 2023 in Alesa, Ogale, and Ebubu. In general, the groundwater NO 3 - concentrations in groundwater were higher in the northern part of the study area (Alesa, Ogale, and Ebubu), where municipal sewage was frequently present. In contrast, in the southern part (Alode and Okochiri), where municipal sewage was absent, concentrations were comparably lower, not reaching the WHO guidelines (Fig. 2). In the sewage samples, NO 3 - concentrations, up to 145, 131 and 100 mg/L, were detected in Alesa, Ogale and Ebubu, respectively. Eleven groundwater samples had nitrite (NO 2 - ) concentrations that exceeded the NSDWQ (2015) drinking water guideline value of 0.2 mg/L. Concentrations up to 1 mg/L, 0.2 mg/L, 1 mg/L, 2 mg/L and 0.2 mg/L were detected in Alesa, Ogale, Ebubu, Alode and Okochiri groundwaters, respectively. Ammonia (NH 4 + ) was detected in five samples of the Alesa groundwater. The estimated concentration ranged from 0.02 to 1.6 mg/L. In Ogale, NH 4 + was detected in all the samples, with concentration estimates ranging from < 0.02 to 12.7 mg/L. In Ebubu, however, NH 4 was detected in only one sample (0.6 mg/L). Additionally, two sewage samples from Alesa were examined: EF 5 contained an estimated 4.8 mg/L, while NH 4 + in EF 2 exceeded the instrument’s detection limit. These values might have been altered due to prolonged storage. Hence, those were considered only qualitatively. 3.2. d 15 N-NO 3 - and d 18 O-NO 3 - In the groundwater, the d 15 N-NO 3 - isotopic signatures varied between +8.9 to +25.6 ‰, and d 18 O-NO 3 - varied between +4.0 to +15.2 ‰ (Fig. 4, Table. 1). Overall, the variation in the d 15 N-NO 3 - and d 18 O-NO 3 - values across Alesa, Ogale, Ebubu, and Alode was small (Table 1). The d 18 O-NO 3 - values tended to increase with the d 15 N-NO 3 - values for the groundwater samples collected in Alesa and Ogale, while this trend was not observed in Ebubu. The d 15 N-NO 3 - and d 18 O-NO 3 - values in the shallow groundwater fitted the regression lines for Alesa and Ogale (y = 0.58x + 0.21, r 2 = 0.71). In municipal sewage samples ( n = 2), d 15 N and d 18 O values varied largely, ranging from -0.5 to +7.9 ‰ for d 15 N, and +1.9 to +10.5 ‰ for d 18 O. Table 1: d 15 N, d 18 O and NH 4 (estimates) values at each of the target communities ID Site d 15 N (‰) d 18 O (‰) NH 4 (µmol/L) estimates d 15 N /d 18 O NO₃ - (mg/L) Cl - (mg/L) NO₃ - /Cl - Ln (NO 3 - ) 1 Alesa 13.4 6.2 N.D. 2.17 106 41 2.6 4.66 2 Alesa 25.6 15.2 B.D.L 1.68 34 17 2 3.53 3 Alesa 13.9 8.8 N.D. 1.58 83 27 3.1 4.42 4 Alesa 13.6 9.4 0.03 1.45 17 9 1.9 2.83 5 Alesa 12.3 7.3 1.6 1.68 73 24 3 4.29 6 Alesa 13.7 7.8 B.D.L 1.76 53 18 2.9 3.97 7 Alesa 13.9 7.4 1.7 1.89 97 38 2.6 4.57 8 Alesa 15.1 9.9 0.4 1.52 32 11 2.9 3.47 9 Alesa 11.7 4 1.5 2.92 66 24 2.8 4.19 10 Alesa 10.2 4.1 N.D. 2.5 22 6 3.7 3.09 15 Ogale 18.8 11.5 13.5 1.64 64 49 1.3 4.16 21 Ogale 11.7 7.6 2.7 1.54 81 21 3.9 4.39 25 Ogale 9 8.2 N.D. 1.1 28 7 4 3.33 26 Ogale 11.5 8 0.6 1.45 22 6 3.7 3.09 27 Ogale 12.5 8.5 0.004 1.47 62 19 3.3 4.13 33 Ebubu 12.1 9 B.D.L 1.35 44 9 4.9 3.78 34 Ebubu 11.8 9 B.D.L 1.33 10 6 1.7 2.3 35 Ebubu 13.5 6.9 0.7 1.97 148 42 3.5 5 39 Ebubu 13.6 10.6 B.D.L 1.28 28 8.4 3.3 3.33 49 Alode 10.3 9.2 2.9 1.13 55 12 4.6 4.01 EF5 Alesa 7.9 10.5 5.1 0.75 145 123 1.1 4.98 EF2 Alesa - 0.5 1.9 A.D.L. -0.27 131 99 1.3 4.88 Notes: N.D. = Not Determined, B.L.D. = Below Detection Limit, A.D.L. = Above Detection Limit 4. Discussion 4.1. Source, transport, and fate of NO 3 - in the groundwater The groundwater samples with elevated ions were predominantly from Alesa, Ogale, and Ebubu. Here, 90 %, 87 %, and 92 % of the samples exceeded the Na + , K + , and Cl − concentrations measured at the reference sites, respectively. The elevated Na + , K + , Cl - , and Ca 2+ levels in the groundwater and sewage reflect the anthropogenic influence related to the discharge of animal/human waste effluents in the area (Minet et al. 2017). Furthermore, several strong positive correlations existed between the NO 3 − concentrations and the Na + , K + , Cl - , Sr 2+ , and Ca 2+ concentrations. NO 3 − had a strong positive correlation with Na + (R = 0.9), indicating possible impacts from the municipal sewage on the NO 3 − loading (Liu et al. 2006). NO 3 − was positively correlated with Cl − (R = 0.94), indicating that the NO 3 − was possibly derived from septic effluents, human excrement, or animal wastes in the area (Liu et al. 2006). Also, NO 3 − was positively correlated with K + (R = 0.83), Sr 2+ (R = 0.88) and Ca 2+ (R = 0.87) concentrations. These correlations, however, were mostly weak (R values ranged from 0.05 to 0.2) in all the groundwater quality parameters at the reference site. Notably, NO 3 − levels were consistently low (< 0.01 to 3 mg/L) at the reference sites. Therefore, the Na + , K + , Cl - and Ca 2+ concentrations suggest that the land use effect, i.e., leachate infiltration from domestic and municipal sewage and unlined pit latrine systems, is the source of NO 3 - loading (e.g., Minet et al. 2017). Similarly, although the Mg 2+ , F - , and SO 4 3- levels were relatively low in the groundwater and sewage, their concentrations exceeded the reference site values in several samples (Table 2S and 3S). Here, the Mg 2+ , F - , and SO 4 3- levels showed that 42 %, 34 %, and 18 % of the groundwater samples exceeded their respective reference site concentrations. NO 3 − vs Mg had a strong positive correlation (R = 0.9), likewise NO 3 − vs F - (R = 0.53), and NO 3 − vs SO 4 3- (R = 0.64). Those elevated ion levels also indicated a possible anthropogenic influence, most likely sewage infiltration into the aquifer. In line with the elevated ion concentration, Alesa, Ogale, and Ebubu groundwater showed an influence from anthropogenic activities (e.g., indiscriminate waste disposal into the municipal drainages). Leachates from municipal sewages often contain various contaminants, including salts and chloride compounds, which may infiltrate the aquifer (e.g., Aweto et al. 2023). The EC and TDS values were consistent with the findings by Eyankware et al. (2022) and Abam andNwankwoala (2020). EC and TDS, which principally comprises Ca 2+ , Mg 2+ , K + , Na + , Cl − , SO 4 3- , HCO 3 - and small amounts of dissolved organic matter (WHO 2017), strongly correlated with NO 3 − (R = 0.93). Reference sites lacked such strong correlations, corroborating the influence of domestic and municipal sewage infiltration. Furthermore, anthropogenic sources of NO 3 − can be identified using the NO 3 − /Cl − molar ratio since Cl − is widely distributed in natural waters (Torres-Martínez et al. 2021). According to Anornu et al. (2017) and Liu et al. (2006), this approach compares the molar ratios for NO 3 − and Cl − with the assumption that halides, such as Cl −, are chemically inert when introduced into the environment. This property makes Cl − , which usually has minimal interaction with the subsoil (Guo et al. 2020), an ideal indicator of sewage, manure, and fertilizer when plotted against NO 3 − (Gibrilla et al. 2020). Generally, groundwater with high values of Cl − against low NO 3 − /Cl − ratios are associated with NO 3 − inputs from sewage and organic wastes, whereas high NO 3 − /Cl − ratios with low Cl − values suggest NO 3 − inputs from agrochemicals (Anornu et al. 2017). Moreover, the NO 3 − /Cl − ratio is low in groundwater unaffected by anthropogenic activities (Jiang et al. 2016). In this study, the NO 3 − /Cl − ratios in the groundwater across Alesa, Ogale, and Ebubu overall were elevated, suggesting an anthropogenic influence. The relationship of the NO 3 − /Cl − vs Cl − concentration appears to be constant, implying that the groundwater has a consistent, non-variable source of NO 3 − (Cao et al. 2021). All the samples showed significantly higher Cl − levels with lower NO 3 − /Cl − (Fig. 3), suggesting that the NO 3 − was derived from the ongoing anthropogenic activities in the area, which are likely leachates from the municipal sewage and the pit latrine systems. Interestingly, a subset of 4 samples deviated from the general trend, which we regard as an indication of nitrate removal in the study area, possibly due to denitrification (Fig. 3). 4.2. Parallel occurrence of nitrification and denitrification? While dual NO 3 - isotopes have been widely used for NO 3 - source assessment in various environments, a precise attribution is complicated by overlapping processes (e.g., nitrification and denitrification) (Granger &Wankel 2016), fractionation effects (Yu et al. 2020), and mixing of different sources (Harris et al. 2022). For each NO 3 - source, there is a distinct dual isotope signature. For instance, d 18 O and d 15 N derived from nitrification of manure and sewage range from -10 to +15 ‰ and +8 to +25 ‰ for O and nitrogen isotopes, respectively (Kendall et al. 2007). In Alesa, Ogale, and Ebubu, groundwater DO content ranged from 1.5 to 8.9 mg/L, and such oxic conditions can favor nitrification as a nitrate source in the aquifer. A first source attribution based on Kendall et al. (2007) shows that the data plotted in the “manure and sewage” zone (Fig. 4a). As mentioned, the communities with elevated NO 3 - concentrations were characterized by drainage systems filled with domestic and municipal sewage (Fig. 6) and pit latrine toilets. Additional high NH 4 + concentrations in the groundwater and sewage across the study communities can rapidly be converted to NO 3 - by nitrifiers in oxic groundwater. The high NO 3 − /Cl − molar ratios (> 1), as well as the elevated δ 15 N-NO 3 - values (> 5) in the groundwater (Fig. 4b), further support that NH 4 + from sewage or manure, is, upon nitrification, a significant source of NO 3 - in the groundwater. Thus, while nitrification appears to be the primary biogeochemical process across the three sites, there is evidence for simultaneous denitrification. Denitrification, regarded as all nitrate respiration processes, is vital for NO 3 - removal in groundwater by transforming the dissolved NO 3 − to N 2 O and N 2 (Cantrell et al. 2023), as expressed in Equation 1 below (Appelo &Postma 2005). It is, however, more likely to occur under limited oxygen conditions and available organic carbon (Xue et al. 2009). NO 3 − (aq) → NO 2 − (aq) → NO (enzyme complex) → N 2 O (gas) → N 2(gas) (1) Despite the elevated groundwater DOC of up to 54 mg/L, our results (i.e., lack of distinct positive or negative correlation between δ 15 N-NO 3 − or δ 18 O-NO 3 − and NO 3 − ) suggest that denitrification is not the primary process for nitrogen transformation in the area (Zaryab et al. 2023). Given oxic conditions in most samples, this is plausible. In most samples, DO was above the threshold oxygen level for denitrification of 2 mg/L (Xue et al. 2012). Nevertheless, the build-up of NO 2 − (0.2 to 2 mg/L, n = 8) and the observed slope of δ 15 N-NO 3 − vs δ 18 O-NO 3 − of 0.58 in the groundwater of Alesa and Ogale, are indicators of potential denitrification. In groundwater, denitrification theoretically follows a dual isotope slope of 0.5 (Mayer et al. 2002). Furthermore, in Alsea and Ogale, a strong positive correlation between δ 15 N-NO 3 − and δ 18 O-NO 3 − indicated the occurrence of biological fractionation, likely due to denitrification (Anornu et al. 2017). The weak negative correlations between δ 15 N-NO 3 − and δ 18 O-NO 3 − vs. ln(NO 3 − ) (Fig. 5a) suggested that the isotopic enrichment of NO 3 - in the groundwater should have been caused by denitrification rather than dilution or mixing of NO 3 − from different sources (Xia et al. 2017). Also, the ratio of δ 15 N-NO 3 − vs δ 18 O-NO 3 − varied between 1.45 to 2.92. Those values are consistent with the reported ratios for groundwater denitrification, suggesting that simultaneous re-oxidation of NO 2 − occurred concurrently with NO 3 − reduction (Harris et al. 2022). In contrast, there was no relationship between DOC concentration and δ 15 N-NO 3 − in the groundwater or between DOC concentration and NO 3 − . This suggests that the dissolved C fraction was not consumed during denitrification. The lack of a clear correlation between DOC and δ 15 N-NO 3 − is consistent with the findings of Hinkle et al. (2007). They suggested that the dissolved carbon fraction is less relevant for denitrification than solid-phase organic carbon within the aquifer matrix. All the groundwater samples had elevated DOC levels of up to 54 mg/L, which we attribute to the ongoing oil and gas extraction activities in the Niger Delta. While such high DOC may not be consumed directly during denitrification, the labile organic carbon can act as a potential electron donor during groundwater denitrification by providing the necessary electrons needed for the reduction of NO 3 − or NO 2 − to N 2 O or N 2 under depleted oxygen condition. However, no conclusive evidence showed that DOC, as an electron donor, controlled the denitrification process in the study area. 4.3. Biogeochemical processes of redox reaction on nitrogen (N) behavior in the groundwater The oxidation/reduction (redox) reaction potential (Eh) is fundamental for most geochemical processes in aqueous environments. While nitrogen compounds actively undergo biogeochemical reactions in groundwater, changes in Eh and pH conditions are controlling the occurrence and stability of the various nitrogen species (i.e., NO 3 − , NO 2 − , and NH 4 + ) (Lidman et al. 2017, Reddy &D'angelo 1997). In the Eh–pH diagram (Fig. 7), aqueous species of nitrogen in groundwater under standard conditions (25°C and 1 atm) are dominated by NO 3 − under highly oxidizing conditions, NH 4 + under highly reducing conditions and NH 3 + under highly basic and reducing conditions. At the same time, N 2 occupies a large area due to atmospheric influence (Fig. 7). In this study, changes in Eh and pH have been identified as an important controlling factor for the dominance of a particular aqueous species of nitrogen predicted to be present at 25°C with an activity value of 1 x 10 -3 M dissolved nitrogen in the groundwater. The 1 x 10 -3 M used is the NO 3 − activity value commonly found in NO 3 − polluted groundwaters (Appelo &Postma 2005). It is, however, essential to note that changes in pressure do not necessarily introduce substantial errors in the Eh-pH boundaries calculated for 1 bar. Similarly, the influence of temperature on nitrogen transformation is usually in the same direction (e.g., the rate of chemical reaction speeds up with high temperature) (e.g., Thiagalingam &Kanehiro 1973). This implies that slight fluctuations in temperature from 25 to 29°C may not significantly alter the stability fields in the Eh-pH diagram (Fig. 7). The Eh values in the Alesa, Ogale, and Ebubu groundwater ranged from 113 to 641 mV (pH = 4 to 6.6), where the NO 3 − contamination was observed, and higher in Alode and Okochiri (Eh = 117 to 801 mV, pH = 4.4 to 6.9) where NO 3 − levels were relatively low. Both sites are characterized by highly oxidizing conditions favorable for nitrification. This explains the high NO 3 − and NO 2 − concentrations in the groundwater (Takatert et al. 1999, Zhao et al. 2016). As shown in Fig. 7, the groundwater samples plotted in the field of N 2(aq) stability between the boundary lines for NH 4 + and NO 3 − . This supports the idea that nitrifying and denitrifying could be possible in groundwater at our study sites. This result is consistent with a similar investigation in the coastal aquifer of Lagos, Nigeria (Aladejana et al. 2020). 4.4. NO 3 - export potential and management implications The schematic diagram of the NO 3 − source in the groundwater of the eastern Niger Delta is given in Fig. 8. The sewage availability, DO, and groundwater flow direction were the controlling factors influencing the distribution of NO 3 − . The nitrogen derived from the sewage likely migrated into the aquifer where it was subsequently nitrified. The NO 3 − was transported along the groundwater flow direction within the groundwater system, presenting the potential for export to the nearby surface and seawater. During the 2022 and 2023 sampling campaigns, some minor flooding occurred, which were due to the frequent heavy rainfall, improper design and maintenance of the drainage channels, and blockage of the municipal drainages. Consequently, sewage from the various drainages was transported to other areas within the community when the soil infiltration capacity was exceeded. This could cause a continuous rise of the groundwater levels facilitating the export of NO 3 − from shallow groundwater to surface water. Although the groundwater has shown evidence of denitrification, the prevailing redox conditions and the high groundwater NO 3 − load did not support a complete attenuation of NO 3 − in the affected communities. Denitrification, therefore, should not be relied upon for the effective NO 3 − reduction. Also, anthropogenic activities responsible for the elevated groundwater NO 3 − are still ongoing, posing the risk of further increases in the groundwater NO 3 − concentration. Hence, there is a need for urgent groundwater management measures to protect the groundwater. The management measures should focus on the following: 1. Safe domestic and municipal sewage management practices should be introduced to reduce the amount of anthropogenic nitrogen reaching the aquifer via municipal and domestic sewage infiltration into the groundwater. Furthermore, responsible municipal and domestic sewage disposal will assist in freeing the clogged drainages, reducing the frequently occurring flooding and minimizing the potential spread of NO 3 − contamination. 2. Measures to encourage immediate discontinuous use of the contaminated PSW while alternative safe drinking water sources (e.g., sachet or bottled) are explored. The groundwater NO 3 − levels should, however, be monitored continuously to ensure that NO 3 − levels are within safe limits. 5. Conclusion The study revealed that the groundwater of Alesa, Ogale, and Ebubu in the eastern Niger Delta is contaminated with NO 3 - , at levels up to 142 mg/L, 211 mg/L, and 120 mg/L, respectively. The groundwater NO 3 - concentration decreased downgradient in Alode (58 mg/L) and Okochiri (11 mg/L). Similarly, NO 2 - groundwater contamination was observed in a few samples at up to 2 mg/L. To further assess the source of NO 3 - , we applied a dual isotope (d 15 N-NO 3 - and d 18 O-NO 3 - ) and hydrochemical markers (major ions and NO 3 - /Cl - ratio) approach. Our isotopic data were consistent with a sewage source of groundwater NO 3 - . It also showed that nitrification is the primary biogeochemical process controlling the groundwater NO 3 - levels. Our hydrochemical markers also revealed that the NO 3 - contamination is derived from sewage effluents, which likely released N-containing compounds before being nitrified. While nitrification is the primary ongoing biogeochemical process, the data also revealed that denitrification co-occurs in the groundwater, especially in Alesa and Ogale. Given the oxidizing condition of the groundwater, denitrification should not be relied upon for the complete attenuation of NO 3 − in the affected communities. Therefore, there is an urgent need to introduce safe domestic and municipal sewage management practices to protect groundwater. This will also prevent the potential NO 3 - movement into the surface and nearshore seawater. Declarations Ethics approval and consent to participate Not applicable Consent for publication Not applicable Availability of data and materials All data generated or analyzed during this study are included in this published article [and its supplementary information files]. Competing interests The authors declare that they have no competing interests Funding This study was funded by the Deutsche Forschungsgemeinschaft (DFG) through grant PI 746/19-1 to Pichler and the Petroleum Technology Development Fund (Nigeria) to Dogo.. Authors' contributions Dogo carried out the fieldwork andparticipated in interpretation and wrote the first draft. Dähnke carried out the isotope analyses participated in interpretation and edited the first draft. Pichler conceptualized the study, supervised study design and edited the final draft. Acknowledgements We thank Dr. Kay Hammer (University of Bremen) for his continuous contribution to this work. We thank Associate Professor Obrike Stephen Ewoma (Nasarawa State University, Keffi), Clement Domgbara, Peter Tamunoigoni, and Faith Nyerma Aleku for their invaluable field support. 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Environmental Science and Pollution Research 23, 6483-6496 Supplementary Files 2024Nitratesupplementarydata.docx Cite Share Download PDF Status: Published Journal Publication published 20 Nov, 2024 Read the published version in Environmental Science and Pollution Research → Version 1 posted Editorial decision: Major Revision 26 Jul, 2024 Reviewers agreed at journal 19 Jun, 2024 Reviewers invited by journal 14 Jun, 2024 Editor invited by journal 29 May, 2024 Editor assigned by journal 13 May, 2024 First submitted to journal 10 May, 2024 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. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4390029","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":314587825,"identity":"8d6412e6-ce16-4395-860b-6b1d6b83bde5","order_by":0,"name":"Dogo Lawrence Aleku","email":"","orcid":"","institution":"University of Bremen: Universitat Bremen","correspondingAuthor":false,"prefix":"","firstName":"Dogo","middleName":"Lawrence","lastName":"Aleku","suffix":""},{"id":314587826,"identity":"00a84bc8-8854-4eab-9c2a-44bf561fba6b","order_by":1,"name":"Kirsten Dähnke","email":"","orcid":"","institution":"Helmholtz-Zentrum Geesthacht Zentrum fur Biomaterialentwicklung: Helmholtz-Zentrum Hereon Institut fur Aktive Polymere","correspondingAuthor":false,"prefix":"","firstName":"Kirsten","middleName":"","lastName":"Dähnke","suffix":""},{"id":314587827,"identity":"52e17a56-1332-4d6e-a87e-6488e382be62","order_by":2,"name":"Thomas Pichler","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6UlEQVRIiWNgGAWjYDACZgY2hgcMDHIMDIwNzDBBCYJaEhgYjEnQwgDRktgA1k6MFnN25mcPEirupW+43dzAXLjHJppfuoHxxgc8Wiyb2cwNEs4U5264c7CBecaztNyZcw4wW87Ao8XgMA+bRGJbQu6GG4ntv3kOHAYyEtikeQhq+ZeQbnAjsYGZ58D/3P0gLX8IamlISIBqOZC7QQKoBZ/3gX4xk0g4lmA4E6RlxoHk3Bk3Epste/BoMec//EziQ02CPN+N9AfMBQfscvtnJB+88QOfw7CIMTbgcxdWLaNgFIyCUTAKUAEAK2pOUy6lbV0AAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0003-3327-2451","institution":"University of Bremen: Universitat Bremen","correspondingAuthor":true,"prefix":"","firstName":"Thomas","middleName":"","lastName":"Pichler","suffix":""}],"badges":[],"createdAt":"2024-05-08 14:40:37","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4390029/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4390029/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11356-024-35499-6","type":"published","date":"2024-11-20T15:57:43+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":60201018,"identity":"0fdd53e2-53e1-4834-b145-ec26833904e0","added_by":"auto","created_at":"2024-07-13 02:41:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":101999,"visible":true,"origin":"","legend":"\u003cp\u003eThe spatial distribution of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentrations in the groundwater and municipal sewage across the study area. Only samples with elevated NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentrations were labeled on the map for clarity. For color interpretation in this map, the reader is referred to the Web version of this article.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/205d6cf4e0c3f5dd059695b6.png"},{"id":60200548,"identity":"05462843-cd0f-44eb-92c5-e4d3e70f9060","added_by":"auto","created_at":"2024-07-13 02:33:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":10818,"visible":true,"origin":"","legend":"\u003cp\u003eBox plot of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentrations in the study area. Alesa, Ogale, and Ebubu are in the northern part of the study site, Alode is in the central part, whereas Okochiri is in the south. An outlier (211 mg/L) in Ogale was excluded from the plot. The edges of the box represent the 75\u003csup\u003eth\u003c/sup\u003e and 25\u003csup\u003eth\u003c/sup\u003e percentiles, respectively. The ‘x’ sign in the box represents the mean value. The solid line represents the median value. The branch gives the range of the data except for the outliers. Twenty samples were selected across Alesa, Ogale, and Ebubu for the \u003cstrong\u003ed\u003c/strong\u003e\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e- \u003c/sup\u003eand \u003cstrong\u003ed\u003c/strong\u003e\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e- \u003c/sup\u003eanalyses.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/69bed6d8021ac833a8eb772c.png"},{"id":60200546,"identity":"a645e907-3425-4542-bc50-b8766885fc88","added_by":"auto","created_at":"2024-07-13 02:33:14","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":15825,"visible":true,"origin":"","legend":"\u003cp\u003eRelationship between Cl\u003csup\u003e− \u003c/sup\u003econcentrations vs NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e/Cl\u003csup\u003e−\u003c/sup\u003e molar ratios in Alesa, Ogale, and Ebubu groundwater in the study area.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/417ff1ed7ac2d3416f96d522.png"},{"id":60201017,"identity":"c36667ba-1061-4df8-8982-e1e97ea92abe","added_by":"auto","created_at":"2024-07-13 02:41:14","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":29751,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) A plot \u003c/strong\u003eof δ\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e vs δ\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e isotopic values\u003cstrong\u003e \u003c/strong\u003ein Alesa, Ogale, and Ebubu groundwater and sewage. The diagram was modified from Kendall et al. (2007), showing typical values of δ\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e and δ\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e derived or nitrified from leachates originating from the municipal sewage and pit latrine systems. The arrow shows the expected denitrification of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e in the area. \u003cstrong\u003e(b)\u003c/strong\u003e A plot of δ\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e vs. NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e/Cl\u003csup\u003e−\u003c/sup\u003e molar ratio variation in the Alesa, Ogale, and Ebubu groundwater.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/ef502722c4b7a1b07b49de3c.png"},{"id":60200547,"identity":"7a9b552d-fa1e-4d19-b782-bb2ba51872df","added_by":"auto","created_at":"2024-07-13 02:33:14","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":15023,"visible":true,"origin":"","legend":"\u003cp\u003eThe plot of \u003cstrong\u003e(a) \u003c/strong\u003eδ\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e vs ln(NO₃) (mg/L N), and \u003cstrong\u003e(b)\u003c/strong\u003e and δ\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e vs ln(NO₃) (mg/L N) for Ogale and Ebubu in the study areas.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/0c9e1e3c075a1fc6d303f362.png"},{"id":60201020,"identity":"ce47f96e-b2cf-4b14-8e33-007ab3f14e64","added_by":"auto","created_at":"2024-07-13 02:41:15","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":615492,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a)\u003c/strong\u003e and \u003cstrong\u003e(b)\u003c/strong\u003e municipal sewage in Alesa, \u003cstrong\u003e(c)\u003c/strong\u003eand \u003cstrong\u003e(d)\u003c/strong\u003e municipal sewage in Ogale with restricted flow, and \u003cstrong\u003e(e) \u003c/strong\u003edomestic sewage in Alesa with restricted flow. During rainfall, the sewage is transported to various parts of the community, infiltrating the aquifer. In flooding events, the sewage is transported to the nearby surface waters.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/55d09cbb3f14a73035ccc929.png"},{"id":60200550,"identity":"21422240-a359-4235-9174-4236dd0a17bf","added_by":"auto","created_at":"2024-07-13 02:33:15","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":40517,"visible":true,"origin":"","legend":"\u003cp\u003eEh – pH for nitrogen species in Alesa, Ogale, and Ebubu groundwater. The diagram was generated using the Geochemist’s Workbench software (17.0 edition).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/0367a497a83d391188222b5d.png"},{"id":60200553,"identity":"82d236dc-dac4-4307-af1b-0c90aeae3a31","added_by":"auto","created_at":"2024-07-13 02:33:15","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":191667,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram (not to scale) of the \u003cstrong\u003eNO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003csup\u003e\u003cstrong\u003e-\u003c/strong\u003e\u003c/sup\u003e source, transport, and fate in the groundwater of the eastern Niger Delta Region of Nigeria.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/69aab29b32e14522132ee928.png"},{"id":69835757,"identity":"c80c04fb-8b69-45e5-bdb6-947f45bcffc1","added_by":"auto","created_at":"2024-11-25 16:14:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2156432,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/481b6256-c039-45a2-a79c-3f84a3c24c71.pdf"},{"id":60201638,"identity":"eab8ea3d-eaec-46d1-9e71-5eda9a12a629","added_by":"auto","created_at":"2024-07-13 02:49:15","extension":"docx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":1074938,"visible":true,"origin":"","legend":"","description":"","filename":"2024Nitratesupplementarydata.docx","url":"https://assets-eu.researchsquare.com/files/rs-4390029/v1/d573784038b244718b098abd.docx"}],"financialInterests":"","formattedTitle":"Source, transport and fate of nitrate in shallow groundwater in the eastern Niger Delta","fulltext":[{"header":"1.\tIntroduction","content":"\u003cp\u003eGlobally, excess nitrate (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e) in groundwater is an environmental problem threatening human health, either directly due to its adverse health effects or by inducing the release of toxic metals, such as cadmium (Cd) from the aquifer matrix\u0026nbsp;(e.g., Kubier et al. 2020, Kubier et al. 2019, Ward et al. 2018). Migration of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003efrom groundwater\u0026nbsp;to surface waters and subsequently into the coastal ocean has also become a cause of concern worldwide\u0026nbsp;(e.g., Guo et al. 2021, Harris et al. 2022). As a result,\u0026nbsp;various global organizations have implemented measures to reduce\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003elevels in groundwater. For instance,\u0026nbsp;the European Union (EU) established a range of measures to reduce\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e contributions from agricultural and non-agricultural sources in the EU\u0026nbsp;(Stark \u0026amp;Richards 2008). To this effect, the EU selected a concentration of 50 mg/L as the\u0026nbsp;guideline value for\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003ein groundwater. Similarly, several countries, including Nigeria\u0026nbsp;(NSDWQ 2015), and organizations like the World Health Organization\u0026nbsp;(WHO 2017)\u0026nbsp;have set\u0026nbsp;50 mg/L as the guideline for NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e in drinking water.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eVarious authors investigated the groundwater quality and geochemistry in Nigeria\u0026apos;s urban and rural areas\u0026nbsp;(Abanyie et al. 2023, Eludoyin \u0026amp;Fajiwe 2023, Obrike et al. 2022, Raimi et al. 2023). Some studies included NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and concentrations reported were up to 2.1 mg/L in Nnewi, 4.2 mg/L in Awka\u0026nbsp;(Ayejoto \u0026amp;Egbueri 2023), 21.1 mg/L in Umunya\u0026nbsp;(Egbueri et al. 2023), 36 mg/L in Ogbaru\u0026nbsp;(Unigwe et al. 2022), 157 mg/L in Gboko\u0026nbsp;(Omonona \u0026amp;Okogbue 2021), and up to 770 mg/L in Maiduguri\u0026nbsp;(Goni et al. 2019). NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e sources were not identified in those studies, although agricultural activities, pit latrines and animal waste were hypothesized as possible sources.\u0026nbsp;Similar investigations in other parts of the world suggested that, most commonly, anthropogenic sources such as nitrogen fertilizer, manure, municipal and domestic sewage discharge, pit latrines, soil organic nitrogen and atmospheric deposition contribute to NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e loading of groundwaters (Biddau et al. 2023, Kendall et al. 2007).\u003c/p\u003e\n\u003cp\u003eTo effectively reduce the excess levels of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e in groundwater, improved groundwater management practices that minimize the release of nitrogen compounds into the environment are required. Determining\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e sources and variability is essential to improve nitrogen management practices.\u0026nbsp;However, identifying a given source and its contribution can be complicated when multiple nitrogen\u0026nbsp;sources\u003csup\u003e\u0026nbsp;\u003c/sup\u003eexist. Such uncertainty, for instance, is typical in urban areas where intensive agricultural activities involving nitrogen fertilizers are common\u0026nbsp;(Minet et al. 2017). Hence, accurately identifying the\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003esource(s) in groundwater, evaluating the ongoing biogeochemical process in the aquifer and calculating\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e contributions from different potential sources are necessary for effective management measures to reduce\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003elevels in groundwater.\u003c/p\u003e\n\u003cp\u003eStable oxygen and nitrogen isotopic signatures of\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e have been effectively applied to identify NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e sources while also detecting nitrification, denitrification or dilution in groundwater (e.g., Anornu et al. 2017, Carrey et al. 2021, Guo et al. 2020, Harris et al. 2022). However, uncertainties remain during data interpretation, which include (1) significant overlaps resulting from multiple nitrogen sources during the early leaching process within unsaturated zones or as nitrification proceeds and (2) a mixing process between the multiple nitrogen sources, subsequent NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eremoval due to denitrification (Kendall et al. 2007), and the concurrent productions of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eduring anaerobic\u0026nbsp;ammonium oxidation under limited oxygen conditions (Granger \u0026amp;Wankel 2016). This complicates nitrogen source identification in groundwaters\u0026nbsp;(Kendall et al. 2007), hence the need for an approach that combines major ion and isotope data to reduce such uncertainties\u0026nbsp;(Minet et al. 2017).\u0026nbsp;This is possible because, for instance, municipal sewage and animal wastes are typically enriched with chloride (Cl\u003csup\u003e-\u003c/sup\u003e), potassium (K\u003csup\u003e+\u003c/sup\u003e), and sodium (Na\u003csup\u003e+\u003c/sup\u003e), amongst many other contaminants, which are all released by decomposing organic matter (e.g., Ranjbar \u0026amp;Jalali 2012).\u003c/p\u003e\n\u003cp\u003eWith this in mind,\u0026nbsp;we combined stable\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eisotope data with hydrochemical markers (i.e., Ca\u003csup\u003e2+\u003c/sup\u003e, Na\u003csup\u003e+\u003c/sup\u003e,\u0026nbsp;K\u003csup\u003e+\u003c/sup\u003e, and Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e) for nitrogen source identification.\u0026nbsp;It is important to note that there are no available studies on groundwater NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e in the eastern Niger Delta, despite the widespread nitrogen-related anthropogenic activities.\u0026nbsp;Hence, this study presents a\u0026nbsp;unique opportunity to investigate\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e source, transport, and fate across the eastern Niger Delta groundwater systems to improve management and remediation efforts.\u003c/p\u003e"},{"header":"2.\tMaterials and methods","content":"\u003cp\u003e\u003cstrong\u003e2.1.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eSite description, geology, and hydrogeology\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study site is in the eastern Niger Delta Region of Nigeria (Latitude 4°44ˈ57ˈˈN to 4°47ˈ42ˈˈN and Longitude 7°05ˈ26ˈˈE to 7°09ˈ54ˈˈE) and comprises the following communities: Alesa, Ogale, Ebubu, Alode and Okochiri. The sampling locations are shown in Fig. 1. Alesa, Ogale, and Ebubu are in the northern part of the study area, commonly characterized by the presence of (1) municipal and domestic sewage in the drainage systems, and (2) unlined pit latrine toilets for human excrement. In these communities, the sewage flow was hindered by blockage resulting from indiscriminate solid waste disposal and the gentle nature of the topography. The municipal sewage was more commonly observed in Alesa than in Ogale and Ebubu. In contrast, sewage was not observed in the Alode and Okochiri drainage systems. The steep nature of the topography appears to play an essential role in aiding the free flow and eventual absence of municipal sewage.\u003c/p\u003e\n\u003cp\u003eThree \u003cstrong\u003emajor lithostratigraphic units have been identified within the Niger Delta Basin: the Benin Formation, Agbada Formation and Akata Formation\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e(Obaje 2009)\u003c/strong\u003e\u003cstrong\u003e. The Oligocene to Recent Benin Formation is about 2 km thick and predominantly consists of clay units, coarse-grained, sub-angular to well-rounded, poorly sorted coastal plain sand and alluvial deposits of\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eabout 95 % to 99 % quartz grains\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;at shallow depths\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e(Nwajide 2013)\u003c/strong\u003e\u003cstrong\u003e. The Formation serves as a groundwater reservoir for the region\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e(Adelana et al. 2008)\u003c/strong\u003e\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eThe aquifer is recharged mainly by direct precipitation at\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e2,532 mm/year\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;and exfiltration from major regional rivers\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e(Abam \u0026amp;Nwankwoala 2020)\u003c/strong\u003e\u003cstrong\u003e. The\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003esandy and permeable nature of the aquifer further facilitates rapid infiltration into the upper units of the formation\u003c/strong\u003e\u003cstrong\u003e(Abam \u0026amp;Nwankwoala 2020)\u003c/strong\u003e\u003cstrong\u003e.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eHowever, the anthropogenic activities in the region have left the shallow groundwater vulnerable to pollution\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e(Adeniran et al. 2023)\u003c/strong\u003e\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2. Groundwater sampling\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eThe groundwater samples for this study were collected from shallow wells (1 to 30 m) in the Benin Formation in April 2022 and April 2023. Groundwater and sewage samples were collected from communities with municipal and domestic sewage and areas considered relatively unaffected by municipal wastewater (i.e., reference sites 1 to 5). The r\u003c/strong\u003eeference samples were collected from Alode (Ref 1 and 2), Okochiri (Ref 3), Okrika Island (Ref 4) and Ogale (Ref 5) within the same geological unit \u003cstrong\u003ein relatively new residential areas without municipal wastewater or other potential anthropogenic contamination sources.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eThe groundwater samples were collected either (1) manually, using a water bailer made of\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003epolyvinyl chloride\u003c/strong\u003e,\u003cstrong\u003e\u0026nbsp;or (2) with an\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eelectric submersible\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;pump in cases where those were installed in the wells.\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;First, groundwater was pumped into the overhead storage tank to purge the wells for 30 minutes before sampling directly from the wellhead\u003c/strong\u003e\u003cstrong\u003e. The bailer\u0026nbsp;\u003c/strong\u003ewas rinsed three times with the groundwater before sampling. Sampling was conducted during the early hours (between 6:00 and 8:00 a.m.) when the wells were actively used to ensure that fresh samples were collected. In total, 180 samples (105 in 2022 and 75 in 2023) were collected from private supply wells (PSW) and community supply wells (CSW) next to municipal or domestic sewage drainages. In private residences, most wells were next to pit latrine toilets, usually between 2 and 9 m apart.\u003c/p\u003e\n\u003cp\u003eImmediately after collection, the samples were filtered through 0.45 μm cellulose acetate (CA) membranes and separated into aliquots for the different chemical analyses (isotopes, major ions, and dissolved organic carbon). The samples were stored in 25 mL glass vials for DOC, 30 mL brown HDPE vials for major cations and 20 mL clear HDPE vials for anions and isotopes. The sub-samples for DOC and major cations were preserved with 2 % concentrated nitric acid (HNO\u003csub\u003e3\u003c/sub\u003e). All samples were stored at 4 °C until laboratory analyses.\u003c/p\u003e\n\u003cp\u003eThe pH, conductivity (EC), total dissolved solids (TDS), temperature, dissolved oxygen (DO), salinity, redox potential (ORP), and resistivity were determined immediately \u003cem\u003ein situ\u003c/em\u003e using aHanna instrument HI98494 multiparameter. In the field, the total alkalinity (CaCO\u003csub\u003e3\u003c/sub\u003e) was determined by colorimetric titration with 0.16 N H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e in combination with a bromcresol green-methyl red indicator. The bromcresol green-methyl red indicator powder was added to 100 mL of the groundwater sample and titrated using a Hach digital titrator to a light pink color. The total alkalinity was reported as mg/L CaCO\u003csub\u003e3\u003c/sub\u003e.\u003c/p\u003e\n\u003cp\u003eAdditionally, 8 samples were collected from municipal and domestic sewage in Alesa, Ogale, and Ebubu.The samples were filtered through 0.45 μm cellulose acetate (CA) membrane filters and collected into 20 mL clear HDPE vials.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3. Analytical procedures\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3.1. Cation, anion and DOC measurements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMajor cations and trace elements were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) using a Perkin Elmer Optima 7300 DV instrument. The precision of the measurement was checked using EnviroMAT Groundwater Low (ES-L-2) and High (ES-H-2) certified water from SCP Science, Canada, showing errors of \u0026lt; 3 % for all analytes. Major anions (including\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e)were determined using a Metrohm 883 Basic IC plus instrument with a 5 μL injection loop and a Metrosep A Supp5 (150 × 4.0 mm; 5 μm) column. An internal standard was used to check the accuracy and precision of the measurement, and errors of less than 10 % were recorded.\u003c/p\u003e\n\u003cp\u003eDissolved organic carbon (DOC), the fraction of organic carbon that can pass through a 0.45 μm pore size, was determined using a Shimadzu TOC analyzer TOC-V CPN (Shimadzu Corporation). A certified Total Organic Carbon Standard of 50 mg/L (Aqua Solutions) was used for quality control, and the measurement error was determined to be less than 6 %.\u003c/p\u003e\n\u003cp\u003eThe ammonium (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) was determined photometrically at 655 nm with salicylate following standard procedures (DIN 38406 1983).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3.2. Determination of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eisotopes (\u003c/strong\u003e\u003cstrong\u003ed\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eand\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003ed\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA subset of 20 groundwater and 2 municipal wastewater samples were analyzed for stable isotopes, specifically from wells where their owners had granted permission to collect samples. The \u003csup\u003e15\u003c/sup\u003eN/\u003csup\u003e14\u003c/sup\u003eN and \u003csup\u003e18\u003c/sup\u003eO/\u003csup\u003e16\u003c/sup\u003eO ratios in dissolved \u003cstrong\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e\u003c/strong\u003e were measured and expressed as d\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eand d\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. Isotope ratios were determined following the denitrifier method (Casciotti et al., 2002; Sigman et al., 2001). \u003cstrong\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e\u003c/strong\u003e and NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e are quantitatively converted to nitrous oxide (N\u003csub\u003e2\u003c/sub\u003eO) by the denitrifying bacteria (\u003cem\u003ePseudomonas aureofaciens\u003c/em\u003e, ATCC#13985) that lack N\u003csub\u003e2\u003c/sub\u003eO reductase. The sample volume for isotope determination was adjusted to achieve 10 nmol of N\u003csub\u003e2\u003c/sub\u003eO. N\u003csub\u003e2\u003c/sub\u003eO was extracted from the sample vials by purging with helium and measured with a GasBench II (Thermo, Germany), coupled to an isotope ratio mass spectrometer (Delta Plus XP, Thermo, Germany). For quality assurance, two external standards (USGS34: δ \u003csup\u003e15\u003c/sup\u003eN: -1.8 ‰, δ \u003csup\u003e18\u003c/sup\u003eO: -27.9 ‰; IAEA-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e: d\u003csup\u003e15\u003c/sup\u003eN: +4.7 ‰, d\u003csup\u003e18\u003c/sup\u003eO: +25.6 ‰) and one internal standard were measured with each sample batch. The standard deviation of samples and standards was \u0026lt; 0.2 ‰ for δ \u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e (\u003cem\u003en\u003c/em\u003e = 4) and \u0026lt; 0.5 ‰ for δ \u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e (\u003cem\u003en\u003c/em\u003e = 4). Note that this method yields combined isotope values for \u003cstrong\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e\u003c/strong\u003e + NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. In two samples, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentration exceeded 5 % of the nitrate concentration. These samples were excluded from the isotopic analysis.\u003c/p\u003e"},{"header":"3. Results","content":"\u003cp\u003e\u003cstrong\u003e3.1. Field measurements and chemical data\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe supplemental information (SI) Tables 1S and 2S present data for all samples, including minimum, maximum, median and average. The\u0026nbsp;pH ranged from 3.5 to 6.9, temperature from 25 to 34 \u0026deg;C, EC from 16 to 852 \u0026micro;S/cm, TDS from 8 to 427 mg/L, DO from 0.7 to 8.9 mg/L and salinity from 0.01 to 0.4 PSU. The EC ranged from 17 to 69 \u0026micro;S/cm at the reference site\u003cstrong\u003e, and the TDS ranged from 9 to 32 mg/L.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMost groundwater quality\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eparameters at the contaminated\u0026nbsp;and reference sites were in accordance with WHO (2017) guidelines for drinking water. Nevertheless, the parameters associated with contamination from NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e fertilizer and animal/human waste effluents (Cl\u003csup\u003e-\u003c/sup\u003eand K\u003csup\u003e+\u003c/sup\u003e) or animal/human wastes (Na\u003csup\u003e+\u003c/sup\u003e) (Minet et al. 2017) showed higher concentrations at the contaminated sites than those at the reference sites. The concentrations of \u003cstrong\u003eNa\u003c/strong\u003e\u003csup\u003e+\u003c/sup\u003e\u003cstrong\u003e\u0026nbsp;in groundwater ranged from\u0026nbsp;\u003c/strong\u003e1 to 56 mg/L, and 57 % of the samples exceeded the measured reference value range of 1 to 2 mg/L. The concentrations of K\u003csup\u003e+\u003c/sup\u003e ranged from 0.1 to 59 mg/L, and 33 % of the samples exceeded the 0.3 to 0.6 mg/L range at the reference sites. The concentrations of Cl\u003csup\u003e-\u003c/sup\u003e ranged from 1 to 66 mg/L, with 25 % of the samples exceeding the 2 to 5 mg/L range at the reference site. Ca\u003csup\u003e2+\u003c/sup\u003e ranged from 0.2 to 51 mg/L, and 71 % of the samples exceeded the 1 mg/L range at the reference sites. In the sewage, Na levels ranged from 5 to 363 mg/L, K\u003csup\u003e+\u003c/sup\u003e concentration from 1 to 74 mg/L, Cl\u003csup\u003e-\u003c/sup\u003e concentrations from 28 to 242 mg/L and Ca\u003csup\u003e2+\u003c/sup\u003e concentrations from 22 to 65 mg/L.\u003c/p\u003e\n\u003cp\u003eThe concentration of dissolved\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e in the water samples ranged from less than 0.01 up to 211 mg/L. Out of the 180 samples collected, 24 had concentrations that exceeded the maximum guideline value of 50 mg/L for NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e in drinking water. Elevated concentrations were observed in 2022 and 2023 in Alesa, Ogale, and Ebubu. In general, the groundwater NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentrations in groundwater were higher in the northern part of the study area (Alesa, Ogale, and Ebubu), where municipal sewage was frequently present. In contrast, in the southern part (Alode and Okochiri), where municipal sewage was absent, concentrations were comparably lower, not reaching the WHO guidelines (Fig. 2). In the sewage samples, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003econcentrations, up to 145, 131 and 100 mg/L, were detected in Alesa, Ogale and Ebubu, respectively.\u003c/p\u003e\n\u003cp\u003eEleven groundwater samples had nitrite (NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e) concentrations that exceeded the NSDWQ (2015) drinking water guideline value of 0.2 mg/L. Concentrations up to 1 mg/L, 0.2 mg/L, 1 mg/L, 2 mg/L and 0.2 mg/L were detected in Alesa, Ogale, Ebubu, Alode and Okochiri groundwaters, respectively.\u003c/p\u003e\n\u003cp\u003eAmmonia (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) was detected in five samples of the Alesa groundwater. The estimated concentration ranged from 0.02 to 1.6 mg/L. In Ogale, NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e was detected in all the samples, with concentration estimates ranging from \u0026lt; 0.02 to 12.7 mg/L. In Ebubu, however, NH\u003csub\u003e4\u003c/sub\u003e was detected in only one sample (0.6 mg/L). Additionally, two sewage samples from Alesa were examined: EF 5 contained an estimated 4.8 mg/L, while NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e\u003csub\u003e\u0026nbsp;\u003c/sub\u003ein EF 2 exceeded the instrument\u0026rsquo;s detection limit. These values might have been altered due to prolonged storage. Hence, those were considered only qualitatively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003ed\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eand\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003ed\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn the groundwater, the d\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e isotopic signatures varied between +8.9 to +25.6 \u0026permil;, and d\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e varied between +4.0 to +15.2 \u0026permil; (Fig. 4, Table. 1). Overall, the variation in the d\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eand d\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e values across Alesa, Ogale, Ebubu, and Alode was small (Table 1). The d\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e values tended to increase with the d\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003evalues for the groundwater samples collected in Alesa and Ogale, while this trend was not observed in Ebubu. The d\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eand d\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e values in the shallow groundwater fitted the regression lines for Alesa and Ogale (y = 0.58x + 0.21, r\u003csup\u003e2\u003c/sup\u003e = 0.71).\u003c/p\u003e\n\u003cp\u003eIn municipal sewage samples (\u003cem\u003en\u0026nbsp;\u003c/em\u003e= 2),\u0026nbsp;d\u003csup\u003e15\u003c/sup\u003eN\u0026nbsp;and\u0026nbsp;d\u003csup\u003e18\u003c/sup\u003eO\u0026nbsp;values varied largely, ranging from\u0026nbsp;-0.5 to +7.9 \u0026permil; for d\u003csup\u003e15\u003c/sup\u003eN, and +1.9 to +10.5 \u0026permil; for d\u003csup\u003e18\u003c/sup\u003eO.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1:\u0026nbsp;\u003c/strong\u003ed\u003csup\u003e15\u003c/sup\u003eN,\u0026nbsp;d\u003csup\u003e18\u003c/sup\u003eO and\u0026nbsp;NH\u003csub\u003e4\u003c/sub\u003e (estimates) values at each of the target communities\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"580\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003eID\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eSite\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003ed\u003csup\u003e15\u003c/sup\u003eN\u0026nbsp;(\u0026permil;)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003ed\u003csup\u003e18\u003c/sup\u003eO\u0026nbsp;(\u0026permil;)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003e (\u0026micro;mol/L) estimates\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003ed\u003csup\u003e15\u003c/sup\u003eN\u0026nbsp;/d\u003csup\u003e18\u003c/sup\u003eO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003eNO₃\u003csup\u003e-\u003c/sup\u003e (mg/L)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003eCl\u003csup\u003e-\u003c/sup\u003e (mg/L)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003eNO₃\u003csup\u003e-\u003c/sup\u003e/Cl\u003csup\u003e-\u003c/sup\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003eLn (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e13.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e6.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eN.D.\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e2.17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e106\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e41\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e2.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.66\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e25.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e15.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eB.D.L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.68\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e34\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e3.53\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e13.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e8.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eN.D.\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.58\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e83\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e3.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.42\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e13.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e9.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.45\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e1.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e2.83\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e12.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e7.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e1.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.68\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e73\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e24\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.29\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e13.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e7.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eB.D.L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.76\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e53\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e2.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e3.97\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e13.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e7.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e1.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.89\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e38\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e2.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.57\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e15.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e9.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e0.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e32\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e2.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e3.47\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e11.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e1.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e2.92\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e66\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e24\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e2.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.19\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e10.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e4.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eN.D.\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e2.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e22\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e3.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e3.09\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eOgale\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e18.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e11.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e13.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e49\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.16\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e21\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eOgale\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e11.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e7.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e2.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.54\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e81\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e21\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e3.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.39\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eOgale\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e8.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eN.D.\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e28\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e3.33\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e26\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eOgale\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e11.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e0.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.45\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e22\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e3.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e3.09\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eOgale\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e12.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e8.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e0.004\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.47\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e62\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e3.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.13\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e33\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eEbubu\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e12.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eB.D.L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e4.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e3.78\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e34\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eEbubu\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e11.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eB.D.L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.33\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e1.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e2.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eEbubu\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e13.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e6.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e0.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e148\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e39\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eEbubu\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e13.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e10.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eB.D.L\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.28\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e28\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e8.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e3.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e3.33\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003e49\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlode\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e10.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e9.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e2.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e55\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e4.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003eEF5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e7.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e10.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003e5.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e0.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e145\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e123\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e1.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.98\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"7.081174438687392%\" valign=\"top\"\u003e\n \u003cp\u003eEF2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"9.844559585492227%\" valign=\"top\"\u003e\n \u003cp\u003eAlesa\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e- 0.5\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e1.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.962003454231432%\" valign=\"top\"\u003e\n \u003cp\u003eA.D.L.\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.808290155440414%\" valign=\"top\"\u003e\n \u003cp\u003e-0.27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e131\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.053540587219343%\" valign=\"top\"\u003e\n \u003cp\u003e99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"7.944732297063903%\" valign=\"top\"\u003e\n \u003cp\u003e1.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"8.635578583765112%\" valign=\"top\"\u003e\n \u003cp\u003e4.88\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003cem\u003eNotes: N.D. = Not Determined, B.L.D. = Below Detection Limit, A.D.L. = Above Detection Limit\u0026nbsp;\u003c/em\u003e\u003c/p\u003e"},{"header":"4. Discussion ","content":"\u003cp\u003e4.1. Source, transport, and fate of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e in the groundwater\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe groundwater samples with elevated ions were predominantly from Alesa, Ogale, and Ebubu. Here, 90 %, 87 %, and 92 % of the samples exceeded the Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, and Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations measured at the reference sites, respectively. The elevated Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Cl\u003csup\u003e-\u003c/sup\u003e, and Ca\u003csup\u003e2+\u0026nbsp;\u003c/sup\u003elevels in the groundwater and sewage reflect the anthropogenic influence related to the discharge of animal/human waste effluents in the area (Minet et al. 2017). Furthermore, several strong positive correlations existed between the\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations and the Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Cl\u003csup\u003e-\u003c/sup\u003e, Sr\u003csup\u003e2+\u003c/sup\u003e, and Ca\u003csup\u003e2+\u003c/sup\u003e concentrations. NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e had a strong positive correlation with Na\u003csup\u003e+\u003c/sup\u003e (R = 0.9), indicating possible impacts from the municipal sewage on the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e loading (Liu et al. 2006). NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was positively correlated with Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e (R = 0.94), indicating that the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003ewas possibly derived from septic effluents, human excrement, or animal wastes in the area\u0026nbsp;(Liu et al. 2006). \u0026nbsp;Also, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was positively correlated with K\u003csup\u003e+\u003c/sup\u003e (R = 0.83), Sr\u003csup\u003e2+\u003c/sup\u003e (R = 0.88) and Ca\u003csup\u003e2+\u003c/sup\u003e (R = 0.87) concentrations. These correlations, however, were mostly weak (R values ranged from 0.05 to 0.2) in all the groundwater quality parameters at the reference site. Notably, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e levels were consistently low (\u0026lt; 0.01 to 3 mg/L) at the reference sites. Therefore, the Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Cl\u003csup\u003e-\u003c/sup\u003e and Ca\u003csup\u003e2+\u003c/sup\u003e concentrations suggest that the land use effect, i.e., leachate infiltration from domestic and municipal sewage and unlined pit latrine systems, is the source of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e loading (e.g., Minet et al. 2017).\u003c/p\u003e\n\u003cp\u003eSimilarly, although the Mg\u003csup\u003e2+\u003c/sup\u003e, F\u003csup\u003e-\u003c/sup\u003e, and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e levels were relatively low in the groundwater and sewage, their concentrations exceeded the reference site values in several samples (Table 2S and 3S). Here, the Mg\u003csup\u003e2+\u003c/sup\u003e, F\u003csup\u003e-\u003c/sup\u003e, and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e levels showed that 42 %, 34 %, and 18 % of the groundwater samples exceeded their respective reference site concentrations. NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e vs Mg had a strong positive correlation (R = 0.9), likewise NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e vs F\u003csup\u003e-\u0026nbsp;\u003c/sup\u003e(R = 0.53), and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e vs SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u0026nbsp;\u003c/sup\u003e(R = 0.64). Those elevated ion\u0026nbsp;levels also indicated a possible anthropogenic influence, most likely sewage infiltration into the aquifer. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn line with the elevated ion concentration, Alesa, Ogale, and Ebubu groundwater showed an influence from anthropogenic activities (e.g., indiscriminate waste disposal into the municipal drainages). Leachates from municipal sewages often contain various contaminants, including salts and chloride compounds, which may infiltrate the aquifer\u0026nbsp;(e.g., Aweto et al. 2023). The EC and TDS values were consistent with the findings by\u0026nbsp;Eyankware et al. (2022)\u0026nbsp;and\u0026nbsp;Abam andNwankwoala (2020). EC and\u0026nbsp;TDS, which principally comprises Ca\u003csup\u003e2+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Na\u003csup\u003e+\u003c/sup\u003e, Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3-\u003c/sup\u003e, HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and small amounts of dissolved organic matter (WHO 2017), strongly correlated with NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (R = 0.93). Reference sites lacked such strong correlations, corroborating the influence of domestic and municipal sewage infiltration.\u003c/p\u003e\n\u003cp\u003eFurthermore, anthropogenic sources of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e can be identified using the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e molar ratio since Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e is widely distributed in natural waters (Torres-Mart\u0026iacute;nez et al. 2021). According to Anornu et al. (2017) and Liu et al. (2006), this approach compares the molar ratios for NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and Cl\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003ewith the\u003csup\u003e\u0026nbsp;\u003c/sup\u003eassumption that halides, such as Cl\u003csup\u003e\u0026minus;,\u003c/sup\u003e are chemically inert when introduced into the environment. This property makes Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, which usually has minimal interaction with the subsoil (Guo et al. 2020), \u003csup\u003e\u0026nbsp;\u003c/sup\u003ean ideal indicator of sewage, manure, and fertilizer when plotted against NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (Gibrilla et al. 2020). Generally, groundwater with high values of Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e against low NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e ratios are associated with NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003einputs from sewage and organic wastes, whereas high NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e ratios with low Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e values suggest NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003einputs from agrochemicals\u0026nbsp;(Anornu et al. 2017). Moreover, the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e ratio is low in groundwater unaffected by anthropogenic activities (Jiang et al. 2016). In this study, the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e ratios in the groundwater across Alesa, Ogale, and Ebubu overall were elevated, suggesting an anthropogenic influence. The relationship of the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e vs Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e concentration appears to be constant, implying that the groundwater has a consistent, non-variable source of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003e(Cao et al. 2021). All the samples showed significantly higher Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e levels with lower NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e (Fig. 3), suggesting that the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was derived from the ongoing anthropogenic activities in the area, which are likely leachates from the municipal sewage and the pit latrine systems. Interestingly, a subset of 4 samples deviated from the general trend, which we regard as an indication of nitrate removal in the study area, possibly due to denitrification (Fig. 3).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e4.2. Parallel occurrence of nitrification and denitrification?\u003c/p\u003e\n\u003cp\u003eWhile dual\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e isotopes have been widely used for\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e source assessment in various environments, a precise attribution is complicated by overlapping processes (e.g., nitrification and denitrification)\u0026nbsp;(Granger \u0026amp;Wankel 2016), fractionation effects\u0026nbsp;(Yu et al. 2020), and mixing of different sources\u0026nbsp;(Harris et al. 2022). For each NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e source, there is a distinct dual isotope signature. For instance, d\u003csup\u003e18\u003c/sup\u003eO and d\u003csup\u003e\u0026nbsp;15\u003c/sup\u003eN derived from nitrification of manure and sewage range from -10 to +15 \u0026permil; and +8 to +25 \u0026permil; for O and nitrogen isotopes, respectively\u0026nbsp;(Kendall et al. 2007).\u0026nbsp;In Alesa, Ogale, and Ebubu, groundwater DO content ranged from 1.5 to 8.9 mg/L, and such oxic conditions can favor nitrification as a nitrate source in the aquifer. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA first source attribution based on Kendall et al. (2007) shows that the data plotted in the \u0026ldquo;manure and sewage\u0026rdquo; zone (Fig. 4a). As mentioned, the communities with elevated NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e concentrations were characterized by drainage systems filled with domestic and municipal sewage (Fig. 6) and pit latrine toilets. Additional high NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e concentrations in the groundwater and sewage across the study communities can rapidly be converted to NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e by nitrifiers in oxic groundwater. The high NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e molar ratios (\u0026gt; 1), as well as the elevated \u0026delta;\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e values (\u0026gt; 5) in the groundwater (Fig. 4b), further support that NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e from sewage or\u003csub\u003e\u0026nbsp;\u003c/sub\u003emanure, is, upon nitrification,\u003csub\u003e\u0026nbsp;\u003c/sub\u003ea significant source of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003ein the groundwater.\u003c/p\u003e\n\u003cp\u003eThus, while nitrification appears to be the primary biogeochemical process across the three sites, there is evidence for simultaneous denitrification. Denitrification, regarded as all nitrate respiration processes, is vital for NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e removal in groundwater by transforming the dissolved NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to N\u003csub\u003e2\u003c/sub\u003eO and N\u003csub\u003e2\u003c/sub\u003e (Cantrell et al. 2023),\u0026nbsp;as expressed in Equation 1 below (Appelo \u0026amp;Postma 2005). It is, however, more likely to occur under limited oxygen conditions and available organic carbon (Xue et al. 2009).\u003c/p\u003e\n\u003cp\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u003csub\u003e(aq)\u003c/sub\u003e \u0026rarr; NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u003csub\u003e(aq)\u003c/sub\u003e \u0026rarr; NO \u003csub\u003e(enzyme complex)\u003c/sub\u003e \u0026rarr; N\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e(gas)\u003c/sub\u003e \u0026rarr; N\u003csub\u003e2(gas)\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;\u003c/sub\u003e(1)\u003c/p\u003e\n\u003cp\u003eDespite the elevated groundwater DOC \u0026nbsp;of up to 54 mg/L, our results (i.e., lack of distinct positive or negative correlation between \u0026delta;\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e or \u0026delta;\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) suggest that denitrification is not the primary process for nitrogen transformation in the area (Zaryab et al. 2023). Given oxic conditions in most samples, this is plausible.\u0026nbsp;In most samples, DO was above the threshold oxygen level for denitrification of 2 mg/L (Xue et al. 2012). Nevertheless, the build-up\u0026nbsp;of\u0026nbsp;NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (0.2 to 2 mg/L, \u003cem\u003en\u003c/em\u003e = 8) and the observed slope of\u0026nbsp;\u0026delta;\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e vs \u0026delta;\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e of 0.58 in the groundwater of Alesa and Ogale, are indicators of potential denitrification. In groundwater, denitrification theoretically follows a dual isotope slope of\u0026nbsp;0.5 (Mayer et al. 2002).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFurthermore, in Alsea and Ogale, a strong positive correlation between\u0026nbsp;\u0026delta;\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and \u0026delta;\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026nbsp;indicated the occurrence of biological fractionation, likely due to denitrification\u0026nbsp;(Anornu et al. 2017). \u0026nbsp;The\u0026nbsp;weak negative correlations between\u0026nbsp;\u0026delta;\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and \u0026delta;\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e vs. ln(NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) (Fig. 5a) suggested that the isotopic enrichment of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e in the groundwater should have been caused by denitrification rather than dilution or mixing of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e from different sources (Xia et al. 2017). Also, the ratio of\u0026nbsp;\u0026delta;\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e vs \u0026delta;\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003evaried between 1.45 to 2.92. Those values are\u0026nbsp;consistent with the reported ratios for groundwater denitrification, suggesting that simultaneous re-oxidation of\u0026nbsp;NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e occurred concurrently with\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction\u0026nbsp;(Harris et al. 2022).\u003c/p\u003e\n\u003cp\u003eIn contrast, there was no relationship between DOC concentration and \u0026delta;\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in the groundwater or between DOC concentration and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. This suggests that the dissolved C fraction was not consumed during denitrification. The lack of a clear correlation between DOC and \u0026delta;\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e is consistent with the findings of Hinkle et al. (2007). They suggested that the dissolved carbon fraction is less relevant for denitrification than solid-phase organic carbon within the aquifer matrix. All the groundwater samples had elevated DOC levels of up to 54 mg/L, which we attribute to the ongoing oil and gas extraction activities in the Niger Delta. While such high DOC may not be consumed directly during denitrification, the labile organic carbon can act as a potential electron donor during groundwater denitrification by providing the necessary electrons needed for the reduction of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e or NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e to N\u003csub\u003e2\u003c/sub\u003eO or N\u003csub\u003e2\u003c/sub\u003e under depleted oxygen condition. However, no conclusive evidence showed that DOC, as an electron donor, controlled the denitrification process in the study area.\u003c/p\u003e\n\u003cp\u003e4.3. Biogeochemical processes of redox reaction on nitrogen (N) behavior in the groundwater\u003c/p\u003e\n\u003cp\u003eThe oxidation/reduction (redox) reaction potential (Eh) is fundamental for most geochemical processes in aqueous environments. While nitrogen compounds actively undergo biogeochemical reactions in groundwater, changes in Eh and pH conditions are controlling the occurrence and stability of the various nitrogen species (i.e., NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, and NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) (Lidman et al. 2017, Reddy \u0026amp;D\u0026apos;angelo 1997). In the Eh\u0026ndash;pH diagram (Fig. 7), aqueous species of nitrogen in groundwater under standard conditions (25\u0026deg;C and 1 atm) are dominated by NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003eunder highly oxidizing conditions, NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u0026nbsp;\u003c/sup\u003eunder highly reducing conditions and NH\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e+\u0026nbsp;\u003c/sup\u003eunder highly basic and reducing conditions. At the same time, N\u003csub\u003e2\u003c/sub\u003e occupies a large area due to atmospheric influence (Fig. 7). In this study, changes in Eh and pH have been identified as an important controlling factor for the dominance of a particular aqueous species of nitrogen predicted to be present at 25\u0026deg;C with an activity value of 1 x 10\u003csup\u003e-3\u003c/sup\u003e M dissolved nitrogen in the groundwater. The 1 x 10\u003csup\u003e-3\u003c/sup\u003e M used is the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e activity value commonly found in NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003epolluted groundwaters (Appelo \u0026amp;Postma 2005). It is, however, essential to note that changes in pressure do not necessarily introduce substantial errors in the Eh-pH boundaries calculated for 1 bar. Similarly, the influence of temperature on nitrogen transformation is usually in the same direction (e.g., the rate of chemical reaction speeds up with high temperature) (e.g., Thiagalingam \u0026amp;Kanehiro 1973). This implies that slight fluctuations in temperature from 25 to 29\u0026deg;C may not significantly alter the stability fields in the Eh-pH diagram (Fig. 7).\u003c/p\u003e\n\u003cp\u003eThe Eh values in the Alesa, Ogale, and Ebubu groundwater ranged from 113 to 641 mV (pH = 4 to 6.6), where the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e contamination was observed, and higher in Alode and Okochiri (Eh = 117 to 801 mV, pH = 4.4 to 6.9) where NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e levels were relatively low. Both sites are characterized by highly oxidizing conditions favorable for nitrification. This explains the high NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations in the groundwater (Takatert et al. 1999, Zhao et al. 2016). As shown in Fig. 7, the groundwater samples plotted in the field of N\u003csub\u003e2(aq)\u0026nbsp;\u003c/sub\u003estability between the boundary lines for NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. This supports the idea that nitrifying and denitrifying could be possible in groundwater at our study sites. This result is consistent with a similar investigation in the coastal aquifer of Lagos, Nigeria (Aladejana et al. 2020).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e4.4. NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e export potential and management implications\u003c/p\u003e\n\u003cp\u003eThe schematic diagram of the\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e source\u0026nbsp;in the groundwater of the eastern Niger Delta is given in Fig. 8. The sewage availability, DO, and groundwater flow direction were the controlling factors influencing the distribution of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. The nitrogen derived from the sewage likely migrated into the aquifer where it was subsequently nitrified. The NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e was transported along the groundwater flow direction within the groundwater system, presenting the potential for export to the nearby surface and seawater.\u003c/p\u003e\n\u003cp\u003eDuring the 2022 and 2023 sampling campaigns, some minor flooding occurred, which were due to the frequent heavy rainfall, improper design and maintenance of the drainage channels, and blockage of the municipal drainages. Consequently, sewage from the various drainages was transported to other areas within the community when the soil infiltration capacity was exceeded. This could cause a continuous rise of the groundwater levels facilitating the export of\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003efrom shallow groundwater to surface water. Although the groundwater has shown evidence of denitrification, the prevailing redox conditions and the high groundwater\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026nbsp;load did not support a complete\u0026nbsp;attenuation of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e in the affected communities. Denitrification, therefore, should not be relied upon for the effective NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e reduction. Also, anthropogenic activities responsible for the elevated groundwater NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e are still ongoing, posing the risk of further increases in the groundwater NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003econcentration. Hence, there is a need for urgent groundwater management measures to protect the groundwater.\u003c/p\u003e\n\u003cp\u003eThe management measures should focus on the following:\u003c/p\u003e\n\u003cp\u003e1.\u0026nbsp; \u0026nbsp;Safe domestic and municipal sewage management practices should be introduced to reduce the amount of anthropogenic nitrogen reaching the aquifer via municipal and domestic sewage infiltration into the groundwater. Furthermore, responsible municipal and domestic sewage disposal will assist in freeing the clogged drainages, reducing the frequently occurring flooding and minimizing the potential spread of\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e\u0026nbsp;contamination.\u003c/p\u003e\n\u003cp\u003e2.\u0026nbsp; \u0026nbsp;Measures to encourage immediate discontinuous use of the contaminated PSW while alternative safe drinking water sources (e.g., sachet or bottled) are explored. The groundwater\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003elevels should, however, be monitored continuously to ensure that\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e levels are within safe limits.\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eThe study revealed that the groundwater of Alesa, Ogale, and Ebubu in the eastern Niger Delta is contaminated with NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, at levels up to\u0026nbsp;142 mg/L, 211 mg/L, and 120 mg/L, respectively. The groundwater\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e\u0026nbsp;concentration decreased downgradient in Alode (58 mg/L) and Okochiri (11 mg/L). Similarly, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003egroundwater\u003csup\u003e\u0026nbsp;\u003c/sup\u003econtamination was observed in a few samples at up to 2 mg/L. To further assess the source of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, we applied a dual isotope (d\u003csup\u003e15\u003c/sup\u003eN-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eand d\u003csup\u003e18\u003c/sup\u003eO-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e)\u0026nbsp;and\u0026nbsp;hydrochemical markers (major ions and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e/Cl\u003csup\u003e-\u003c/sup\u003e ratio) approach. Our isotopic data were consistent with a sewage source of groundwater\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. It also showed that nitrification is the primary biogeochemical process controlling the groundwater\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003elevels. Our hydrochemical markers also\u0026nbsp;revealed that\u0026nbsp;the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003econtamination is derived from sewage effluents, which likely released N-containing compounds before being nitrified. While nitrification is the primary ongoing biogeochemical process, the data also revealed that denitrification co-occurs in the groundwater, especially in Alesa and Ogale. Given the oxidizing condition of the groundwater, denitrification should not be relied upon for the complete\u0026nbsp;attenuation of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u0026nbsp;\u003c/sup\u003ein the affected communities. Therefore, there is an urgent need to introduce safe domestic and municipal sewage management practices to protect\u0026nbsp;groundwater. This will also prevent the potential\u0026nbsp;NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003emovement into the surface and nearshore seawater.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eEthics approval and consent to participate\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eNot applicable\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eConsent for publication\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eNot applicable\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eAvailability of data and materials\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eAll data generated or analyzed during this study are included in this published article [and its supplementary information files].\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eCompeting interests\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eThe authors declare that they have no competing interests\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eThis study was funded by the Deutsche Forschungsgemeinschaft (DFG) through grant PI 746/19-1 to Pichler and the Petroleum Technology Development Fund (Nigeria) to Dogo..\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eAuthors' contributions\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eDogo carried out the fieldwork andparticipated in interpretation and wrote the first draft. Dähnke carried out the isotope analyses participated in interpretation and edited the first draft. Pichler conceptualized the study, supervised study design and edited the final draft.\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eAcknowledgements\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eWe thank Dr. Kay Hammer (University of Bremen) for his continuous contribution to this work. We thank Associate Professor Obrike Stephen Ewoma (Nasarawa State University, Keffi), Clement Domgbara, Peter Tamunoigoni, and Faith Nyerma Aleku for their invaluable field support. We thank Markus Ankele (Institute of Carbon Cycles, Aquatic Nutrient Cycles, Geesthacht, Germany), Dr. Henning Fröllje, Dr. Christian Hansen, Dr. Tobias Himmler, Janin Scheplitz, and Luis Fernandes-Nogueira (University of Bremen, Germany) for their continuous laboratory support.\u003c/em\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAbam T, Nwankwoala H (2020): Hydrogeology of Eastern Niger Delta: A Review. Journal of Water Resource and Protection 12, 741-777\u003c/li\u003e\n\u003cli\u003eAbanyie SK, Apea OB, Abagale SA, Amuah EEY, Sunkari ED (2023): Sources and factors influencing groundwater quality and associated health implications: A review. 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Environmental Science and Pollution Research 23, 6483-6496\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"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},"keywords":"Nitrate, nitrite, groundwater, source, sewage, isotopic, hydrochemical, contamination, anthropogenic.","lastPublishedDoi":"10.21203/rs.3.rs-4390029/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4390029/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe eastern Niger Delta region in Nigeria is a hotspot for reactive nitrogen pollution due to extensive animal husbandry, pit latrine usage and agricultural practices. Despite the high level of human activity, the sources and processes affecting nitrogen in groundwater remain understudied. Groundwater nitrate (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) concentrations are highly variable, with some areas recording values well above the safe drinking water threshold of 50 mg/L. This is particularly true near municipal sewage systems. Elevated nitrite (NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) and ammonium (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) concentrations were also detected in the study area.\u003c/p\u003e \u003cp\u003eSewage analysis revealed NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e concentrations ranging from 1 to 145 mg/L, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e from 0.2 to 2 mg/L, and notably high NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e concentrations. A comparison of major ions indicated that 71%, 90%, 87%, and 92% of groundwater samples surpassed reference site levels for calcium (Ca\u003csup\u003e2+\u003c/sup\u003e), sodium (Na\u003csup\u003e+\u003c/sup\u003e), potassium (K\u003csup\u003e+\u003c/sup\u003e), and chloride (Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e), respectively, pointing to sewage as a likely source of contamination. The NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e ratios at several sites suggested that most groundwater NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e originates from human waste.\u003c/p\u003e \u003cp\u003eStable isotope analysis of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e showed a general enrichment in \u003csup\u003e15\u003c/sup\u003eN and, in some cases, a depletion in \u003csup\u003e18\u003c/sup\u003eO, indicating that the NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e originates from sewage-derived NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e nitrification. Although denitrification, a process that reduces NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, is present, the high dissolved oxygen (DO) and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e levels in the groundwater suggest that denitrification is insufficient to fully mitigate NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e pollution. Consequently, there is a risk of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e leaching from shallow aquifers into the Niger Delta\u0026rsquo;s surface waters and ultimately into the coastal ocean.\u003c/p\u003e","manuscriptTitle":"Source, transport and fate of nitrate in shallow groundwater in the eastern Niger Delta","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-13 02:33:09","doi":"10.21203/rs.3.rs-4390029/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2024-07-26T04:42:34+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2024-06-19T04:06:37+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-14T16:25:11+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Environmental Science and Pollution Research","date":"2024-05-29T10:16:19+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-13T05:03:22+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2024-05-10T04:29:06+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":"bbbe4622-7382-4597-b3d2-598dae7ed0ba","owner":[],"postedDate":"July 13th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-11-25T16:10:28+00:00","versionOfRecord":{"articleIdentity":"rs-4390029","link":"https://doi.org/10.1007/s11356-024-35499-6","journal":{"identity":"environmental-science-and-pollution-research","isVorOnly":false,"title":"Environmental Science and Pollution Research"},"publishedOn":"2024-11-20 15:57:43","publishedOnDateReadable":"November 20th, 2024"},"versionCreatedAt":"2024-07-13 02:33:09","video":"","vorDoi":"10.1007/s11356-024-35499-6","vorDoiUrl":"https://doi.org/10.1007/s11356-024-35499-6","workflowStages":[]},"version":"v1","identity":"rs-4390029","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4390029","identity":"rs-4390029","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}