Source tracing analysis of the exceedance of NH₃-N and COD_Mn in shallow groundwater in the central typical area of the Yangtze river delta

The safety of groundwater is fundamental for maintaining human health and ecological balance and ensuring sustainable agricultural development¹. However, with the rapid increase in human activities, groundwater pollution has become increasingly severe, attracting global attention^2,3. Industrial effluent, agricultural irrigation, and household water usage all significantly affect the quality and quantity of groundwater⁴. Despite regional differences in economic development and environmental protection efforts, safeguarding water resources remains a fundamental task for maintaining human health.

The Yangtze River Delta region is a key area for economic development identified by the Chinese government, and environmental issues in this area have received significant attention^5,6. Maanshan is a representative city situated in the central region of the Yangtze River Delta. The primary sources of shallow groundwater in this urban area are rainfall and recharge from surface rivers (as shown in Fig. 1). According to data from a national monitoring well (QY10A) established in 2017, 2020, and 2021, the NH₃-N and COD_Mn concentrations reached 22.2 and 17.9 mg/L, and 39.1 and 14.6 mg/L, respectively. These values significantly exceed the Class V values (1.5 mg/L ≤ NH₃-N ≤ 2 mg/L; 10 mg/L ≤ COD_Mn ≤ 15 mg/L) of the Standard for Groundwater Quality (GB/T 14,848–2017), likely due to the impact of rapid urbanization on the groundwater quality. Nevertheless, there is a paucity of studies examining the safety risks associated with groundwater contamination resulting from such impacts. Notably, COD_Mn and NH₃-N are important indicators for assessing organic pollution levels and nitrogen pollution levels, respectively, in water bodies, thereby revealing the extent of water pollution. In previous studies, untreated industrial wastewater, agricultural runoff from fertilizers, and domestic sewage have been identified as the main sources of these pollutants^7,8,9, but the influence of geological structures on groundwater cannot be disregarded¹⁰. For typical areas at the center of the Yangtze River Delta, identifying and analyzing the sources of these pollutants are crucial.

Location of the national monitoring well (QY10A0 in the central part of the Yangtze River Delta.

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The present study monitored and analysed the groundwater quality in the study area through hydrogeological drilling and multipoint sampling. The aim of this study was to identify the sources and extent of contamination. Meanwhile, the morphology of contaminants in different strata was explored through the use of laboratory leaching analysis. The isotope analyses were combined with the determination of the concentrations of COD_Mn and NH₃-N in the shallow groundwater in the central part of the Yangtze River Delta, the analysis of the hydraulic connections and the tracing of the sources of these contaminants. The principal research objectives of this study can be summarised as follows: (1) To identify the primary sources of NH₃-N and COD_Mn contamination in shallow groundwater in the central part of the Yangtze River Delta. (2) To analyse the distribution of pollutants in different strata. (3) To explore the influence of environmental factors on the formation mechanism of pollutants, with a view to providing the government and environmental protection agencies with a scientific basis for policy formulation.

To investigate the water quality exceeding indices of QY10A and the reasons for its exceedance, the study area was divided based on the geological background. The divided areas were tested for water quality, and sampling points and related research methods were established.

Study area

Study area and geological background

Located in the eastern part of Anhui Province within East China, He County of Maanshan City is located in the alluvial plains of the lower reaches of the Yangtze River. The geographical coordinates range from 117°53′ to 118°52′ east longitude and from 31°24′ to 32°02′ north latitude. The Yangtze River flows across the region from east to west, which features a northern subtropical humid monsoon climate characterized by warmth and moisture. The region has an annual average temperature of 16.6 °C and an annual average precipitation of 1091.5 mm, with summer rains accounting for 44% of the total annual rainfall. The regional topography primarily encompasses hills and plains, with flatter terrain in areas north of the river, sloping generally towards the northeast. The regional terrain is shown in Fig. 2a. The main surface water bodies include the Yangtze River, Shuangqiao River, Shiba River, and Desheng River. The groundwater types and burial conditions in this area are complex, and groundwater can be classified as unconsolidated rock pore water or bedrock fracture water. The regional hydrogeology is shown in Fig. 2b. Quaternary strata are well developed, with no exposed bedrock, and gully slopes and river terraces have formed as a result of stratum movement. The bedrock geology and regional geomorphology are shown in Figs. 2c and d, respectively. Figure 3 shows the hydrogeological parameters of the study area obtained through hydrogeological and engineering geological investigations.

(a) Topography of the study area. (b) Hydrigeology of the area. (c) Bedrock geology of the study area. (d) Regional geomorphology of the study area.).

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Hydrogeologic profile of the study area.

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The QY10A national monitoring well, which was established in 2017 in Xinwei village in Wujiang town, He County, was selected for pollution source monitoring. This monitoring site is located on the northern side of the urban area of He County, along the northern bank of the Yangtze River, with geographical coordinates of 118°24′43.65"E and 31°46′2.46"N and a borehole depth of 35.10 m. The well site is approximately 3.5 km from the Economic and Technological Development Zone of He County and approximately 2.5 km from the Yangtze River. The local geomorphology can be categorized as a flood plain within the Yangtze River alluvial plain. The primary activities around the QY10A monitoring well are agricultural and fishery activities, with no industrial pollution sources present. In accordance with the Technical Guidelines for Environmental Impact Assessment–Groundwater Environment (HJ 610–2016), the hydrogeological unit at the monitoring site was determined on the basis of a topographical delineation of a watershed, thereby defining the study area of the QY10A monitoring site, which spans an area of 7.30 km² (blue area in Fig. 2).

Groundwater exploitation in the study area

Historically, no large-scale centralized groundwater exploitation has occurred in the research area. On the basis of the 1981 Maanshan Hydrogeological Census Report, NH₃-N < 0.5 mg/L and COD_Mn < 3 mg/L in the area. The surface water quality was generally satisfactory, and villagers relied mainly on surface water as their domestic water source. However, with economic development, the quality of the surrounding surface water has deteriorated. As a result, since the 1990s, local villagers have drilled wells to extract shallow groundwater. In 2001, a county-level water supply plant was established. In approximately 2010, the drinking water supply plant in Wujiang town began to provide water to the surrounding villages.

The study area consists mainly of Xinyu Village in Wujiang Township, Hexian County. It includes several groups of villages, including Zhao Village, Laodun Village, Yin Toufang Village, Lidai Village, Xiaohodun Village, Zhang Toufang Village, Xinyu Village, Chen Dacun Village, Chai Village and Dali Village. Each of these village groups was researched using a walkthrough survey method together with in-depth on-site observations. The findings indicate that all village groups have achieved full piped water coverage, providing residents with stable access to tap water from the Yangtze River for domestic use. As a result, most of the civil wells have been abandoned and the remaining wells are mainly used for non-potable purposes such as washing vegetables and laundry. Overall, the level of groundwater development and use in the area is relatively low. Furthermore, a related report on the Maswelife website (https://www.maswelife.com/article-1780-1.html) corroborates these findings, strengthening the argument and confirming the accuracy of the results.

Sampling point setup

In the case of QY10A, which is surrounded by agriculture and has no industrial distribution, the focus of the study area is on the water quality of QY10A, surface water quality monitoring, groundwater monitoring, and exploration of the impact of the stratigraphic structure. On this basis, corresponding sampling points are set up. The methodology employed for the collection, preservation, and analysis of the water samples is outlined in Supplementary Material Text 1–3.

QY10A monitoring point and vicinity

First, water samples from monitoring well QY10A were fully monitored for water quality. Second, a new borehole, QY10A-1, was added 10 m from the QY10A monitoring well to identify possible contamination layers. Sampling and analysis of borehole QY10A-1 will provide an understanding of the stratigraphic structure around the well and its impact on water quality. In addition, two new monitoring wells (QY10A-2 and QY10A-3) were added upstream of monitoring well QY10A for reference water quality data. This approach will assist in assessing the dispersion of pollution sources and impacts on upstream water quality. The locations of the monitoring wells are shown in Fig. 4a.

Sampling point layout map.

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River monitoring points

River water quality was tested to determine the effect of river recharge on groundwater quality. The study area involves two rivers. The monitoring point of the Shuangqiao River is at the Suoyi Bridge, whereas the monitoring point of the Shiba River is located at the Huangba Bridge. The locations of the two monitoring points are shown in Fig. 4b.

Groundwater monitoring points

Shallow groundwater contamination was determined by analyzing the water quality of domestic wells in the study area. The groundwater quality and its relationship to stratigraphic structures were evaluated. The water quality was tested in 15 domestic wells (labeled MJ ##) in the study area. The locations of the sampling points are shown in Fig. 4c.

Source tracing analysis of NH₃-N and COD_Mn

A total of 18 sets of samples were collected for the traceability investigation of water quality exceedances, including 5 sets of surface water samples and 13 sets of groundwater samples. The groundwater samples were collected from hydrogeologic boreholes and a domestic well. Specifically, QY10A-1, QY10A-2, and QY10A-3 are the aforementioned hydrogeologic boreholes, whereas D01-D08 were collected from new hydrogeologic boreholes added in September 2022. Sample M01 was collected from a domestic well in the northwestern part of the study area. The exact locations of the sampling sites are shown in Fig. 4d.

Analytical methods

Calculation of the influence range

The estimation of groundwater flow velocity (V) can be determined using Darcy’s Law, which allows for the calculation of the pollutant influence range. Darcy’s Law is an equation describing groundwater movement in aquifers based on three variables: permeability coefficient (K), hydraulic gradient (I), and effective porosity (n). From groundwater flow velocity Eq. (1), the influence range calculation Eq. (2) is derived. To investigate the impact of the pollution sources around QY10A on QY10A, the influence range is calculated using Eq. (2).

where L is the upstream distance from the well site, m; α is the coefficient of variation, α ≥ 1, which is usually set to 2; K is the permeability coefficient, which is usually the average value, 5 m/d in this case; I is the hydraulic gradient, which is measured, 0.002 in this case; n_e is the effective porosity, which is determined empirically, 0.35; and T is the particle migration time, which is not less than 5000 d.

Water quality tests

The water sample collection process used in this study was carried out with reference to the “Water quality-Guidance on sampling techniques” (HJ 494–2009). The preservation of water samples was performed in accordance with the “Water quality sampling—technical regulation of the preservation and handling of samples” (HJ 493–2009). Sampling, transportation, and testing were carried out by the Sinosteel Maanshan General Institute of Mining Research Co., Ltd. The methodology employed for the collection and preservation of water samples is delineated in Supplementary Material Text 1.

Dissolving and leaching tests

Dissolution and leaching tests were performed to analyze the cores from hydrogeological boreholes. All tests were carried out by Suzhou Hongyu Environmental Polytron Technologies Inc. Samples were collected and transported according to the requirements of this unit. The methodology employed for the collection and preservation of soil samples is delineated in Supplementary Material Text 2. The experimental process was carried out in strict accordance with the “Soil-Determination of ammonium, nitrite and nitrate by extraction with potassium chloride solution—spectrophotometric methods” (HJ 634–2012), and “Soil quality-Determination of total nitrogen-Modified Kjeldahl method” (HJ 717–2014).

Isotope detection analysis

Isotope testing and analysis were performed on the collected samples. Hydrogen and oxygen isotopes, as well as Nitrogen isotopes, were detected by the Third Institute of Oceanography, Ministry of Natural Resources. The samples were collected and transported in accordance with the relevant requirements of the testing organization. The instrumentation used in the isotope testing process, the detailed testing procedures, the methods for data calculation, and the requirements for data accuracy are all described in Supplementary Material Text 3.

QY10A water quality and influence range

Water quality monitoring results for monitoring well QY10A

On the basis of the collected water quality data from the QY10A monitoring well and measurements, the concentrations of NH₃-N and COD_Mn consistently exceeded the Class IV level of the groundwater quality standard GB/T14848-2017, as indicated in Table 1. Notably, the NH₃-N concentration during the dry season of 2021 was 1.79 mg/L, which is likely due to improper sampling. Additionally, the iron concentration exceeded the standard during the dry season of 2018. However, the subsequent monitoring results were satisfactory. A complete data sheet is provided in Supplementary Material Table S3.

Influence range

On the basis of drilling information for the area near monitoring well QY10A, the strata within 35 m comprise mainly silty clay, fine sand, silt, and silty clay with a silt–mud texture. According to the pumping test results of hydrogeological drilling around the area and combined with regional empirical values, the permeability coefficient of the sand layer was assumed to be 5 m/d. The upstream influence range of monitoring well QY10A can be calculated via Eq. (2).

The nearest industrial park in the study area is located 3.5 km from well site QY10A, and the stratigraphic structures at these two locations completely differ. The former is situated in a piedmont stratigraphic environment with poor groundwater flow conditions and extremely low velocities. In contrast, QY10A occurs in a fluvial stratigraphic setting, with better groundwater flow conditions and higher velocities. On the basis of a groundwater flow distance of 285 m over 5,000 d, more than 60,000 d (approximately 168 years) would be needed for pollutants to percolate from He County Industrial Park and reach the QY10A monitoring site.

Therefore, there are only three plausible explanations for the pollution observed at monitoring site QY10A: first, the presence of a local contaminant source; second, persistent widespread regional pollution that has impacted the groundwater quality at other locations within the hydrogeological unit; and third, geological factors that might influence the conditions at the site.

Water quality monitoring analysis

To comprehensively analyze the water quality in the study area, it was determined whether the exceedance levels of NH₃-N and COD_Mn in groundwater at the QY10A monitoring site were influenced by the water quality conditions in the surrounding environment.

Analysis of NH₃-N and COD_Mn in river water in the study area

The major surface water bodies near QY10A are the Shuangqiao River and Shiba River. Water quality monitoring data were collected between January 2020 and August 2022, as detailed in Table S4 and Fig. 5.

Water quality monitoring data collected between January 2020 and August 2022.

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The NH₃-N content at the Shuangqiao River Suoyi Bridge monitoring point ranged from 0.027 to 5.7 mg/L, with an average value of 1.212 mg/L, exceeding the Class IV water quality standard limit of GB/T14848-2017. There were several instances of excess NH₃-N at this monitoring point, primarily occurring during the dry season. The COD_Mn content ranged from 3.50 to 11.50 mg/L, with an average value of 6.66 mg/L, exceeding the Class IV water quality standard by a factor of two. Compared with that at the QY10A monitoring point, the highest NH₃-N concentration at the Shuangqiao River Suoyi Bridge monitoring point accounted for only 14.58%, with an average concentration of 3.10%, whereas the highest COD_Mn concentration accounted for 78.77% of that at the QY10A monitoring point, with an average concentration of 45.62%.

According to the water quality monitoring data from January 2020 to August 2022, the NH₃-N content at the Shiba River Huangba Bridge monitoring point ranged from 0.03 to 1.97 mg/L, with an average value of 0.391 mg/L. This average value exceeded the Class IV water quality limit, with two instances of exceedance, mainly during the dry season. The COD_Mn content ranged from 2.6 to 37.0 mg/L, with an average value of 7.39 mg/L, which exceeded the standard. Notably, the highest ammonia nitrogen concentration at the Shiba River Huangba Bridge monitoring point accounted for 5.04% of that at the QY10A monitoring point, with an average of 1.00%.

A comprehensive analysis of the collected data indicated that although industrial and agricultural activities and sewage discharge from residential areas impacted the Shuangqiao River and Shiba River, resulting in slight water quality deterioration, the two rivers did not experience significant pollution. By comparing the data from the two monitoring points, we can conclude that the ammonia nitrogen concentrations in the Shuangqiao River and Shiba River are significantly lower than those at the QY10A monitoring point, suggesting that during the monitoring period, severe pollution did not occur in these areas.

Analysis of NH₃-N and COD_Mn in groundwater in the study area

As indicated in Table 2, water quality monitoring based on NH₃-N and COD_Mn was conducted in domestic wells in the research area. Monitoring data from 15 domestic wells showed that ammonia nitrogen was not detected in shallow groundwater within the study area, and all levels remained below the detection limit of 0.025 mg/L. The measured COD_Mn contents ranged from 0.7 to 2.8 mg/L, all of which were below the Class III water quality standard of GB/T14848-2017, indicating that the overall levels of NH₃-N and COD_Mn in shallow groundwater within the study area are generally favorable.

Further data analysis revealed that the depths of the monitored domestic wells ranged from 2.60 to 8.40 m, with silty clay as the main geological formation. The results indicated that the NH₃-N levels in groundwater within the silty clay layer and above were below the detection limit, and the COD_Mn levels complied with GB/T14848-2017, suggesting the absence of high concentrations of ammonia nitrogen in the area and indicating that no significant shallow groundwater contamination occurred.

The integrated research conclusions and investigations of the surrounding environment revealed no notable distribution of industrial pollution sources, thus eliminating possible pollution sources near well site QY10A. Additionally, as the Shuangqiao River is a surface water body subjected to long-term monitoring in Maanshan City, the historical monitoring data do not indicate severe exceedances of NH₃-N and COD_Mn. Moreover, there have been no reports of pollution accidents in the study area, essentially precluding overall pollution in this region. Therefore, it is hypothesized that the high NH₃-N and COD_Mn levels at site QY10A may originate from geological factors¹¹.

Analysis of the dissolution leaching test results

Analysis of dissolution leaching tests in monitoring wells

To characterize the concentrations of NH₃-N and COD_Mn in the vertical direction of monitoring well QY10A, and to determine the stratigraphic ___location of the “contaminants”, dissolved leach analyses were conducted in drill cores QY10A-1, QY10A-2, and QY10A-3. The data obtained are shown in Table 3 and Figures S1-S2.

QY10A-1

The NH₃-N concentration was lower in the upper silty clay layer and the silty soil layer, whereas it was significantly greater at the bottom of the powdery fine sand layer and in the silty chalky clay layer, with the difference between the two amounts being 10–20 times. There was also a large difference in the COD_Mn indices between the upper part and lower part of the layer, which ranged from 19–24 times. The NH₃-N and COD_Mn concentrations increased significantly in the vertical direction, and the highest concentrations were found in the silty chalky clay layer, reflecting the poor water quality of the layer in this region. The drilled cores revealed that the silty chalky clay layer contained large amounts of ancient plant humus, organic matter and ancient shell fragments. When sampling the QY10A monitoring well in the early stage, the gas at the wellhead was determined to be combustible after the water in the well was drained (see Figure S3), indicating that the silty chalky clay layer contained a large amount of organic matter, which was related to the depositional environment at that time. After a long period of redox action, a “biogas” of sufficient ignition concentration is formed, and this “biogas” can enter the upper powdery fine sand layer, which has a high degree of porosity.

The long-term saturation of the silty chalky clay layer has allowed a large amount of organic matter to enter the groundwater, resulting in high NH₃-N and COD_Mn concentrations and low pH values. This finding is consistent with the water quality monitoring results of monitoring well QY10A. Therefore, it is hypothesized that the exceedance of NH₃-N and COD_Mn in the groundwater of monitoring well QY10A may be caused by the large amount of organic matter in the silty chalky clay layer.

QY10A-2

The stratigraphic structure of borehole QY10A-2 is similar to that of monitoring well QY10A but lacks the top silty clay layer. The silty chalky clay layer has higher NH₃-N and COD_Mn concentrations that are approximately two times higher than those of the upper chalky sand layer (see Table 3 and Figure S2). The water quality test after washing the well revealed that the groundwater NH₃-N concentration was 5.03 mg/L and the COD_Mn concentration was 4.80 mg/L, which exceeded the GB/T 14,848–2017 Class IV water standard. Although QY10A-2 is located upstream of monitoring well QY10A, its monitoring data are significantly lower than those of QY10A, indicating a low potential for contaminant dispersion. The analysis of stratigraphic structure and paleogeographic position revealed that QY10A-2 is located at the edge of the study area and that the stratigraphy is thinner than that of the QY10A area. The silty powdery clay layer in the core of the drilling area did not contain organic matter such as humus, and the organic matter content was relatively low. Simultaneously, clearly revealed that NH₃-N and COD_Mn were lower than those in QY10A-1, which is consistent with the paleogeographic environment evolution law. The upper silty soil layer is closely hydraulically connected to the powdery fine sand layer, and the adsorption properties of the silty soil are better, so its dissolution leaching test results are greater than those of the powdery fine sand layer.

QY10A-3

This site is located in the river floodplain downstream of He County Industrial Park, which is similar to the stratigraphy of the industrial park area and also comprises a low-permeability silty clay and clay layer. According to the water quality monitoring results after the wells were washed, when the concentration of groundwater was NH₃-N < 0.025 mg/L and COD_Mn = 2.60 mg/L, both indicators were lower than the GB/T 14,848–2017 Class III water standard. The quality of groundwater in the downstream area near He County Industrial Park is generally good.

In summary, combined with the results of the aforementioned calculation of the impact area, the pollutant diffusion time is much greater than the time of the establishment of the industrial park. Therefore, the exceedance of the water quality standard at monitoring point QY10A is less relevant to the pollution leakage from the industrial park and may be attributed to the large amount of organic matter and volatile gases contained in the silty chalky clay layer in the area.

Analysis of dissolution leaching tests in the study area

To investigate the effects of stratigraphic structure on water quality exceedances, dissolved leaching tests were conducted in 8 newly constructed hydrogeologic boreholes in the study area. Sedimentary layers of silty clay were observed only in boreholes D01 and D03, whereas borehole D02 exhibited no characteristics of fluvial facies sequence stratigraphy (refer to Table 4). A total of 65 sample sets were collected and analyzed, with the borehole locations shown in Fig. 4d.

The NH₄-N, NO₃-N, total nitrogen, and total organic carbon (TOC) contents in boreholes D01 to D08 were analyzed in detail (refer to supplementary Figures S5−S11, Text 4 and Table S5). Overall, the comparisons revealed a significant stratigraphic dependency of the morphology and nitrogen concentration within the study area, which can be summarized as follows:

Distribution of NH₄-N

The concentration was significantly greater in the silty mud loam and underlying grayish black silty clay layers (approximately ten times greater) than in the upper silty clay and fine sand layers on a per kilogram soil basis. These findings indicate that the nitrogen detected in these strata was primarily ammonium. In contrast, boreholes D04, D05, D06, D07, and D08 presented heterogeneous distributions of NH₄-N across the different stratigraphic layers, although generally at lower levels, especially in the fine sand layers and surface silty clay layers, than did boreholes D01 (15.8–87.5 mg/kg) and D03 (11.5–57.4 mg/kg). This finding that high NH₄-N concentrations were concentrated mainly in layers with relatively high organic matter contents.

Distribution of NO₃-N

The nitrate nitrogen (NO₃-N) concentration was significantly higher in the silty mud loam layers in boreholes D01 (4.04–48.6 mg/kg) and D03 (11.6–51.4 mg/kg), whereas it was lower in the underlying silty clay layers, with an approximately tenfold difference. This distribution pattern is opposite to that of NH₄-N, suggesting possible differences in nitrogen cycling processes within these layers. In the other boreholes, the distribution pattern of NO₃-N exhibited vertical variations similar to those in NH₄-N, likely indicating similar biogeochemical characteristics in these layers. The NO₃-N and NH₄-N data suggest that the silty mud loam layers may have been subjected to reducing conditions that inhibited nitrification¹².

Total nitrogen and organic carbon

The total nitrogen and TOC contents exhibited a vertical distribution pattern similar to that of NH₄-N across the boreholes. This finding is that the variability in nitrogen is closely associated with the presence of organic matter, which may be a significant driving factor of the change in the nitrogen level¹³.

Specific layers

A silty chalky clay layer similar to that observed in drill core QY10A-1 occurs in both D01 and D03, and the formation of this stratum is related to the depositional environment at the time, resulting in it being rich in organic matter. This stratum occurs in the middle of the fine sand and silty clay layers. In borehole D06, the consistent changes in NO₃-N and NH₄-N within the silty clay with silty sand suggested that this layer may exhibit chemical or biogeochemical processes distinct from those in the other layers. Additionally, the enriched zones of ancient plant humus, snails, and clam shells recorded in borehole D06 may reveal unique depositional environments during its geological history. This is similar to the large amount of ancient plant humus observed in the silty chalky clay layer in drill core QY10A-1. This also verifies the conclusion that there is an influence of high organic matter rich strata on shallow groundwater. Photographs of specific layers in D06 are shown in Figure S4.

Correlation analysis with the water quality results revealed that the ammonia nitrogen content in the hydrogeological boreholes suitably conformed with the soil NH₄-N concentration. Particularly in boreholes D01 and D03, higher soil NH₄-N contents corresponded to higher NH₄-N levels in groundwater, indicating the potential migration of NH₄-N from soil to groundwater. In conclusion, soil nitrogen is transformed mainly in the silty mud loam layer, and the ammonia in groundwater likely originates from soluble nitrogen transformations within this layer. The corresponding conclusions of the dissolution leaching experiments on the drill cores of the monitoring wells were also verified.

Hydrochemical type

According to the water quality monitoring results, the main hydrochemical types of groundwater in the study area are HCO₃–Ca and HCO₃–Ca·Mg. Among the various domestic wells, the hydrochemical type in well M01 is HCO₃·SO₄–Ca·Mg·K. Moreover, the main surface water hydrochemical type is HCO₃–Ca, followed by HCO₃–Ca·K and HCO₃·Cl–Ca (Fig. 6 and Table S6).

Hydrochemical types of the study area.

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On the basis of the water quality monitoring data (Table S7), some groundwater parameters, including total hardness, NH₃-N, and COD_Mn, exceeded the Class IV limits specified in GB/T 14,848–2017. The samples with excessive values were located at points QY10A (1) and (2), D01, D03, and D06, totaling 4 sampling sites. The exceedance indicators are provided in Table 5.

The water quality statistical results revealed that the Cl^- content in the surface water samples ranged from 25.00–38.20 mg/L, whereas the content in the groundwater samples ranged from 3.54–112 mg/L. Among these samples, those with excessive NH₃-N contents presented Cl^- contents ranging from 3.54–11.50 mg/L (Fig. 7, Table S6). A comparison of the concentrations of Cl^-, NH₃-N, and COD_Mn revealed a weak correlation between surface water and groundwater with exceedances. The Cl^- concentrations in the samples with exceedances (QY10A (1) and (2), D01, D03, and D06) were notably lower than those in the other groundwater samples, which exhibited significant differences in the Cl^- level. Among them, the Cl^- content in well M01 reached 112 mg/L, with a hydrochemical type of HCO₃·SO₄–Ca·Mg·K, clearly indicating an influence from anthropogenic activities^14,15.

Comparison of Cl^-, NH₃-N, and COD_Mn concentrations.

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In conclusion, the samples from QY10A (1) and (2), D01, D03, D06, and other wells exhibited nonsignificant correlations with the surface water samples.

Isotope analysis

Hydrogen and oxygen isotope analysis

During the water cycle, the hydrogen and oxygen isotopic compositions of different water bodies undergo significant changes due to processes such as evaporation, condensation, and atmospheric mixing¹⁶. These changes can be tracked through systematic variations in isotope ratios (δ¹⁸O/¹⁶O and δ²H/¹H). By analyzing the systematic variations in these isotope ratios, it is possible to identify groundwater recharge sources and differentiate the hydrological connections between surface water and groundwater, as well as among different aquifers.

In global precipitation, a linear relationship between the stable isotopes of hydrogen and oxygen, namely, δ²H and δ¹⁸O, respectively, is observed and is typically expressed as δ²H = 8δ¹⁸O + 10. This relationship is referred to as the global meteoric water line (GMWL), also referred to as the Craig line¹⁷. The δ²H and δ¹⁸O ratios in precipitation are influenced by various factors including temperature, latitude, distance from the ocean, altitude, and season¹⁸. Therefore, the linear relationship between δ²H and δ¹⁸O in precipitation may deviate from the GMWL in different regions. For example, the atmospheric precipitation line in China can be expressed as δ²H = 7.9δ¹⁸O + 8.2, referred to as the China meteoric water line (CMWL)¹⁹. The study area is close to Nanjing, and the atmospheric precipitation line in Nanjing can be expressed as δ²H = 8.47δ¹⁸O + 17.52²⁰.

The stable isotope detection results for δ²H and δ¹⁸O in the 5 sample sets indicated that the data generally conform with the CMWL, showing lower isotope ratios than those associated with the Nanjing atmospheric precipitation line (Fig. 8). However, the QY10A-3 sample exhibited characteristics closer to those of the Nanjing meteoric water line. This finding suggests that groundwater circulation at well site QY10A-3 differs from that at the other four sites, while those four points demonstrate similar hydraulic connections. These 5 sample sets were collected in March 2022, during the dry to normal hydrological period, with reduced precipitation and less influence from marine water vapor. Therefore, surface water in areas such as the Shuangqiao River (SQR), effluent from the He County Sewage Treatment Plant (STP), and groundwater at points with suitable permeability, such as QY10A and QY10A-2, deviates from the Nanjing atmospheric precipitation line in terms of stable hydrogen and oxygen isotope values. In contrast, the QY10A-3 sample point, characterized by a low-permeability dense soil lithology, exhibits low groundwater renewal rates and is less sensitive to atmospheric precipitation, leading to isotope values that are closer to the Nanjing atmospheric precipitation line.

Hydrogen and oxygen isotope results.

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In summary, the groundwater circulation dynamics at QY10A agree with those observed at QY10A-2, the Shuangqiao River, and the effluent outlet of the He County STP, which are primarily sourced from atmospheric precipitation, indicating close hydraulic connections among these sites. Positioned downstream of the other three locations, any lateral spread of groundwater contamination, if present, should manifest as observable changes in historical monitoring data at these points. However, the analysis of historical data from the Shuangqiao River, a long-term surface water monitoring site, revealed no anomalies in NH₃-N or COD_Mn, suggesting that the observed exceedances at site QY10A were not caused by long-term lateral spread of groundwater contamination.

Nitrogen and oxygen isotope analysis

Under natural conditions, groundwater typically has low nitrogen concentrations, particularly in terms of ammonia nitrogen. However, certain polluted groundwater sources may contain elevated levels of ammonia nitrogen stemming from sources such as domestic sewage, industrial effluents, and agricultural fertilizers²¹. ammonia nitrogen from these sources can infiltrate groundwater. Through transport and transformation within the aquifer driven by groundwater flow, a series of complex physical, chemical, and biogeochemical processes can occur, with nitrification and adsorption playing pivotal roles^22,23.

δ ¹⁵N-NH₄ ⁺ isotope analysis

Nitrification is the process whereby NH₄⁺ is converted to NO₃^- through microbial activity in aerobic environments. Analysis of 18 water samples, including measurements of ammonia nitrogen, nitrate, and nitrite contents, revealed significant differences between the groundwater and the surface water samples. The surface water samples exhibited aerobic conditions, which was consistent with the water quality monitoring results (Table 6, Fig. 9).

Cluster diagram of δ¹⁵N-NH₄⁺ isotopes in the study area.

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Based on the water quality test and δ¹⁵N-NH₄⁺ isotope analysis results, the NH₄-N concentration in groundwater is notably high, whereas the NO₃-N and NO₂-N levels are low. This finding suggests an anaerobic environment at site QY10A, where the process of nitrification involving ammonia and nitrogen is likely limited²⁴. Notably, at points where the groundwater δ¹⁵N-NH₄⁺ level exceeded the standard (QY10A, QY10A-2, D01, D03, D06), NO₃-N and NO₂-N were not detected. This absence of nitrate and nitrite compounds in conjunction with the elevated δ¹⁵N-NH₄⁺ levels suggests that the NH₄-N source at QY10A is not the anaerobic transformation of nitrate.

δ ¹⁵N-NO₃ ^- isotope analysis

Ammonia nitrogen adsorption primarily occurs on the surfaces of negatively charged mineral particles, affecting the migration of pollutants through cation exchange processes and impeding their movement within media. The adsorption capacity is influenced by factors such as medium properties and pH. Indoor experiments conducted by Ning et al.²⁵ demonstrated that clay cation exchange typically results in the adsorption of δ¹⁵NH₄⁺ from the solution and the enrichment of the heavy isotope δ¹⁵N, resulting in a solution depleted in δ¹⁵N. Conversely, nitrification processes lead to products exhibiting δ¹⁵N depletion, while the residual NH₄⁺ solution becomes enriched in δ¹⁵N.

The 18 groups of groundwater samples collected in this study, including the QY10A-2, QY10A-3, W02, and W03 locations, presented negative δ¹⁵N-NH₄⁺ values (Table 7), indicating δ¹⁵N depletion. This finding suggests that the NH₄-N in the groundwater at these locations primarily undergoes adsorption processes, indicating differences in the redox environments between these locations and other locations. The QY10A-2, QY10A-3, W02, and W03 locations are characterized by anaerobic conditions, whereas the remaining locations present aerobic conditions. Under anaerobic conditions, nitrogen-containing organic pollutants such as human and animal feces are difficult to decompose into ammonia nitrogen, thereby excluding the possibility of domestic sewage as a nitrogen source.

Summary and analysis

In the transport processes of nitrogen and oxygen pollutants, various physicochemical and biological reactions alter the isotope ratios of nitrogen and oxygen, leaving behind the isotopic fingerprints of such processes. Therefore, by utilizing the characteristic stable isotopes of nitrogen and oxygen in NO₃^-, it is possible to identify the sources and migration–transformation processes of nitrate pollution effectively in water bodies. Owing to nitrogen isotope fractionation, the δ¹⁵N values of nitrate from different sources vary (refer to supplementary Text 5). The distributions of the δ¹⁵N and δ¹⁸O values of nitrogen pollutants in groundwater are provided in Table S8.

On the basis of the combined analysis of the δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- results obtained in this study (Table 7, Fig. 10), the sources of NO₃^- at assessment points D01, D03, D06, and QY10A fall within the range associated with NH₄⁺ in soil, indicating that NH₄⁺ in groundwater at assessment point QY10A originates from the transformation of soluble nitrogen in soil. This finding is consistent with the leaching experiment and water quality test results, suggesting favorable consistency. Therefore, the elevated levels of NH₄⁺ in groundwater at assessment point QY10A may be due primarily to the high content in the natural geological environment rather than anthropogenic pollution sources. A preliminary range of primary NH₃-N exceedance values in the study area is shown in Fig. 11.

Point distribution of the source values of nitrogen and oxygen isotopes of NO₃^- in the study area.

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Preliminary range of the primary ammonia exceedances in the study area.

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The results of studies investigating the contamination of groundwater vary to some extent. Lu et al.’s²⁶ research on shallow groundwater pollution in the Yangtze River Delta indicated that urban living and runoff, industrial production, and the application of fertilizers and pesticides in agricultural production are the primary sources of pollution. Leaching, transport enrichment, agricultural and urban runoff, and industrial pollution significantly affect groundwater quality. In the study of the Tano River Basin, Ghana, Edjah, A. K. et al.²⁷ identified geological processes and underlying geology as significant influences on groundwater chemistry. Human activities and mineral dissolution play crucial roles in groundwater chemistry. Integrating these relevant studies, it becomes clear that geological factors are indispensable in groundwater pollution research. However, for the same basin, study areas should be delineated at different scales according to specific research objectives to derive more convincing conclusions.

The objective of this study is to identify the sources of excessive NH₃-N and COD_Mn in shallow groundwater in the central part of the Yangtze River Delta. A variety of methods were employed, including field sampling, hydrogeological drilling, leaching experiments, combined water quality testing, and isotope analyses, with the objective of exploring the sources of these contaminants and assessing their impact on regional groundwater quality. In light of the aforementioned study objectives, the following conclusions can be drawn:

Source identification

The excess NH₃-N in groundwater at the QY10A monitoring site and its surrounding research area is attributed primarily to nitrogen transformation processes under natural geological conditions. Similarly, the exceedance of COD_Mn is caused by similar processes rather than direct anthropogenic pollution. This conclusion is supported by the results obtained from onsite monitoring, solution leaching experiments, and isotope analysis.

Distribution of contaminants

The silty chalky clay layer and the silty clay with silty sand layer were the main sources of high concentrations of NH₃-N and COD_Mn. The gray-black chalky clay layer in the lower part of the silty chalky clay was also significantly high. Vertical changes in nitrogen content and TOC content were generally consistent. This finding indicates that the high concentration of NH₃-N is closely related to the organic matter content in this layer. The NO₃-N concentration in this layer is also high, exhibiting a vertical distribution pattern similar to that of NH₃-N. This finding highlights the layer-dependent nature of nitrogen forms and concentrations in subterranean strata.

Pollution mechanism

The analysis of stable isotopes and geochemical data indicates that the increase in NH₃-N is due mainly to the interaction of groundwater with soil layers containing high levels of organic matter, leading to the transformation of soluble nitrogen in the soil. Furthermore, long-term redox processes can culminate in the generation of methane gas. The migration of high concentrations of this gas to strata with greater porosity results in the exacerbation of further redox reactions. Accordingly, the elevated NH₃-N and COD_Mn levels are derived from the primary environmental soil, rather than external sources. This phenomenon is closely related to the depositional environment and hydrogeological conditions of the area.

Risk assessment and management recommendations

The formulation of groundwater pollution regulatory strategies should consider the multidimensional impacts of geological and hydrogeological conditions. In the central typical region of the Yangtze River Delta, understanding both the natural and anthropogenic factors underlying groundwater pollution by NH₃-N and COD_Mn will aid in developing effective groundwater protection and pollution control measures, especially in sensitive areas with active transformation of soil organic matter.

Overall, this study successfully elucidated the sources and transformation mechanisms of excess NH₃-N and COD_Mn in the shallow groundwater of the central Yangtze River Delta through the comprehensive application of stable isotope analysis techniques and environmental geochemistry methods. This research provides a systematic foundation and guidance for groundwater pollution prevention and control in this region and similar geological settings. Future research should explore broader distribution patterns of these contaminants and assess their environmental impacts, aiming to optimize groundwater management strategies.