Evolution of hydrochemistry and isotopic characteristics in shallow alluvial aquifers of Poyang Lake Basin, Eastern China

Poyang Lake, located in the north of Jiangxi Province, China, is an important feeder for the Yangtze River and an important ecological economic zone (Shvartsev et al. 2016; Xu & Wang 2016). However, with rapid economic development, Poyang Lake has become polluted with nitrogen and phosphorus. The main sources of pollution in Poyang Lake are chemical fertilizers, livestock breeding activities, and urban sewage treatment plants, with pollutants entering the lake through surface runoff (Soldatova et al. 2017; Liao et al. 2018). However, groundwater is an important component of the water resources around Poyang Lake, and has a close hydraulic relationship with surface water. In fact, previous studies have shown that the surface water of the Poyang Lake basin (Ganjiang, Fuhe, Xiushui, Xinjang) is composed of direct runoff, of which 23% is from precipitation and 77% is from the base flow of shallow groundwater (Hu et al. 2013; Liao et al. 2018). To understand the potential impact of groundwater on the water quality of Poyang Lake, it is necessary to study the changes in the chemical characteristics of groundwater due to the infiltration of surface water and atmospheric precipitation.

In recent years, researchers have analyzed the influence of the formation and evolution of groundwater on hydrochemical characteristics, which can effectively reveal the mechanisms involved in the interaction between groundwater and the environment and further identify the factors affecting groundwater quality (Gao et al. 2020; Solovey et al. 2021). Moreover, the types of surface water and groundwater hydrochemistry can be revealed using statistical diagrams from hydrochemistry analyses (Javadzadeh et al. 2020). The ratios of the main ions, Ca²⁺ and Mg²⁺, and their combinations have been widely used to analyze the process of groundwater circulation and evolution, and to identify the factors influencing the chemical composition of groundwater and the degree of water-rock interaction (Xing et al. 2013; Xiao et al. 2017; Ramaroson et al. 2020). Stable hydrogen and oxygen isotopes (δD and δ¹⁸O) are ideal natural tracers for studying groundwater sources and evolution (Zhen et al. 2017; Li et al. 2019). Nitrate levels are generally low in groundwater, and their increase is also indicative of human activity (Wang et al. 2017).

In this study, we analyzed shallow groundwater hydrochemistry and the major ion ratios, hydrogen and oxygen isotopic composition, and hydrochemical characteristics of groundwater around the Poyang Lake area. Moreover, we investigated the regional groundwater recharge sources and the main water–rock interaction processes. The findings of this study will provide a greater understanding of nitrate pollution sources based on the regional groundwater evolution process, and provide support for regional water quality improvement programs.

The region around Poyang Lake is located in the north of Jiangxi Province. It includes the cities Nanchang, Jingdezhen, and Yingtan, and 38 counties, including Jiujiang, Xinyu, Fuzhou, Yichun, Shangrao, and Ji'an (Figure 1). The total body of Poyang Lake covers an area of 51,200 km². The topography of the study area is a large asymmetric basin, with the Poyang Lake Plain as the base. The groundwater in the area can be divided into four types: loose rock pore water, clastic rock fissure pore water, carbonate-rock karst water, and bedrock fissure water. The depth of the shallow groundwater is 0.63–14.18 m, and the main sources of groundwater are atmospheric precipitation and river lateral recharge. The level of groundwater is affected by the topography. The groundwater in the whole region moves from east to west to the central region and from south to north according to the terrain and the runoff from hills and the low hill plain, with a general trend of migration to the central lake area.

Table 1 presents an overview of the statistical parameters established for the main ions in the phreatic aquifer of the urban agglomeration around Poyang Lake Basin. The results showed that the groundwater was weakly acidic, with a mean pH value of 6.27 (n = 404) and a minimum coefficient of variation of 0.76, indicating that the groundwater had a relatively stable pH. The order of anion and cation concentrations in groundwater was > > Cl⁻ > > and Ca²⁺ > Na⁺ >K⁺ > Mg²⁺ > NH₄⁺, respectively. showed the largest mean concentration, but the coefficient of variation was relatively small. Therefore, was the main ion in the groundwater and was present at high stability. The standard deviation of total dissolved solids (TDS) was higher than the standard deviation of the other components in the study area, indicating that the spatial distribution of TDS was significantly different.

The large coefficients of variation of ⁠, NH₄⁺, Na⁺, Cl⁻, and were mainly due to the large area from which measurements were taken, leading to large differences in ion concentration. This also indicates that these ions are easily affected by natural and anthropogenic activities. The maximum concentration of in the groundwater of the study area was 192.80 mg/L, with an average concentration of 23.77 mg/L, and the concentration exceeded groundwater quality standard Class III (20 mg/L; GB/T 14848 − 2017) in 44.44% of the sampling points. Moreover, the coefficient of variation was smaller for than for NH₄⁺ and ⁠, indicating that the source of in the region was relatively stable, and it may be attributed to human activity.

Piper trigrams can be used to analyze the content and evolution characteristics of the chemical composition of groundwater and further identify changes in the general chemical characteristics and control units in groundwater (Piper 1994; Zhang et al. 2017; Li et al. 2018; Adeyeye et al. 2021). As shown in Figure S1 (Supplementary Information), the groundwater in the study area had a variety of hydrochemical types. The main cations were Ca²⁺, Na⁺, and K⁺ and the main anions were and ⁠. The high relative content of indicated that the dissolution of carbonate rocks played a major role in controlling the hydrochemical characteristics of the groundwater. Strictly speaking, the Na⁺ and K⁺ content of the study area were significantly higher than the Ca²⁺ and Mg²⁺ content. Na⁺ and K⁺ mainly originated from the dissolution of potassium and sodium minerals in the aquifer. The sources of in the groundwater environment were mainly sulfate minerals in sedimentary rocks, oxidized sulfide, meteoric atmospheric precipitation, and artificial input (chemical fertilizers) (Fang et al. 2021). In general, it is difficult to dissolve CaSO₄, which has low solubility, to produce ⁠. However, content was higher than Cl⁻ content in the study area, indicating that sulfate in shallow groundwater was affected by meteoric precipitation and human activity.

Correlation analysis reflects the similarities and differences of the groundwater components, which is helpful in identifying the source consistency of each component. As shown in Figure 2, TDS was significantly correlated with the presence of ions in the groundwater (r > 0.5), and the strength of the correlation was in the order of Na⁺ > > Cl⁻ > Ca²⁺ > Mg²⁺, indicating that Na⁺, ⁠, and Cl⁻ were the main ions contributing to TDS. There was a significant correlation between pH and Ca²⁺ and levels, with correlation coefficients of 0.642 (p < 0.01) and 0.674 (p < 0.01), respectively. Moreover, the correlation coefficient between and Ca²⁺ levels was 0.852 (p < 0.01). These results indicated that pH and Ca²⁺ and levels were related to the weathering and dissolution of carbonate rocks. The correlations between ⁠, NH₄⁺, and the other ions were weak, indicating that the source of nitrogen was different from the sources of other cations and anions. Groundwater nitrogen in the study area was mainly imported from external sources. These results indicated that the formation of the chemical characteristics of the groundwater around the Poyang Lake area was mainly affected by meteoric precipitation, water–rock interactions, and human activity.

The shallow groundwater samples around Poyang Lake mainly lie in the water–rock interaction zone of the Gibbs diagram, with some points having a tendency to deviate towards meteoric precipitation (Figure 3). These results indicate that the content of the major ions in the groundwater is mostly controlled by mineral dissolution in the aquifer matrix. In addition, meteoric precipitation had a certain effect on the ion content, but evaporation had no obvious effect. It can also be observed from Figure 3 that many samples fall outside the model block diagram, which may be due to the influence of human activity.

Changes in the ion ratio of soluble mineral components effectively indicates leaching in the aquifer (Figure 2, Supplementary Information). When the equivalent ratio of (Ca²⁺ + Mg²⁺) to (SO₄²⁺ + ⁠) levels >1, Ca²⁺ and Mg²⁺ mainly originate from the dissolution of carbonate in the groundwater, but when this ratio <1, Ca²⁺ and Mg²⁺ mainly originate from the dissolution of silicate and evaporation (Qian et al. 2016; Guo et al. 2020; Liu et al. 2020). As shown in Figure S2 (Supplementary Information), these ratios were mainly located on both sides of the salt dissolution line (1:1). These results indicated that Ca²⁺ and Mg²⁺ content of the groundwater are mainly influenced by water–rock interactions and are mainly produced by the dissolution of carbonate and silicate and by evaporation (Zhu et al. 2021). More than 88% of the points for the relationship between (Ca²⁺ + Mg²⁺) and levels were distributed below the salt rock dissolution line (1:1 line), indicating that gypsum dissolution, but not carbonate mineral dissolution, generally occurs in the groundwater. Approximately 12% of the sample sites fell above the 1:1 line, which demonstrated that sodium (potassium) feldspar dissolution occurred in some aquifers in the study area. The Ca²⁺ formed after the dissolution of feldspar may affect the chemical balance of calcium bicarbonate in groundwater, and thus have a certain influence on the enrichment of ⁠.

The correlation between (Na⁺ + K⁺) and Cl⁻ levels reflects the dissolution of salt and feldspar minerals in the aquifer and the input of exogenous chloride ions (Sun et al. 2020). This ratio was distributed on both sides of the 1:1 line for most of the groundwater sampling sites in the study area. These results indicated that the sources of Na⁺ and K⁺ in the groundwater in the study area were different. In general, the dissolution of feldspar minerals was limited in most aquifers in the study area, indicating that positive cation exchange was the main reason for the relative enrichment of Na⁺ and K⁺. The relative enrichment of Cl⁻ may have been greatly affected by exogenous sources in some of the groundwater sites in the study area.

Cation exchange is an important factor affecting the chemical composition of groundwater. It has been found that the correlation between (Ca²⁺ + Mg²⁺ + + ⁠) and (Na⁺ + Cl⁻) levels may be used to evaluate cation exchange between Ca²⁺, Mg²⁺, and Na⁺ in aquifers (Wei et al. 2021). As shown in Figure 4, there was a significant negative correlation between (Ca²⁺ + Mg²⁺ + + ⁠) and (Na⁺ + Cl⁻) levels in the groundwater (r = 0961, p < 0.01). These results indicated that cation exchange was an important driving factor for the formation of the chemical components of groundwater in the study area.

The composition of the stable isotopes, δD and δ¹⁸O, in water is affected by meteorological processes, and their content and distribution characteristics are the basis for investigating and analyzing the source of groundwater recharge. The different positions of the δD and δ¹⁸O data points of the water samples on the δD − δ¹⁸O diagram reflect the different sources or formation processes of the water samples. If the data points fall on or near the meteoric water line, it indicates that the water sample originates from atmospheric water (Crawford et al. 2014). According to Craig (1961), there is a linear correlation between δD and δ¹⁸O levels in global meteoric precipitation; therefore, the GMWL can be represented by δD = 8δ¹⁸O + 10. Chen et al. (2019) calculated the local meteoric water line (LMWL) in Lushan using the equation: δD = 7.45δ¹⁸O + 8.36. The slope and intercept of the LMWL were slightly smaller than those of the GMWL, which may be due to a certain degree of evaporation and may also indicate that water vapor circulation was more frequent in the study area than in previously studied regions. As shown in Figure 5(a), the slopes and intercepts of the evaporation lines of the surface water and groundwater were similar, and they were both smaller than those of the LMWL. The stable hydrogen and oxygen isotope compositions of the shallow groundwater and surface water were generally distributed on the upper right side of the LMWL, with a small number of samples from the region around Poyang Lake on the upper left side. These results indicated that surface water and groundwater interact with each other, are both recharged by precipitation, and are more susceptible to evaporation than to precipitation. The stable hydrogen and oxygen isotope levels in the surface water and shallow groundwater had a high degree of correlation, which further indicated a strong hydraulic relationship between groundwater and surface water.

The d-excess reflects the deviation of the hydrogen and oxygen isotopic compositions from the GMWL. The d-excess distribution of groundwater in the study area ranged from 3.42 to 17.39, with an average value of 10.01 (Figure 5(b)). The d-excess distribution of surface water ranged from 2.63 to 16.46, with an average value of 9.00. The d-excess value of groundwater was similar to that of the global average meteoric precipitation. Unexpectedly, the evaporation rate was lower for surface water than for groundwater. This may be because the sampling period was during the wet season, and surface water is easily affected by rainfall, whereas the groundwater was delayed due to evaporation in the dry season, which resulted in a large d-excess value. These results showed that groundwater is greatly affected by water–rock interactions.

Under natural conditions, the nitrate content of groundwater is usually low and constitutes a minor component. Therefore, the change in nitrate concentration in groundwater was used as an indicator of the effects of human activity. In addition, urban sewage contains ⁠, which increases the concentration of in the groundwater (Zhou et al. 2019).

The source of in groundwater was identified based on the ratio of (⁠/Cl⁻) to Cl⁻. When the Cl⁻ content was high and the (⁠/Cl⁻) content ratio was low, the main source of in groundwater was manure and domestic sewage, but when the (⁠/Cl⁻) content ratio was high and the Cl⁻ content was low, the main source was chemical fertilizer (Egbi et al. 2020; Zhang et al. 2020a, 2020b). As shown in Figure 6(a), the study area was a traditional agricultural area, and the sampling points fell within the range of a high /Cl⁻ content ratio and low Cl⁻ content. The results showed that the main source of was chemical fertilizer. Some sampling points were distributed near the 1:1 line, indicating that mainly originated from chemical fertilizer, manure, and domestic sewage. Thus, these sampling points may be located in an urban–rural transitional area. Similarly, previous studies have found that domestic sewage and fertilizers are the principal sources of nitrogenous compounds (Soldatova et al. 2017).

The /Cl⁻ content ratios in most groundwater samples were less than 1 (Figure 6(b)), and the results showed that the content was less affected by urban domestic sewage, which further indicated that the content of the groundwater was also less affected by domestic sewage and more affected by chemical fertilizers.

The shallow groundwater around Poyang Lake was mainly derived from meteoric water. The average pH value was 6.27. The main cations were Ca²⁺, Na⁺ and K⁺ in groundwater, and the main anions were and ⁠. There were significant differences in ⁠, NH₄⁺, Na⁺, Cl⁻, and ⁠, which demonstrates that these ions were susceptible to natural and human activity.

The hydrochemical characteristics of shallow groundwater were influenced by both natural factors and human factors. Natural factors include rock weathering, dissolution of dissolved minerals such as carbonate silicate or evaporite, and ion exchange. Human factors such as domestic sewage and chemical fertilizer lead to the difference of nitrate in groundwater in this region. In addition, precipitation also had a certain degree of influence on it, but the influence of evaporation was not obvious.

mainly comes from chemical fertilizers, feces and domestic sewage in the shallow groundwater around Poyang Lake. However, the value of /Cl⁻ in most groundwater sampling points was less than 1 in the study area. This result indicated that was less affected by urban domestic sewage. Therefore, was more susceptible to the influence of chemical fertilizers in the groundwater.

Xiaodong Chu: Conceptualization, Investigation, Formal analysis, Software, Writing – original draft, Visualization, Writing – review & editing. Hao Wang: Supervision, Writing – review & editing. Liangzhong Li: Data curation, Writing – original draft, Funding acquisition. Zhifei Ma: Data curation, Writing – original draft. Daishe Wu: Validation, Data curation, Writing – original draft, Writing – review & editing. Ting Chen: Conceptualization, Funding acquisition.

This work was financially supported by the National Nature Science Foundation of China (No. 41907168 and 42007390) and the ‘Investigation and monitoring of groundwater pollution in urban agglomerations around Poyang Lake' provincial projects.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict of interest.