Comprehensive analysis of groundwater hydrochemistry and nitrate health risks in the Baiquan basin, Northern China

Drinking water quality

The entropy weight water quality index (EWQI) is an effective method for evaluating water quality, capable of employing multidimensional analysis of multiple parameters to more accurately assess water quality conditions (Ahmad, Umar & Ahmad, 2024; Dashora et al., 2022; Liu et al., 2021). Compared to the traditional water quality index (WQI) method, the EWQI utilizes information entropy to ascertain the weights of each water quality parameter. This approach minimizes the impact of human factors on weight allocation, thereby enhancing the objectivity of the evaluation results. The EWQI method for assessing water quality primarily involves five steps (Fig. 2): establishing an initial water quality matrix, normalizing the data, determining weights using the entropy weight method, establishing grading quantitative standards, and calculating and classifying water quality indices. This study employed the EWQI method to evaluate the quality of groundwater from BQB as drinking water.

Irrigation water quality

Globally, groundwater not only serves as a source of drinking water but also plays a crucial role in agricultural irrigation (Kushwah & Singh, 2024; Liu et al., 2023; Mohammed et al., 2022). This study assessed the suitability of BQB groundwater for irrigation using two methods: the sodium adsorption ratio (SAR) and the sodium percentage (%Na). The calculation equations for these methods are as follows: (2)${\displaystyle \mathrm{SAR}=\frac{N{a}^{+}}{\sqrt{\frac{C{a}^{2+}+M{g}^{2+}}{2}}}}$ (3)${\displaystyle \text{\%}\mathrm{Na}=\frac{N{a}^{+}}{C{a}^{2+}+M{g}^{2+}+N{a}^{+}+K^{+}}.}$

In Eqs. (2) and (3), the unit for ions is milligram equivalents per liter (meq/L). For SAR values, the categories are as follows: values less than 10, between 10 and 18, between 18 and 26, and greater than 26 correspond to water quality grades of excellent, good, doubtful, and unsuitable, respectively. For Na%, values less than 20, between 20 and 40, between 40 and 60, between 60 and 80, and greater than 80 represent water quality grades of excellent, good, permissible, doubtful, and unsuitable, respectively.

Health risk assessment

As a source of drinking water, groundwater is also critical to human health risks (Mao et al., 2023). The human health risk assessment model of USEPA was adopted to judge whether the local water source was suitable for drinking Eqs. (6) and (7). The calculation formula is as follows: (4)${\displaystyle ADD=\frac{C_s\times IR\times EF\times ED}{BW\times AT}}$ (5)${\displaystyle \mathrm{HI}=\frac{\mathrm{ADD}}{\mathrm{RfD}},}$

when HI < 1, it indicates that the non-carcinogenic risk is an acceptable level of non-carcinogenic risk. An HI > 1 indicates a significant non-carcinogenic risk, and the higher the value, the higher the risk (Samuel et al., 2024). The evaluation parameters can be found in Table 1.

Gibbs graphical model

Groundwater hydrochemical characteristics result from a combination of natural processes and human influences (Alvarez et al., 2020; Gao et al., 2023a; Liu et al., 2024; Zhang et al., 2022). Gibbs (1970) developed a graphical model for analyzing surface water hydrochemistry, which has been extensively applied in groundwater studies (Liu et al., 2024; Masoud & Abu El-Magd, 2022). The Gibbs model (Fig. 5) utilizes a semi-logarithmic coordinate system divided into three regions, representing key factors affecting hydrochemical characteristics: rock weathering, evaporation, and atmospheric precipitation (Gibbs, 1970). However, the Gibbs model has limitations in assessing the impact of human activities on water chemistry. Projection of the BQB groundwater sample data onto the Gibbs model (Fig. 5) shows that the majority of the data points fall within the central region, corresponding to the rock weathering control zone. This distribution indicates that the hydrochemical composition of BQB groundwater is predominantly influenced by rock weathering processes.

Ion ratios analysis

Ion ratio end-member plots are useful for identifying the types of rock weathering sources influencing hydrochemical composition (Liu et al., 2024; Masoud & Abu El-Magd, 2022; Ren et al., 2020). According to Gaillardet et al. (1999), the end-member diagram is divided into three areas: evaporite rocks in the bottom left, silicate rocks in the middle left, and carbonate rocks in the top right. When the groundwater hydrochemical data of BQB are projected onto the end-member plot (Fig. 6), the sample points are predominantly distributed between the carbonate rock and silicate rock regions. This distribution indicates that the water chemistry of the BQB groundwater is predominantly influenced by the weathering of carbonate and silicate rocks.

In theory, if Na⁺ and Cl⁻ in groundwater primarily originate from the dissolution of halite (NaCl), the Na⁺/Cl⁻ ratio should be approximately 1 (Li et al., 2018; Ren et al., 2020). As illustrated in Fig. 7A, the majority of groundwater sampling sites at BQB are distributed on either side of the Na⁺/Cl⁻ = 1 line, with several sites in close proximity to this line. This distribution suggests that rock salt dissolution is not the sole source of Na⁺ and Cl⁻. Water samples located above the 1:1 line exhibit Cl⁻ concentrations that exceed Na⁺ levels, indicating potential anthropogenic influences (Liu et al., 2021). Conversely, water samples below the 1:1 line show higher Na⁺ concentrations, likely due to the weathering and dissolution of silicate minerals, which can result in excess Na⁺. Additionally, cation-exchange processes within the aquifer may also impact Na⁺ concentrations (Zhang et al., 2022). Figure 7B shows that BQB groundwater samples are mainly located to the right of the Ca²⁺/SO${}_4^{2-}$ = 1 line. This distribution implies that SO₄²⁻ is not sufficient to balance the Ca²⁺ concentration, suggesting that gypsum dissolution is not the sole source of Ca²⁺ and SO${}_4^{2-}$ in the groundwater. Given that the study area’s hydrogeology is primarily composed of carbonate rocks, the excess Ca²⁺ is likely derived from the dissolution of carbonate minerals (Qi et al., 2015).

The relationship between Ca²⁺ and HCO₃⁻ can be utilized to determine the contribution of carbonate mineral dissolution to the concentrations of Ca²⁺ and HCO₃⁻ in groundwater (Li et al., 2018; Zhang et al., 2022). As depicted in Fig. 7C, the majority of the groundwater samples from BQB are distributed predominantly on the right side of the Ca²⁺/HCO₃⁻ = 1 line, with some samples located near this line. This distribution suggests that carbonate mineral (e.g., calcite) (Yang et al., 2016) dissolution contributes to the levels of Ca²⁺ and HCO₃⁻, although Ca²⁺ also originate from other sources. If the dissolution of carbonate and sulfate minerals is the dominant hydrogeochemical reaction in a groundwater system, the molar ratio (Ca²⁺+Mg²⁺)/(HCO₃⁻+SO${}_4^{2-}$) = 1 is theoretically approximated to 1. Ratios greater than 1 suggest that silicate weathering predominantly influences the groundwater chemistry. Conversely, ratios less than 1 indicate that the hydrochemistry of groundwater is primarily driven by the weathering of carbonate rocks (Liu et al., 2024). Figure 7D illustrates that the water sample points are primarily situated below the (Ca²⁺+Mg²⁺)/(HCO₃⁻+SO${}_4^{2-}$) = 1 line, indicating that the main hydrogeochemical process affecting the groundwater in BQB is silicate rocks weathering, with some influence from carbonate rocks weathering as well. The ionic components of the Baotu spring are primarily influenced by the dissolution of rock salt, carbonate, and sulfate minerals (Wang et al., 2024a; Wang et al., 2024b).

Cation exchange, alongside mineral dissolution and precipitation, frequently contributes to the regional hydrochemical composition of groundwater (Gao et al., 2023b; Hu et al., 2024; Ige, Adewoye & Obasaju, 2021; Ren et al., 2020). This process can be identified and quantified through the relationship between (Na⁺+K⁺-Cl⁻) and (Ca²⁺+Mg²⁺-HCO₃⁻-SO₄²⁻). If cation exchange is the primary process influencing the hydrogeochemical properties of groundwater, the relationship between the relevant variables should be linear, with a slope approximating −1 (Zhang et al., 2024). The application of the chlor-alkali index (CAI), as outlined in Eqs. (6) and (7), enables the determination of the direction of cation exchange. If both CAI-1 and CAI-2 are negative, cation exchange is occurring within the groundwater system. Conversely, positive values for both indices indicate the occurrence of reverse cation exchange (Hu et al., 2024; Liu et al., 2024). (6)${\displaystyle \mathrm{CAI}-1=\frac{{\mathrm{Cl}}^{-}-\left({\mathrm{Na}}^{+}+{\mathrm{K}}^{+}\right)}{{\mathrm{Cl}}^{-}}}$ (7)${\displaystyle \mathrm{CAI}-2=\frac{{\mathrm{Cl}}^{-}-\left({\mathrm{Na}}^{+}+{\mathrm{K}}^{+}\right)}{{\mathrm{HCO}}_3^{-}+{\mathrm{SO}}_4^{2-}+{\mathrm{CO}}_3^{2-}+{\mathrm{NO}}_3^{-}}.}$

As illustrated in Fig. 8A, the distribution of water sample points near the (Na⁺+K⁺-Cl⁻)/(Ca²⁺+Mg²⁺-HCO₃⁻-SO${}_4^{2-}$) = -1 line suggests that cation exchange significantly influences the hydrochemical characteristics of groundwater in BQB. Furthermore, Fig. 8B indicates that the majority of groundwater sample points fall within the area where CAI > 0, revealing that reverse cation exchange is predominant in the BQB groundwater environment, characterized by the exchange of Na⁺ in groundwater with Ca²⁺ in the aquifer medium.

The impact of anthropogenic activities on groundwater environments is increasingly evident, particularly in regions with advanced industrial and agricultural activities. Human activities are a significant factor influencing the hydrochemical composition of groundwater (Abdulsalam et al., 2022; Adjéï Kouacou et al., 2024). Nitrate (NO₃⁻) is the primary anthropogenic source in groundwater and can serve as an indicator of the intensity of human activities to some extent (Garrard et al., 2023; Liu et al., 2021). NO₃⁻ and Cl⁻ are commonly used to identify and analyze disturbances in groundwater chemical features due to anthropogenic activities (Hua et al., 2020; Liu et al., 2021).

As depicted in Fig. 9A, the majority of groundwater sampling sites in the BQB are situated in the lower left corner, within the fertilizer control area. Furthermore, Fig. 9B shows that water sample points are predominantly clustered near the agricultural impact area in the upper-right corner, indicating that the NO₃⁻ in BQB’s groundwater is largely attributed to the use of chemical fertilizers in agricultural activities.

Drinking water quality

Evaluating the water quality of groundwater, which serves as a primary source of water for industry and agriculture, is crucial for the rational development and utilization of regional groundwater resources (Ahmad, Umar & Ahmad, 2024; Ghosh & Bera, 2023; Kammoun, Abidi & Zairi, 2022; Kushwah & Singh, 2024). In this study, EWQI was employed to assess groundwater quality in the study area. Based on the calculated EWQI values, groundwater quality was categorized into five grades, ranging from “very poor” to “very good.” Generally, EWQI values exceeding 100 indicate that the water quality is unsuitable for drinking purposes (Liu et al., 2023). The assessment of groundwater drinking water quality using the EWQI for BQB revealed that EWQI values during the wet season ranged from 27.01 to 78.75, with an average of 40.41. In contrast, during the dry season, EWQI values ranged from 25.12 to 93.46, with an average of 43.62. Overall, BQB exhibits good groundwater quality and is considered a suitable source of drinking water.

The water quality assessment results indicated that the water quality during both the dry and wet seasons was essentially consistent, falling into two primary categories: good and moderate, with good being the predominant classification. During the dry season, 42 water samples (80.77%) were classified as good, and 10 (19.23%) as medium. Similarly, during the wet season, 41 water samples (78.85%) were classified as good, and 11 (21.15%) as medium. Figure 10 illustrates the spatial distribution of EWQI values for groundwater quality in BQB. The results indicate that the spatial distribution of groundwater quality assessment results is generally consistent between the dry and wet seasons, with higher EWQI values observed in the northwestern and southern parts of the BQB. This pattern may be attributed to the higher frequency of human activities in these regions, such as industrial and agricultural pollution, as well as over-exploitation of groundwater (Kong et al., 2015). Additionally, the northern part of the BQB exhibited lower EWQI values during the rainy season compared to the dry season, likely due to elevated TDS and fluoride concentrations resulting from seasonal evapotranspiration and concentration effects in the dry zone. Based on these findings, the government can prioritize targeted water quality control and improvement measures in the study area.

Irrigation water quality

High-quality irrigation water provides essential nutrients and moisture for plant growth and development. However, substandard irrigation water quality can result in issues such as soil salinization, soil acidification, and plant toxicity (Bhatnagar & Thakral, 2024; Kushwah & Singh, 2024; Liu et al., 2023). To assess the suitability of BQB’s groundwater for agricultural irrigation, this study used SAR and Na% as key indicators.

The results indicate that the SAR values of BQB groundwater range from 0.13 to 2.34 during the dry season and from 0.09 to 2.11 during the wet season, with mean values of 0.39 and 0.40, respectively. All water samples fall into the excellent category (SAR < 10), suggesting that BQB’s groundwater is suitable for agricultural irrigation. Additionally, the Na% values range from 2.97 to 37.34 during the dry season and from 2.44 to 32.19 during the wet season, with mean values of 8.35 and 8.99. During the dry season, all water samples except for four, which are classified as good (20 < Na% < 40), are categorized as excellent (Na% < 20). During the wet season, all water samples except for two, which are classified as good, also fall into the excellent category. Thus, based on Na%, BQB’s groundwater also exhibits excellent water quality for irrigation purposes.

The United States Salinity Laboratory (USSL) irrigation water quality classification diagram (Liu et al., 2023) reveals that the groundwater samples are primarily clustered in two zones, namely C₂S₁ and C₃S₁ (Fig. 11A). This distribution pattern suggests that BQB’s groundwater qualifies as an ideal source for agricultural irrigation. Furthermore, according to the Wilcox irrigation water quality classification diagram (Akakuru et al., 2022; Liu et al., 2023), the groundwater points are predominantly located in regions ① and ② (Fig. 11B). This distribution indicates the excellent irrigation water quality of BQB’s groundwater. In summary, BQB’s groundwater stands out as a superior source for agricultural irrigation water supply.

Health risk assessment

The extensive use of inorganic nitrogen fertilizers in agriculture has significantly increased nitrate concentrations in water. Nitrate is highly soluble and stable in water. Human exposure to nitrates occurs primarily through drinking water and food, with nitrates being readily absorbed and excreted by organisms, playing an essential role in the nitrogen cycle of river basins. The health risk assessment model developed by the USEPA (2001) was employed to evaluate nitrate-related health risks in drinking water for infants, children, and adults, using the hazard quotient (HQ) as the characterization metric.

The calculation results demonstrate that during the wet season, the health HQ values for nitrate-related health risks in groundwater exhibit a range of 0.31 to 3.44 for infants, 0.20 to 2.29 for children, and 0.18 to 1.96 for adults, with respective mean values of 1.25, 0.83, and 0.71. Conversely, during the dry season, these ranges are 0.40 to 2.30 for infants, 0.27 to 1.53 for children, and 0.23 to 1.31 for adults, yielding average HQ values of 1.14, 0.76, and 0.65, respectively. Overall, the health risk associated with nitrate in groundwater appears to be higher during the wet season, possibly due to increased precipitation in the wet season facilitating the leaching of nitrates from soil into groundwater.

A spatial distribution map of nitrate health risks in the groundwater of BQB was generated using GIS software, as depicted in Fig. 12. The figure shows that the non-carcinogenic risk is mainly concentrated in the north-western and south-eastern parts of the study area, while higher EWQI values are observed in the north-western and southern parts of the BQB, which may be due to the high level of anthropogenic activities in these areas, such as agricultural fertilisers, manure, and domestic wastewater, which can degrade water quality (Peng, 2023). Overall, HQ values were slightly higher in infants compared to children and adults. In the dry season, 65% of the sampling sites posed a significant non-cancer risk to infants, while in the wet season, this proportion was 56%. Comparing the health risks associated with BQB groundwater to those of the Jinan spring area, it was observed that the nitrate non-carcinogenic health risk index exhibited an increasing trend with decreasing receptor age in both regions. However, the health risk index was comparatively higher in the BQB than in the Jinan spring area (Dou et al., 2022). These findings underscore the necessity for further investigation and mitigation strategies.

By comparing the EWQI water quality evaluation with the health risk assessment, it was observed that water samples classified as good or moderate still exhibited health risks exceeding permissible limits. This discrepancy was attributed to the use of the USEPA human health risk assessment model, which employs nitrate limits that differ from those utilized in the EWQI. It is recommended that a unified health risk assessment system be established to enhance the accuracy of groundwater health risk assessments. Additionally, protecting groundwater from pollution is essential. This involves reducing the overuse of fertilizers and pesticides in agriculture. Regular assessments of groundwater quality are conducted, particularly for contaminants that pose significant risks to children’s health.