Characterization and Health Risks of Groundwater Hydrochemistry in the Upper Weihe River Basin

Abstract

Groundwater is a vital and invaluable resource on our planet, serving as a critical water supply for human life, industrial activities, and agricultural production. It plays a pivotal role in sustaining human existence and driving societal progress. In this study, we conducted a comprehensive analysis of the hydrochemical characteristics and controlling factors of groundwater in the Upper Weihe River (UWR) using statistical analysis, Piper diagrams, Gibbs diagrams, correlation analysis, and ion ratio analysis. To evaluate the suitability of the regional groundwater for potable use, we employed the entropy weight water quality index (EWQI). Additionally, the sodium adsorption ratio (SAR) and percentage of soluble sodium (Na%) were utilized to evaluate the groundwater’s adaptability to irrigation. Furthermore, this study also assessed the health risks faced by adults and children in the UWR. The findings indicate that the main cations and anions in groundwater are Ca²⁺ and HCO₃⁻, respectively. The hydrochemical types are predominantly HCO₃-Ca, Cl-Ca, and mixed types. The composition of groundwater is primarily influenced by the dissolution of silicate and carbonate minerals, with cation exchange also playing a significant role in shaping its hydrochemical characteristics. The water quality assessment indicates that the majority of groundwater in UWR is classified as “excellent” or “good”, rendering it suitable for human consumption. However, 7.17% of the water samples were of poor quality and unsuitable for drinking; these were primarily located in a few areas in the northern and western parts of the study area. Regarding irrigation, 94.83% of the groundwater is deemed very suitable; however, a small fraction is not appropriate for such use. Additionally, non-carcinogenic risks are generally higher across most parts of the study area for both children and adults, with children exhibiting significantly higher risks than adults. These findings offer crucial insights regarding the sustainable management and environmental conservation of groundwater resources in the UWR.

1. Introduction

Water resources form the foundation of both production and ecology. Water is an essential natural resource that sustains human production, daily life, and social development. Industries, agriculture, domestic consumption, and ecological systems all depend on a reliable supply of clean and stable water [1,2,3,4]. Groundwater, a critical component of water resources, serves as a vital source of human sustenance and societal advancement, and it is also integral to the ecological environment. Ensuring good groundwater quality is a focal point of concern, as this is crucial for environmental sustainability [5,6,7,8]. Groundwater, an essential component of the water cycle, has chemical properties that are closely linked to its storage environment. Groundwater experiences complex hydrochemical interactions with the surrounding environment during its flow, leading to continual variations in its chemical composition [9,10,11,12].

In recent years, the swift growth in the population, economy, and society has led to a substantial increase in the discharge of industrial and agricultural wastewater, as well as domestic sewage, exerting significant pressure on groundwater systems globally. Furthermore, several broad categories of emerging pollutants are now receiving increased attention, many of which are not yet regulated by water quality standards. Consequently, groundwater pollution has emerged as a pressing global environmental issue [13,14,15]. Furthermore, urbanization has decreased the infiltration of rainwater, thereby reducing groundwater recharge. The scarcity and degradation of, as well as imbalances in, groundwater resources have constrained the sustainable and healthy development of the social economy and led to significant environmental issues over the past few decades [16,17]. Analyzing the hydrochemical characteristics and identifying the factors controlling a region can help to provide a scientific and effective basis for the rational development and optimal utilization of water resources. Thus, there is an urgent need to investigate the chemical composition of groundwater and the influences of natural and anthropogenic activities to assess its quality and forecast its future trends [18,19,20]. These studies have great practical significance and contribute to the understanding and management of water resources.

Wu et al. analyzed the hydrochemical types and distribution characteristics of groundwater in Weifang using the Piper trilinear diagram, Gibbs diagram, and mathematical statistics, and evaluated the groundwater quality in Weifang using the multi-component evaluation method, Brown water quality index method, and Nemero pollution index method [21]. Wang studied the main ionic characteristics and influencing factors of groundwater in the urban area of Weifang, and selected the single-factor evaluation method and the comprehensive evaluation method to carry out a comprehensive evaluation of groundwater quality [22]. Liu et al. also evaluated the health risks associated with nitrates in Weifang [23]. However, most of the studies focused on Weifang as a whole, and there is a lack of groundwater hydrochemistry studies focusing on the upper end of the Weihe River. Furthermore, the rapid development of the national economy has led to an increase in the exploitation and use of groundwater, resulting in significant hydrological issues in the affected cities. Therefore, it is necessary to analyze the groundwater hydrochemistry and water quality in the study area.

Groundwater serves as the primary source of drinking and production water in the UWR, yet research on its hydrochemical characteristics and the distribution of water quality for drinking and irrigation is limited. The scarcity of studies on the UWR limits the ability to provide a comprehensive picture of its groundwater’s hydrochemical characteristics and the water quality. For instance, Wang et al. analyzed the hydrogeology of the northwestern part of Zhucheng, which does not fully reflect the entire study area [24]. To address this gap, our study conducted a comprehensive analysis of 232 groundwater chemical datasets, employing statistical and hydrochemical analyses to systematically investigate the hydrochemical characteristics and the driving mechanisms of groundwater in the UWR. Additionally, the study included assessments of water quality and the health risks associated with groundwater, offering further insights for regional research. The present study addresses the gap in the research on groundwater hydrochemistry in the study area, evaluates water quality and health risks, and refines previous findings. These findings are crucial for evaluating groundwater quality, exploring its sources and circulation, and facilitating the rational exploitation of groundwater resources.

2. Study Area

The study area is situated in the southeastern sector of the Shandong Peninsula, specifically in the southern region of Weifang, at the confluence of the Taiyi Mountains and the Jiaowei Plain. Geographically, its latitude spans from 35°42′23″ to 36°21′05″ and its longitude spans from 119°0′19″ to 119°43′56″, as depicted in Figure 1. The upper Weihe river (UWR) is situated within the Zhucheng Basin of the Ludong fault block. The predominant lithologies in this region are sandstone, shale, and conglomerate. Its geological structure is intricate, characterized predominantly by fold and fault structures. The topography of the UWR is characterized by higher elevations in the south and lower elevations in the north. The southern region of the study area is characterized by rolling low mountains and hills, while the central part consists of undulating plains that transition to low mountains and gentle hills on the periphery. Zhucheng hosts over 50 rivers, which are categorized into the Weihe river, Jili river, and Jiaolai river systems. Notably, the Weihe river system is the largest in China, with its main channels and tributaries arranged in a dendritic pattern. The aquifers within the region are classified as shallow groundwater. The UWR falls within the semi-humid climate zone of the warm temperate continental monsoon region. The annual average temperature is 13.2 °C, the annual sunshine rate is 54%, and the annual average relative humidity is 67%.

The UWR is a quintessential resource-based water-deficient region in northern China, with its water supply predominantly sourced from precipitation during the flood season and water retained in hydraulic infrastructure [25]. The region records an average annual precipitation of 718.1 mm, and the average annual total water resources amount to 561 million m³. In the UWR, most areas consist of weakly permeable strata, resulting in relatively scarce groundwater resources. Groundwater is predominantly found in river alluvial plains and piedmont plains, with the primary type being pore water, supplemented by fissure and karst waters. Based on the city’s hydrological conditions, the region can be broadly categorized into three hydrogeological zones: the loose rock pore water zone, the bedrock fissure water zone, and the soil pore water zone. The loose rock pore water area is predominantly located along the banks of the Quhe and Weihe rivers. The aquifer thickness typically ranges from 2.00 to 5.00 m, with groundwater levels generally between 5.00 and 9.00 m deep. Recharge to this aquifer is primarily derived from atmospheric precipitation and river leakage, while the principal discharge mechanisms are anthropogenic extraction and river recharge. The bedrock fissure water area is predominantly found in locales such as Jiayue, Mazhuang, and Chenggezhuang within Zhucheng. These areas are characterized by the cretaceous wangshi formation’s red brittle sandstone and interbedded red and yellow-green sand shale. The groundwater levels are typically between 4.00 and 20.00 m below the surface. Recharge primarily derives from atmospheric precipitation, with a minor contribution from river leakage during periods of high extraction in dry years. In the UWR, groundwater is predominantly categorized as soil pore water, with depths varying between 2.00 and 7.00 m. Recharge is exclusively derived from precipitation, while evaporation and human withdrawal are the principal consumption pathways. Although the majority of this groundwater lacks economic exploitation value, it remains the principal source of water for both human consumption and livestock. The primary issue concerning groundwater in the UWR is the scarcity of the water resources.

3. Materials and Methods

3.1. Groundwater Sample Collection and Measurement

In this study, 232 groundwater samples were collected in the UWR. To ensure the acquisition of fresh, uncontaminated groundwater, pumping was initiated for 10 min prior to sampling [26,27]. The sampling bottles were meticulously cleaned, with 3–5 rinses using the same water intended for sampling, ensuring that the bottles were completely filled with this water before sample collection commenced. Subsequently, bubbles were carefully removed to prevent air contamination, and the bottle was sealed with a film. Nitric acid was added to the sampling bottle to lower the pH of the water sample to below 2, thereby preventing the precipitation or adsorption of metal ions onto the container walls. Immediately following sampling, the water samples were placed in a cooler with an ice pack to maintain proper refrigeration. The samples were then expeditiously transported to the laboratory for comprehensive water quality analysis.

The pH was measured using an acidity meter (PHS–3C). In the laboratory, major cations (K⁺, Na⁺, Ca²⁺ and Mg²⁺), total dissolved solids (TDS), and total hardness (TH) were measured using an inductively coupled plasma emission spectrometer (optima7000DV). Anions (SO₄²⁻, NO₃⁻, Cl⁻, and F⁻) were measured using an ion chromatograph (ICS-600), and chemical oxygen demand (COD) and HCO₃⁻ were measured using a titration method.

In order to ensure the reliability of the water quality data, a preliminary assessment was conducted before data analysis. The reliability of the data was evaluated by using the electric neutrality equation to calculate the charge balance error (%CBE) (Equation (1)).

$$\%CBE={\displaystyle \frac{{\displaystyle \sum Z·{\mathrm{m}}_{\mathrm{cation}}-{\displaystyle \sum Z·{\mathrm{m}}_{\mathrm{anion}}}}}{{\displaystyle \sum Z·{\mathrm{m}}_{\mathrm{cation}}+{\displaystyle \sum Z·{\mathrm{m}}_{\mathrm{anion}}}}}}$$

(1)

Here, m_cation and m_anion refer to the molar concentration of cations and anions, and Z represents the charge of the ions. The %CBE values of the water samples analyzed in this study were calculated and it was found that all values were less than 5%. These results show the reliability of the data.

In addition, in order to ensure the reliability and accuracy of the test results, each sample was tested repeatedly in this study. Specifically, this study conducted at least three parallel tests on each sample and calculated the mean and standard deviation. Only when the standard deviation was within the acceptable range was it considered to be reliable.

3.2. Groundwater Quality Evaluation

3.2.1. Drinking Water

In this study, the entropy weight water quality index (EWQI) was employed to assess regional groundwater quality. Unlike the traditional water quality index (WQI), 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 [28]. Moreover, the EWQI method, leveraging the information entropy approach, offers a more comprehensive evaluation of water quality by considering the interrelationships between parameters. Compared to the conventional single-factor assessment method, the EWQI considers multiple water quality parameters and integrates their contributions, providing a more comprehensive reflection of the water quality status. Consequently, the EWQI offers a more holistic view of water quality, reduces human influence, and increases the reliability and accuracy of the evaluation outcomes.

The entropy weight water quality index (EWQI) method offers an effective assessment of water quality by analyzing multiple parameters in various dimensions, thereby improving evaluation accuracy. In the selection of water quality indicators for this study, priority was given to those with a significant influence on groundwater quality variations and pronounced exceedances. Consequently, the key indicators chosen were pH, TDS, TH, Na⁺, Cl⁻, SO₄²⁻, and NO₃⁻. The process consists of five steps: creating an initial water quality matrix, data normalization, weight determination using the entropy weight method, setting quantitative grading standards, and calculating and classifying the water quality index [29]. A flowchart illustrating these steps is presented in Figure 2.

• Establish the initial water quality matrix:

$$X=\left(\begin{array}{ccc}{x}_{11}& \dots & x_{1n}\\ ⋮& \ddots & ⋮\\ x_{m1}& \cdots & x_{mn}\end{array}\right)$$

(2)

• Standardize the data:

$${\mathrm{y}}_{\mathrm{ij}}={\displaystyle \frac{x_{ij}-\min (x_j)}{\max (x_j)-\min (x_j)}}\in (0,1)$$

(3)

$$Y=\left(\begin{array}{ccc}{y}_{11}& \dots & y_{1n}\\ ⋮& \ddots & ⋮\\ y_{m1}& \cdots & y_{mn}\end{array}\right)$$

(4)

In general, the hydrochemical indicators in the initial water quality data vary greatly in magnitude and dimension, and the calculated weights are also very different. Therefore, it is necessary to standardize the initial water quality data.

• Determine the ratio, information entropy and weight:

$$P_{ij}={\displaystyle \frac{1+y_{ij}}{{\displaystyle \sum _{i=1}^m(1+y_{ij})}}}\in (0,1)$$

(5)

$$E_j={\displaystyle \frac{1}{\ln m}}\times {\displaystyle {\sum }_{i=1}^m{P}_{ij}\ln P_{ij}}$$

(6)

$$w_j={\displaystyle \frac{1-E_j}{{\displaystyle \sum _{j=1}^{n}1-E_j}}}\in (0,1)$$

(7)

In the equations, E_j is the information entropy of the jth hydrochemical index, and w_j is the entropy weight of the jth hydrochemical index.

• Determine the quality level of the groundwater:

$$q_j={\displaystyle \frac{c_{ij}}{s_j}}\times 100$$

(8)

c_ij is the jth index measured in the ith sample, and s_j is the standard allowable value of the jth index of China’s groundwater quality standard.

• Determine the entropy weight water quality index (EWQI):

$$EWQI={\displaystyle {\sum }_{j=1}^n{w}_j{q}_j}$$

(9)

3.2.2. Irrigation Water

Groundwater in the study area is also the main water source for agricultural activities. High salinity in irrigation water can adversely impact soil structure, diminish soil water permeability, and consequently directly affect crop growth. In this context, the sodium adsorption ratio (SAR) and the soluble sodium percentage (Na%) were utilized as pivotal indicators for the assessment of irrigation water quality [30]. The calculation equations for these methods are as follows:

$$SAR={\displaystyle \frac{N{a}^{+}}{\sqrt{\frac{C{a}^{2+}+M{g}^{2+}}{2}}}}$$

(10)

$$Na\%={\displaystyle \frac{N{a}^{+}}{C{a}^{2+}+M{g}^{2+}+N{a}^{+}+K^{+}}}$$

(11)

In Equations (10) and (11), the ionic unit is in meq/L. According to the SAR value, the irrigation water quality can be divided into four categories: excellent (SAR < 10), good (10 < SAR < 18), suspicious (18 < SAR < 26), and unsuitable (SAR > 26). For Na%, less than 20, 20 to 40, 40 to 60, 60 to 80 and more than 80 represent excellent, good, allowable, suspicious and inappropriate water quality grades, respectively.

3.3. Health Risk Assessment

As a source of drinking water, groundwater is critical to human health risks. In addition, humans make skin contact with groundwater through swimming and bathing, among other ways. Health risk assessments are used to quantitatively describe the risk of harm caused by human exposure to a polluted environment [9]. This study used the internationally recognized procedures and standards introduced by the U.S. Environmental Protection Agency (EPA) to evaluate the health risks to adults and children in the study area by combining the non-carcinogenic risks of nitrate with the two exposure routes of oral exposure and percutaneous exposure [31].

3.3.1. Oral Exposure

For the oral exposure pathway, the intake of pollutants through drinking water is mainly considered. The exposure dose is calculated by Equation (12) [32]:

$$\mathrm{CDI}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{W}}\times \mathrm{IR}\times \mathrm{EF}\times \mathrm{ED}}{\mathrm{BW}\times \mathrm{AT}}}$$

(12)

In Equation (12), CDI is the average exposure dose of specific chemical substances ingested by the target individual through drinking water, mg/kg·d; C_w is the concentration of specific chemicals in water, mg/L; IR is the uptake rate, L/d; EF is the exposure frequency, d/a; ED is the duration of exposure, a; BW is the weight of the target individual, kg; AT is the average exposure dose time parameter, d.

3.3.2. Dermal Exposure

Skin contact exposure mainly occurs through swimming and bathing. The equation for calculating the daily average exposure dose is as follows:

$$\mathrm{DAD}={\displaystyle \frac{{\mathrm{DA}}_{\mathrm{event}}\times \mathrm{EV}\times \mathrm{ED}\times \mathrm{EF}\times \mathrm{SA}}{\mathrm{BW}\times \mathrm{AT}}}$$

(13)

Here, DAD is the average daily exposure dose through skin contact, mg/kg·d; DA_event is the absorbed dose per unit area of skin, mg/cm². The equation for DA_event is as follows:

$${\mathrm{DA}}_{\mathrm{event}}={\mathrm{K}}_{\mathrm{p}}\times {\mathrm{C}}_{\mathrm{w}}\times {\mathrm{t}}_{\mathrm{event}}$$

(14)

The exposure calculation parameter values are shown in

Table 1. Exposure calculation parameter values.

Parameter	Parameter Meaning	Unit	Child/Adult
IR	Uptake rate	L/d	1/2
EF	Exposure frequency	day/year	365
ED	Mean exposure time	year	6/30
BW	Average body weight	kg	15/61.75
AT	Mean exposure time	day	365 × ED
SA	Surface area of skin in contact	cm2	8000/16,000
Kp	Skin permeability coefficient	cm/h	0.001
tevent	Single contact time	h	0.33/0.25
RfD	Reference dose	mg/(kg·d)	1.6

[23].

3.3.3. Hazard Index (HI)

The risk of non-carcinogenic substances is quantified using the noncancer hazard quotient (NHQ), which is calculated as follows:

$$\mathrm{NHQ}={\displaystyle \frac{\mathrm{E}}{\mathrm{RfD}}}$$

(15)

In the equation, E is the exposure level or daily average exposure dose, namely the CDI or DAD, mg/kg·d.

The comprehensive risk of non-carcinogens is expressed by the hazard index (HI), which is the sum of the non-carcinogenic hazard quotients. When the HI value is greater than 1, it is considered that there is a certain health risk. The HI calculation equation is as follows:

$$HI={\displaystyle \sum NH{Q}_{\mathrm{i}}}$$

(16)

Figure 3 provides a visual representation and understanding of the technical procedures employed in this study.

4. Results and Discussions

4.1. General Characteristics of Groundwater Hydrochemistry

The general characteristics of groundwater hydrochemistry provide a foundation for the study of water chemistry and an understanding of the chemical formation processes of groundwater, with statistical analysis being the most frequently employed method.

Table 2. Groundwater hydrochemical statistics.

	pH	TH	TDS	Na+	Ca2+	Mg2+	K+	HCO3−	Cl−	SO42−	NO3−
Maximum	10.04	1012.15	1326	369.22	264.93	86.18	54.05	592.63	380.96	702.63	578.32
Mminimum	6.94	8.94	182	1.61	3.29	0.18	0.21	7.56	8.29	20.78	<0.1
Mean	7.96	365.86	567	43.63	98.97	28.83	2.08	201.61	75.79	94.18	104.36
Standard deviation	0.36	175.27	261.53	50.68	54.55	16.25	5.00	82.78	67.45	65.47	98.89
Coefficient of variation (%)	4.52	47.90	46.13	116.16	55.12	56.36	240.38	41.06	89.00	69.52	94.76

shows the statistical data of the main chemical components of groundwater in the UWR. The pH range of groundwater is 6.94~10.04, with an average of 7.96, indicating that the groundwater is weakly alkaline water (Figure 4a). The TDS and TH are two important parameters that comprehensively reflect the quality of the groundwater. The TDS in the UWR’s groundwater varied from 182 mg/L to 1326 mg/L, with a mean concentration of 567 mg/L. The TH levels spanned from 8.94 mg/L to 1012.15 mg/L, averaging at 365.86 mg/L. As depicted in Figure 4a, based on the classification of the TH and TDS, 75.86% of the groundwater in the UWR is classified as hard freshwater and moderately hard freshwater, and soft freshwater and slightly hard freshwater account for 14.66%. As shown in Table 2, the main cations in the groundwater are Ca²⁺ and Na⁺, and their average contents are 98.97 mg/L and 43.63 mg/L, respectively. HCO₃⁻ and NO₃⁻ are the main anions in the groundwater, with an average content of 201.61 mg/L and 104.36 mg/L, respectively. It is worth noting that the concentrations of all major chemical components in the study area are high. According to the box plot, the cation order is Ca²⁺ > Na⁺ > Mg²⁺ > K⁺, and the anion order is HCO₃⁻ > NO₃⁻ > Cl⁻ > SO₄²⁻ > F⁻ (Figure 4b).

The coefficient of variation (CV) is a reliable index to evaluate the dispersion of hydrochemical data [33]. As shown in Table 2, the coefficient of variation of the groundwater pH is small, indicating that the spatial distribution is stable and the variability is small. On the contrary, the CV values of Na⁺, K⁺, Cl⁻ and NO₃⁻ in the groundwater are very large (Table 2), indicating that there is considerable variability in their spatial distribution.

4.2. Hydrochemistry Type

The Piper diagram is a widely utilized hydrochemical classification tool, known for its objectivity and intuitive representation of the general chemical characteristics and hydrochemical types of water samples. Comprising a diamond and two triangles, the diagram uses the diamond to illustrate the overall chemical properties of water samples. The two triangles, in turn, represent the relative concentrations of anions and cations, enabling the determination of the water samples’ chemical types [34]. As shown in Figure 5, the Piper diagram shows that the water samples are mainly distributed in the Ca-type and mixed-type regions, and a small amount is distributed in the sodium type; the anion triangle diagram shows that the water samples are mainly distributed in the mixed and HCO₃-type regions. The water samples in the study area are mainly distributed in diamond-shaped areas 1, 2, and 5, which represent the HCO₃-Ca type, Cl-Ca type, and mixed type regions, respectively.

4.3. Groundwater Hydrochemical Control Factors

4.3.1. Gibbs Model

The hydrochemical characteristics are usually affected by rock weathering, evaporation, and precipitation. Gibbs introduced a semi-logarithmic graphical model that categorizes the chemical composition of global surface water, precipitation, and seawater [35]. The Gibbs diagram employs a semi-logarithmic coordinate system, plotting the Total Dissolved Solids (TDS) on the Y-axis using logarithmic scales, and the ratios Na⁺/(Na⁺ + Ca²⁺) and Cl⁻/(Cl⁻ + HCO₃⁻) on the X-axis. This model delineates three broad regions: rock weathering (middle left), evaporation (upper right), and atmospheric precipitation (lower left) [34]. However, the Gibbs model has limitations in its ability to assess the impact of human activities on water chemistry. As shown in Figure 6, most of the water samples are located in the rock-weathering area, indicating that rock weathering plays a leading role in determining the hydrochemical characteristics of the UWR. A comparison of these results with those from the Weifang area indicates that the hydrochemical characteristics of groundwater in Weifang are predominantly influenced by rock weathering. However, some water samples from the entire Weifang area are situated in evaporation-influenced zones, while certain samples from the coastal region are primarily controlled by seawater intrusion.

4.3.2. Correlation Analysis

Pearson correlation analysis is a commonly used tool in hydrogeochemical research to characterize the relationship between chemical components or parameter indicators. In this study, the Pearson correlation coefficients of various water quality indicators were plotted based on a Pearson correlation analysis. The obtained correlation coefficient matrix is shown in Figure 7. The TDS was significantly correlated with other major ions, indicating that these ions made a significant contribution to the TDS. There is also a positive correlation between Na⁺ and Cl⁻, which may be due to the dissolution of evaporites or atmospheric precipitation [36]. Both Ca²⁺ and Mg²⁺ exhibited a strong correlation with Cl⁻. However, 4.74% of the water samples had Cl⁻ levels that were close to or exceeded the groundwater quality standards, most likely due to human activities.

4.3.3. Ion Ratio Analysis

The ion ratio end-member diagram is used to further identify the potential sources of rock weathering and the mineral dissolution types that affect hydrogeochemical characteristics [36]. As illustrated in Figure 8, approximately 99.14% of the water samples fall between the silicate and carbonate rock end-members, suggesting that the hydrogeochemical composition of groundwater in UWR is primarily influenced by the weathering of silicate and carbonate rocks. Additionally, the proximity of two water samples to the evaporation end-member suggests that evaporite weathering contributes to the groundwater system in the UWR.

An ion ratio analysis is commonly employed to ascertain the types of rock weathering and mineral dissolution that influence the water chemistry within a given region [33]. The end-member diagram reveals that the hydrochemical composition of groundwater in the study area is predominantly influenced by the weathering of silicate and carbonate rock. Theoretically, if the dissolution of salt rocks was the sole contributor to Na⁺ and Cl⁻ in groundwater, the ratio [Na⁺]/[Cl⁻] would be expected to equal 1 [36]. This phenomenon occurs because the dissolution of rock salt yields equal molar amounts of Na⁺ and Cl⁻, as illustrated in Equation (17). However, Figure 9a indicates that water samples are predominantly distributed on either side of the 1:1 line, with some aligning with it, suggesting that salt rock dissolution is not the exclusive source of Na⁺ and Cl⁻ in groundwater. Samples positioned above the 1:1 line, where Cl⁻ exceeds Na⁺, may be influenced by human activities. Conversely, samples below the line, where Na⁺ surpasses Cl⁻, likely result from the weathering and dissolution of silicate minerals, which produce an excess of Na⁺. Additionally, cation exchange processes within the aquifer can also impact Na⁺ concentrations.

If the dissolution of gypsum is the only source of Ca²⁺ and SO₄²⁻ in groundwater, the [Ca²⁺]/[SO₄²⁻] ratio should theoretically be equal to 1 [23]. Figure 9b demonstrates that the water sample points predominantly lie to the right of the 1:1 line, indicating that the SO₄²⁻ is insufficient to balance Ca²⁺, suggesting the presence of additional sources. Consequently, the dissolution of gypsum is not the sole contributor to Ca²⁺ and SO₄²⁻ in groundwater; the excess Ca²⁺ may originate from the dissolution of carbonate minerals or calcium-bearing silicate minerals.

The Ca²⁺ to HCO₃⁻ ratio is commonly employed to assess the impact of carbonate mineral dissolution, including calcite and dolomite, on the hydrochemical composition [36]. Figure 9c illustrates that 85.35% of groundwater samples from the study area are distributed on both sides of the 1:1 line, suggesting that factors other than the dissolution of carbonate minerals influence the Ca²⁺ and HCO₃⁻ concentrations. If the dissolution of carbonate and sulfate minerals was the predominant hydrogeochemical reaction in the groundwater system, the molar ratio [Ca²⁺ + Mg²⁺]/[HCO₃⁻ + SO₄²⁻] would theoretically approximate 1. A ratio of [Ca²⁺ + Mg²⁺]/[HCO₃⁻ + SO₄²⁻] greater than 1 suggests that silicate weathering predominantly influences the chemical composition of groundwater. Conversely, if this ratio is less than 1, the hydrochemical characteristics of groundwater are primarily driven by the weathering of carbonate rocks [36]. Figure 9d demonstrates that groundwater samples predominantly lie below the 1:1 line, indicating that the ratio of (Ca²⁺ + Mg²⁺) to (HCO₃⁻ + SO₄²⁻) is generally greater than 1. This suggests that silicate rock weathering significantly influences the hydrogeochemical composition of groundwater in UWR. Additionally, samples with concentrations above the 1:1 line suggest the impact of carbonate weathering on these characteristics.

$$\mathrm{NaCl}\to {\mathrm{Na}}^{+}{+\mathrm{Cl}}^{-}$$

(17)

4.3.4. Cation Exchange

Cation exchange is a common hydrogeochemical reaction in groundwater systems. It is usually determined using the relationship between [(Na⁺ + K⁺) − Cl⁻] and [(Ca²⁺ + Mg²⁺) − (HCO₃⁻ + SO₄²⁻)]. If cation exchange is a key process affecting the hydrogeochemical properties of groundwater, the relationship between these values should be linear, with a slope close to -1 [36]. Figure 10a reflects the relationship between [(Na⁺ + K⁺) − Cl⁻] and [(Ca²⁺ + Mg²⁺) − (HCO₃⁻ + SO₄²⁻)] of groundwater samples in the study area. The figure clearly demonstrates a significant negative correlation between the milligram equivalents of [(Na⁺ + K⁺) − Cl⁻] and [(Ca²⁺ + Mg²⁺) − (HCO₃⁻ + SO₄²⁻)]. This observation suggests that cation exchange is a significant hydrogeochemical process, exerting a considerable influence on the hydrogeochemical properties of the groundwater system in the UWR.

Furthermore, the occurrence of cation exchanges can be ascertained using the chlor-alkali index (CAI). As per Equations (18) and (19), if both the CAI-1 and CAI-2 are negative, this signifies that a cation exchange is taking place within the groundwater system. Conversely, if both indices are positive, this suggests that a reverse cation exchange is occurring [37]. In Figure 10b, 83.62% of samples are located in the upper right quadrant. The results show that reverse cation exchange is a common form of cation exchange in most samples.

$$\mathrm{CAI}-1={\displaystyle \frac{{\mathrm{Cl}}^{-}-{(\mathrm{Na}}^{+}{+\mathrm{K}}^{+})}{{\mathrm{Cl}}^{-}}}$$

(18)

$$\mathrm{CAI}-2={\displaystyle \frac{{\mathrm{Cl}}^{-}-{(\mathrm{Na}}^{+}{+\mathrm{K}}^{+})}{{\mathrm{HCO}}_3^{-}+{\mathrm{SO}}_4^{2-}+{\mathrm{CO}}_3^{2-}+{\mathrm{NO}}_3^{-}}}$$

(19)

4.3.5. Impact of Human Activities

Nitrate has become a major contaminant in groundwater globally, primarily originating from industrial and agricultural activities, as well as domestic wastewater. To a significant extent, nitrate (NO₃⁻) concentrations reflect the impact of human activities on the hydrochemistry of aquatic systems [33,38]. As shown in Figure 11a, the samples from the UWR exhibit a trend along the x-axis, suggesting that the groundwater in the study area is affected by agricultural practices and the discharge of domestic sewage. Furthermore, Figure 11b illustrates that groundwater samples are predominantly found in areas of agricultural activity, indicating that nitrates in the groundwater of the UWR primarily result from agricultural practices. For instance, the application of fertilizers and pesticides can intensify groundwater nitrate pollution. Additionally, issues related to waste disposal and sewage discharge in rural areas also significantly contribute to the nitrate contamination of groundwater. Therefore, it is imperative to implement measures to reform agricultural practices in the study area. During the rainy season, the leaching effect of precipitation can significantly increase nitrate concentrations in groundwater. Consequently, rationalizing irrigation schedules and water quantities to mitigate the leaching effects of rainfall and irrigation water is an effective strategy to reduce nitrate pollution [39]. Reducing fertilizer usage and optimizing application times and methods are crucial steps in minimizing nitrate contamination. Nitrate pollution poses a significant threat to children’s health, necessitating proactive measures by local governments. For instance, the use of agricultural fertilizers and pesticides should be strictly regulated, and water purification equipment should be installed in high-risk areas to reduce the concentration of pollutants such as nitrates in water.

4.4. Groundwater Quality Evaluation

4.4.1. Drinking Water

According to the calculated EWQI value, the groundwater quality in the study area can be divided into five grades, ranging from “very poor” to “excellent”, as shown in

Table 3. Classification of the EWQI.

EWQI	Level	Category
≤50	I	Excellent
51~100	II	Good
101~150	III	Medium
151~200	IV	Poor
>200	V	Very poor

. In general, an EWQI value greater than 100 indicates that the water quality is not suitable for drinking [40].

In this study, TDS, TH, pH, SO₄²⁻, Cl⁻, NO₃⁻, and Na⁺ were selected as the evaluation factors, and the EWQI of water samples was calculated using the steps shown above. The EWQI values of water samples in the study area were between 8.08 and 216.36, with an average of 57.42. According to the EWQI grade classification, samples in the study area with excellent and good grades accounted for about 87.08%, of the total and the overall water quality was good. However, there were still a small number of water samples with a poor and very poor EWQI classification, accounting for 3.88%, which are not suitable for drinking. In the EWQI distribution map (Figure 12), it can be intuitively seen that the EWQI of most groundwater in the study area is less than 100, and is suitable for consumption. However, in a few areas in the north and west of the study area, the water quality is very poor and highly inadequate for consumption. Upon examining land use patterns in the UWR, it was determined that regions with poor water quality correspond to areas of agricultural and urban land use, where agricultural and domestic wastewater significantly degrade water quality.

Comparing the water quality results of the study area with those of other regions in Weifang, it was observed that groundwater quality was poorer in the coastal and central northern areas, while the southern region, where the study area is situated, exhibited better water quality, corroborating the findings of this study [23]. A comparison with Hainan Island, China, indicated a similar groundwater quality scenario; although the majority of samples demonstrated excellent quality, some were notably poor [9]. This suggests that timely improvements to groundwater quality across China are necessary. The water quality distribution map can be utilized to rigorously manage areas with substandard water quality.

4.4.2. Irrigation Water Quality

In addition to drinking water, groundwater is also the main source of irrigation water. High-quality irrigation water provides the necessary nutrients and water for plant growth and development [30]. In order to evaluate the suitability of groundwater for agricultural irrigation in the UWR, the SAR and Na% were used as key indicators in this study. The findings revealed that the SAR ranged from 0.22 to 85.43, with an average of 7.13. A substantial majority, 88.36%, of the water samples were categorized as excellent (SAR < 10), while 4.47% were considered good. The remaining 6.90% were deemed suspicious or unsuitable for agricultural irrigation. Na% varied from 1.16 to 95.69, averaging at 20.08. Among the water samples, 70.26% were classified as excellent (Na% < 20) and 21.55% were considered good (20 < Na% < 40). However, 5.17% were found to be unsuitable for agricultural irrigation.

Figure 13a presents the USSL irrigation water quality classification diagram, where ‘C’ denotes the salinization level and ‘S’ signifies the alkalization level. The water samples from the study area predominantly fall within the C₂S₁ and C₃S₁ categories, indicating that they are highly suitable for agricultural irrigation. Nevertheless, Figure 13a indicates that a subset of water samples are unsuitable for this purpose. Furthermore, the Wilcox irrigation water quality classification diagram (Figure 13b) shows that the majority of groundwater samples were situated in regions 1 and 2, suggesting that most groundwater in the study area is of excellent quality for irrigation. However, 3.88% of the samples were situated in zones 3 and 4. In conclusion, while the majority of the groundwater in the study area is well-suited to irrigation, a small fraction is not appropriate for such use.

4.4.3. Health Risk Assessment

The extensive application of inorganic nitrogen fertilizers in agricultural practices has led to a notable increase in nitrate levels in water bodies. Human exposure to nitrates in groundwater primarily occurs through skin contact, the ingestion of drinking water, and the consumption of food. Nitrate was selected as the evaluated pollutant in this study due to the relatively high levels of nitrate ions observed in the collected water samples and the correlation between nitrate pollution and other pollutants. In this study, nitrate was designated as a key factor in the assessment of human health risks in the study area, with the non-carcinogenic comprehensive risk quantified using the hazard index (HI) [23]. In this study, the HI of children and adults was statistically analyzed, and the health risk of the groundwater in the study area was evaluated.

The calculated HI values for children ranged from 0 to 24.13, with an average of 4.35, exceeding the non-carcinogenic risk threshold of 1. For adults, the HI values ranged from 0 to 11.72, with an average of 2.11, which also surpasses the threshold of 1. This suggests that there is a significant risk of disease among both age groups in the UWR. Furthermore, children’s non-carcinogenic risk exceeded that of adults, with 82.32% of water samples showing children’s HI values surpassing the threshold of 1, compared to 62.93% for adults. Utilizing ArcGIS, a spatial distribution map of the non-carcinogenic comprehensive risk (HI) of nitrate in groundwater across the UWR was created, as depicted in Figure 14. This figure indicates that the HI value surpasses the non-carcinogenic risk limit in most regions, signifying a high risk. In conjunction with the analysis of human impacts and land use, it was determined that agricultural activities and the substantial proportion of agricultural land in the UWR are the primary sources of nitric acid, which, to some extent, pose health risks to humans. Notably, the risk to children is elevated compared to that for adults. These results highlight the necessity of further research and the implementation of mitigation strategies.

In summary, the study’s results offer valuable insights into the sustainable management of groundwater resources and environmental protection in the UWR. Nitrate, identified as the primary pollutant, predominantly stems from agricultural activities. Therefore, local governments should implement measures to reduce the overuse of chemical fertilizers and pesticides in agriculture, regulate the discharge of industrial wastewater and waste, and conduct regular groundwater quality assessments [41]. Additionally, the health risk assessment results indicate that the UWR poses a relatively high health risk, particularly to children. This may be attributed to children’s ongoing growth and development, which results in a faster metabolic rate than adults, making them more sensitive to contaminants. Additionally, children’s lower body weight and higher pollutant intake per unit of body weight further increase their susceptibility to health risks. Considering children’s increased sensitivity to certain pollutants, it is imperative to implement protective measures, including the provision of safe drinking water in educational institutions and children’s recreational spaces. Regular health check-ups for adolescents, particularly focusing on the blood and digestive systems, are essential for the timely detection of health issues potentially caused by nitrate contamination. Additionally, health education for adolescents and their parents should be enhanced to promote good drinking water habits, discourage the direct consumption of untreated groundwater, and increase awareness of the hazards of nitrate contamination.

5. Conclusions

• The groundwater pH in the study area varies between 6.94 and 10.04, averaging 7.96, which is indicative of weakly alkaline conditions. The TDS and TH values suggest that the groundwater is primarily hard freshwater. The cationic dominance in groundwater is ordered Ca²⁺ > Na⁺ > Mg²⁺ > K⁺, and the anionic order is HCO₃⁻ > NO₃⁻ > Cl⁻ > SO₄²⁻ > F⁻. The high CV values for Na⁺, K⁺, Cl⁻, and NO₃⁻ suggest considerable spatial variability.
• The predominant hydrochemical types in the study area are HCO₃-Ca, Cl-Ca, and mixed types. The formation of groundwater chemistry is primarily influenced by the dissolution of silicate minerals, with a contribution from the weathering of carbonate rocks. Additionally, cation exchange significantly impacts the hydrochemical composition of groundwater, with reverse cation exchange being a prevalent form in the majority of samples. Agricultural activities are the main source of nitrate.
• The assessment of drinking water quality revealed that 92.83% of groundwater in the UWR is classified as excellent or good. However, in certain northern and western regions, the water quality is deemed poor and unfit for drinking. The SAR and Na% indicate that 94.83% of the groundwater possesses excellent irrigation water quality, yet some areas are unsuitable for irrigation purposes.
• The health risk evaluation results indicate a high non-carcinogenic risk for both adults and children in most areas of the UWR. In addition, the non-carcinogenic risk of groundwater in children is much higher than that in adults. Given these findings, it is recommended that proactive measures be implemented to enhance groundwater pollution prevention and control, to manage the use of fertilizers and pesticides scientifically, and to intensify groundwater monitoring and management in the UWR.
• Based on the study’s findings, local governments can utilize the distribution of water quality to pinpoint priority areas for groundwater pollution control, enforce stringent pollution control measures, and devise comprehensive groundwater protection and use plans. Moreover, the health risk assessment indicates that local governments should rigorously control groundwater pollution and implement protective measures for adolescents and children. This study did not conduct a seasonal analysis of the UWR; however, a comparative analysis of the wet and dry seasons will be undertaken in future research.