Characterization of Groundwater Hydrochemistry and Temporal Dynamics of Water Quality in the Northern Baiquan Spring Basin

Abstract

Groundwater represents a critical resource for sustaining the livelihoods of both urban and rural populations, facilitating economic and social development, and preserving ecological equilibrium. This study leverages groundwater quality monitoring data from the northern Baiquan spring basin (NBSB) to elucidate groundwater hydrochemical characteristics and decipher the temporal variability in water quality. Findings suggest that groundwater within the NBSB is predominantly weakly alkaline and characterized as hard-fresh, with HCO₃⁻ and Ca²⁺ as the predominant ions, which collectively demarcate the hydrochemical type as predominantly HCO₃-Ca. The principal constituents of NBSB groundwater are influenced predominantly by the weathering of carbonates and silicates alongside the dissolution of gypsum and halite. Moreover, agricultural operations and similar human activities have exerted an impact on the hydrochemical attributes of NBSB’s groundwater. Generally, fluctuations in groundwater anion concentrations over time are more pronounced than those of cations, exemplified by a significant upward trend in the major ion concentrations at the BQ03 monitoring site in the later stages. While the general groundwater quality within the NBSB is deemed satisfactory, most monitoring sites have experienced an escalation in water quality indices over time, notably at BQ03, which warrants serious attention. The findings of this research contribute to the efficient management and sustainable utilization of groundwater resources in the NBSB.

1. Introduction

Groundwater, as a natural water storage system, is widely distributed, stable, and reliable, with substantial reserves constituting approximately 99% of the Earth’s liquid freshwater [1,2,3]. It plays a critical role in providing drinking water, supporting sanitation systems, agriculture, industry, and ecosystems [4,5,6,7,8]. Globally, groundwater supplies 50% of residential water and approximately 25% of agricultural irrigation water, which supports 38% of the world’s irrigated land. However, rapid population growth, accelerated urbanization, and changing patterns in water usage are posing significant threats and challenges to groundwater resources [3,9,10,11]. Many underground aquifers are being exploited at unsustainable rates, and groundwater pollution is becoming increasingly problematic [12,13,14]. Consequently, the rational development and management of groundwater resources has emerged as an urgent issue that must be addressed.

The formation of groundwater is a prolonged hydrological process influenced by environmental factors such as rainfall, evaporation, infiltration, and seepage [1,13,14,15,16]. Groundwater undergoes intricate geological processes and geochemical reactions within underground aquifers, interacting with subterranean rocks to create distinct geochemical characteristics [2,5,17,18,19]. Analyzing dynamic changes in groundwater’s chemical composition and evaluating water quality can enhance our understanding of groundwater’s composition, trends, and potential issues, thereby guiding the scientific management and protection of groundwater resources. For example, Liu et al. [7] investigated groundwater level and quality fluctuations in the valley plain of Lhasa City on the Qinghai-Tibet Plateau, identifying influencing factors. Mishra et al. [20] examined spatiotemporal variations in groundwater quality in Madhya Pradesh, India, assessed its suitability for drinking, and offered recommendations for groundwater management. Ismail et al. [21] analyzed groundwater quality in Lahore from 2005 to 2021, revealing a general decline associated with urbanization, and providing insights for decision-making on groundwater management and urban expansion in Lahore. Pacheco Castro et al. [22] employed hierarchical clustering analysis to study spatiotemporal changes in groundwater quality of karst aquifers in Yucatan, Mexico, enhancing understanding of how precipitation and human activities impact groundwater quality. These studies collectively contribute valuable insights for the rational development and management of groundwater resources in their respective regions.

Groundwater serves as a vital water source and a key component of the ecological environment in the Baiquan spring basin (BSB), significantly impacting the lives and economic progress of local communities. The groundwater in the BSB mainly consists of carbonate rock fissure karst water and Quaternary loose rock pore water. In previous studies, the groundwater in typical water sources in the study area was classified as Class III water, with the main indicators exceeding the standard being nitrate, sulfate, TDS, and TH. The concentrations of each indicator have significantly increased compared to decades ago, indicating that the groundwater has been polluted to a certain extent. In order to prevent the continuous deterioration of groundwater quality, certain groundwater protection measures need to be taken [23,24]. Despite its importance, there is a notable dearth of research exploring the hydrochemistry and water quality of BSB groundwater. To address this gap, our study capitalized on long-term monitoring data from groundwater sites in the northern Baiquan spring basin (NBSB). Employing a combination of hydrochemical and statistical techniques, we aimed to: (1) delve into the hydrochemical traits and primary ion sources of NBSB groundwater, (2) elucidate the temporal shifts in groundwater’s main ion concentrations, and (3) assess groundwater quality and its evolving trends. The insights gained from this research offer valuable guidance for the sustainable utilization, management, and conservation of groundwater resources in the NBSB region.

Study Area

The BSB (Figure 1), one of the four major spring areas in Jinan, constitutes a relatively independent hydrogeological unit with a total area of 783.48 km². This research area is situated in a mid-latitude region within the warm temperate semi-humid monsoon climate zone. It experiences an average annual temperature of 12.8 °C, average annual precipitation of 671.2 mm, and average annual evaporation of 1804 mm [25]. The region features a well-developed water system, including lakes and reservoirs, and is part of the Yellow River basin. Notable rivers in the area include the Juye River, Xibalou River, and Ganggou River.

The southern boundary of the BSB is delineated by a surface watershed, while the northern boundary corresponds to the contact zone between the Carboniferous and Permian strata and the associated magmatic rocks. The eastern and western limits are defined by the Wenzu fault and Dongwu fault, respectively, both of which exhibit relative water resistance. The altitude of the research area decreases gradually from south to north, with the southern region characterized by hilly terrain and low mountains, transitioning to a plain area in the north. From south to north, the exposed geological formations include metamorphic rocks from the Mount Taishan Group, Cambrian limestone, and Ordovician limestone in succession [23,25].

The primary water types found in the BSB include Quaternary loose rock pore water, clastic rock pore and fissure water, and carbonate rock fissure and karst water. Quaternary loose rock pore water is predominantly located in the central and northern plain areas of the BSB. Clastic rock pore and fissure water mainly occur within Carboniferous–Permian sandstone, sandy shale, and Tertiary strata. Carbonate fissure and karst water are present in thick layers of limestone aquifers, thin layers of limestone, and interbedded shale aquifers [23]. Additionally, bedrock fissure water is sporadically distributed within the BSB (Figure 1). Groundwater recharge, runoff, and discharge in the BSB are influenced by factors such as meteorology, hydrology, stratigraphy, lithology, artificial exploitation, and irrigation practices. The principal source of groundwater recharge is atmospheric precipitation, followed by infiltration from field irrigation. Groundwater runoff generally follows the same direction as surface water, flowing from southeast to northwest. Under natural conditions, discharge occurs primarily through lateral runoff and groundwater evaporation [26]. Currently, the main discharge methods are manual extraction and downstream runoff discharge, with significant variation in water level depths observed from south to north.

2. Materials and Methods

2.1. Sampling

A total of 8 long-term dynamic observation points were set up in the research area, with continuous and reliable data sources, so these 8 points were chosen for sampling, with a time span from 2018 to 2023.The monitoring points, as illustrated in Figure 1, yielded a total of 70 water quality data points. Notably, BQ02 (monitored from 2020 to 2023) and BQ08 (monitored from 2022 to 2023) had shorter monitoring durations, and there are some missing data at other monitoring points. Groundwater samples were collected twice yearly, once in June and once in September, using two 500 mL polyethylene plastic bottles. To ensure the freshness of the samples, a stable pumping was conducted for 5 min prior to sampling. Furthermore, each sampling bottle was rinsed at least three times with the groundwater to be sampled, and the bottles were completely filled, leaving no bubbles. After the sampling bottle was completely filled, the bottle cap was tightened and sealing film was wrapped around it. Acid was added to pH < 2 for detecting NO₃⁻. The distance between the sampling location and the laboratory was relatively close. After the sampling was completed on the same day, the samples were sent to the laboratory for water quality testing. The water chemistry laboratory of Shandong Provincial Geo-mineral Engineering Exploration Institute conducted the water sample testing.

2.2. Testing

Hydrochemical composition analysis of samples in the laboratory was conducted, including major anions (Cl⁻, SO₄²⁻, HCO₃⁻, NO₃⁻, F⁻), cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), pH, total hardness (TH), and total dissolved solids (TDS). The pH value was measured using an acidity meter (PHS-3C, Shanghai Yidian Scientific Instrument Co., Ltd., Shanghai, China), while cations, TDS, and TH were tested using an inductively coupled plasma emission spectrometer (ICP, Optima 7000DV, Thermo Fisher Scientifc, Waltham, MA, USA). No other ions were detected, so they are concentrated in the following main ions. Anions, except for HCO₃⁻ (which was determined by titration), were measured using an ion chromatograph (ICS-600, Thermo Fisher Scientifc, Waltham, MA, USA). Additionally, to ensure the accuracy of the water quality data, charge balance error (CBE) calculations were performed on all water samples. The results indicated that the CBE values for all samples were within ±5%, confirming the reliability of the water quality monitoring conducted in this study.

2.3. Groundwater Quality Evaluation

Groundwater quality evaluation is a comprehensive process that assesses multiple water quality parameters. It involves comparing the actual concentrations of each parameter in groundwater with standard concentrations to determine an evaluation grade that reflects water quality directly [2,6,27,28]. In this study, groundwater quality at the NBSB was evaluated using the entropy weight water quality index (EWQI), based on China’s “Groundwater Quality Standards”. Eight indicators were selected for this evaluation: pH, total hardness (TH), total dissolved solids (TDS), Na⁺, Cl⁻, SO₄²⁻, NO₃⁻, and F⁻. The calculation process for the EWQI is as follows:

Firstly, construct the initial matrix X_ij where i = 1, 2, …, m and j = 1, 2, …, n. Here, m represents the total number of water samples, and n denotes the number of hydrochemical parameters for each sample.

$$x_{ij}=\left[\begin{array}{ccc}{x}_{11}& \cdots & x_{1n}\\ ⋮& \ddots & ⋮\\ x_{m1}& \cdots & x_{mn}\end{array}\right]$$

(1)

$$y_{ij}={\displaystyle \frac{x_{ij}-{\left(x_{ij}\right)}_{min}}{{\left(x_{ij}\right)}_{max}-{\left(x_{ij}\right)}_{min}}}$$

(2)

Secondly, normalize the initial matrix X_ij to obtain the judgment matrix Y.

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

(3)

In the formula, (x_ij)min and (x_ij)max represent the minimum and maximum values of the same hydrochemical parameters across all samples, respectively.

Then, calculate the information entropy (e_j), entropy weight (w_j), and quality evaluation quantity (q_j) using the following equations:

$$\begin{gathered} e_j=-{\displaystyle \frac{1}{\ln m}}{\displaystyle \sum }_{i=1}^m{P}_{ij}\ln P_{ij} \\ {P}_{ij}=1+{\displaystyle \frac{\left(1+y_{ij}\right)}{{{\displaystyle \sum }}_{i=1}^m\left(1+y_{ij}\right)}} \end{gathered}$$

(4)

$$w_j={\displaystyle \frac{1-e_j}{{{\displaystyle \sum }}_{j=1}^n\left(1-e_j\right)}}$$

(5)

$$q_j={\displaystyle \frac{C_j}{S_j}}\times 100$$

(6)

In the formula, c_j represents the concentration of evaluation indicator j (mg/L), and s_j represents the allowable limit value of indicator j (mg/L).

Finally, calculate the EWQI using the following equation:

$$\mathrm{EWQI}={\displaystyle \sum }_{i=1}^m{w}_j{q}_j$$

(7)

Based on the classification criteria for groundwater quality:

EWQI < 25: Groundwater quality is classified as Excellent (Class I).

25 ≤ EWQI < 50: Groundwater quality is classified as Good (Class II).

50 ≤ EWQI < 100: Groundwater quality is classified as Medium (Class III).

100 ≤ EWQI < 150: Groundwater quality is classified as Poor (Class IV).

EWQI > 150: Groundwater quality is classified as Extremely Poor (Class V).

These classifications help in assessing the suitability of groundwater for various uses and determining the need for treatment or remediation.

3. Results

3.1. Overall Characteristics of Groundwater Hydrochemistry

Table 1. Statistical results of groundwater hydrochemical composition of NBSB.

	Maximum	Minimum	Mean	Standard Deviation	Coefficient of Variation
pH	8.1	7.1	7.59	0.23	3.03
TH (mg/L)	950	298	527.80	187.66	35.56
TDS (mg/L)	1215	371	665.63	248.05	37.27
Ca2+ (mg/L)	301	89.6	160.62	58.62	36.50
Mg2+ (mg/L)	66.6	18.1	30.77	11.29	36.69
K+ (mg/L)	3.62	0.46	1.39	0.92	66.19
Na+ (mg/L)	44.8	2.93	19.77	13.75	69.55
Cl− (mg/L)	165	8.25	53.39	47.17	88.35
SO42− (mg/L)	583	58.7	184.39	113.34	61.47
HCO3− (mg/L)	578	187	329.79	93.94	28.48
NO3− (mg/L)	131	12.2	37.76	17.62	46.66
F− (mg/L)	0.57	0.06	0.26	0.11	42.31

and Figure 2a present the statistical results of groundwater quality monitoring data for the NBSB from 2018 to 2023. The pH values of the groundwater range from 7.1 to 8.1, with a mean of 7.59, indicating a weakly alkaline environment. TDS and TH are key indicators of groundwater quality [2,29,30]. In the NBSB, groundwater TH ranges from 298 mg/L to 950 mg/L, and TDS ranges from 371 mg/L to 1215 mg/L, with average values of 527.80 mg/L and 665.63 mg/L, respectively. Except for pH and HCO₃⁻, the coefficient of variation of other ions is greater than 30%, indicating that these ions have a high degree of dispersion and instability, which may be influenced by supply sources or human activities. According to the TDS and TH water quality classification (Figure 2b), the groundwater in the NBSB is predominantly classified as hard-fresh water (82.9%), with a smaller proportion categorized as hard-brackish water (17.1%).

The dominant cation in the groundwater of the NBSB is Ca²⁺, with concentrations ranging from 89.6 mg/L to 301 mg/L and a mean value of 160.62 mg/L. Mg²⁺ is the second most prevalent cation, varying from 18.1 mg/L to 66.6 mg/L and averaging at 30.77 mg/L. Among anions, HCO₃⁻ is the most abundant, with levels between 187 mg/L and 578 mg/L, averaging at 329.97 mg/L. SO₄²⁻ is the next most common anion, ranging from 58.7 mg/L to 583 mg/L and averaging 184.39 mg/L. In general, the cation concentration follows the order Ca²⁺ > Mg²⁺ > Na⁺ > K⁺, while the anion concentration follows HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻ > F⁻ (Figure 2a). The NO₃⁻ content, which serves as an indicator of human activity intensity and its impact on the groundwater environment [10,31,32], ranges from 12.2 mg/L to 131 mg/L in NBSB groundwater from 2018 to 2023, with an average concentration of 37.76 mg/L. Notably, the average NO₃⁻ concentration in NBSB groundwater remains below the standard limit of 88.57 mg/L.

3.2. Hydrochemical Types

The Durov plot [33] is a widely utilized method for characterizing groundwater chemistry. Compared to the Piper diagram, the Durov plot not only depicts the hydrochemical type but also simultaneously represents the TDS and pH values of water samples, which facilitates a more comprehensive understanding of hydrochemical characteristics [2,14,34,35]. Figure 3 illustrates the projection of groundwater monitoring data from the NBSB for the period 2018 to 2023 onto the Durov plot. It is evident that, with the exception of one groundwater sample located in area ①, the majority of the samples are distributed in area ②. Therefore, according to the Durov plot, the predominant hydrochemical type of groundwater in the NBSB is classified as Ca-HCO₃ type.

3.3. Water Quality Evaluation Results

This study evaluated groundwater quality at five monitoring points (BQ01, BQ03, BQ04, BQ06, and BQ07) within the NBSB region, utilizing the EWQI based on comprehensive groundwater dynamic monitoring data. The results indicated that EWQI values ranged from 19.18 to 76.09, with a mean value of 36.65. Specifically, 23.64% of the samples were classified as Class I water (13 samples), 52.73% as Class II water (29 samples), and 13.64% as Class III water (13 samples). Overall, groundwater quality in the NBSB is rated as excellent, making it an ideal water supply source.

4. Discussion

4.1. Sources of Major Ions

The hydrochemical composition of groundwater in a region is primarily influenced by rock weathering, evaporation, and precipitation. Among these, rock weathering—particularly of silicates, carbonates, and evaporites—is often the most significant factor [5,17,27,36]. In coastal areas, seawater intrusion can also play a substantial role [1,37,38]. Accelerated urbanization and population growth in many parts of the world have exacerbated groundwater pollution, mainly through the misuse of groundwater assets and the discharge of domestic and industrial wastewater into groundwater systems. Groundwater pollution that includes radiation, wastewater, heavy metal pollution, microbial contamination and more, can also have an impact on hydrochemical characteristics [39]. Additionally, in regions with significant human activity, anthropogenic factors often significantly impact hydrochemical characteristics and should not be overlooked [4,18,30,32]. Conversely, analyzing the characteristics and interrelationships of major ions in groundwater can help identify their sources.

One of the primary sources of Na⁺ and chloride Cl⁻ in groundwater is the dissolution of halite (NaCl) in sedimentary rocks. Therefore, if halite dissolution is the main source of Na⁺ and Cl⁻ in groundwater, their ratio should theoretically approach 1 [17,32,36]. Figure 4a illustrates the relationship between Na⁺ and Cl⁻ in the groundwater of the NBSB region. The data points are predominantly located above the Na/Cl = 1 line, with some points situated close to both sides of this line. This suggests that while halite dissolution contributes to the Na⁺ and Cl⁻ concentrations, other hydrogeochemical processes—such as cation exchange or anthropogenic inputs—also influence these ions [2,36]. The relationship between Ca²⁺ + Mg²⁺ and HCO₃⁻ + SO₄²⁻ is used to assess the impact of carbonate and silicate rock weathering on groundwater chemistry [6,29,40]. As depicted in Figure 4b, the water sample points are mainly distributed on either side of the (Ca²⁺ + Mg²⁺)/(HCO₃⁻ + SO₄²⁻) = 1 line, with a concentration in the upper region. This indicates that groundwater in the NBSB area is influenced by the dissolution of carbonate and silicate minerals, with a more pronounced effect from carbonates.

Figure 4c illustrates the relationship between Ca²⁺ and SO₄²⁻ in groundwater. The data points predominantly fall below the Ca²⁺/SO₄²⁻ = 1 line, with some points near the line, suggesting gypsum dissolution in the groundwater of the NBSB area. However, gypsum dissolution is not the primary source of Ca²⁺ and SO₄²⁻. In the end-member diagram of the Cl⁻/Na⁺ to NO₃⁻/Na⁺ ratio, the upper right corner indicates areas influenced by agricultural activities, the lower left corner denotes areas controlled by the weathering of carbonates and silicates, and the lower right corner represents areas affected by urban development and evaporite dissolution, as shown in Figure 4d. The end-member diagram of the Cl⁻/Na⁺ to NO₃⁻/Na⁺ ratio is commonly used to identify the sources of Cl⁻ and NO₃⁻ in groundwater [40,41,42]. Projecting the groundwater quality data from the NBSB area onto Figure 4d reveals that the water samples are primarily located in the agricultural, urban, and evaporite-controlled areas. This indicates that NO₃⁻ in the groundwater is predominantly influenced by human activities such as agriculture and urban development, while some Cl⁻ originates from evaporite dissolution.

The saturation index (SI) is defined as the logarithm (base 10) of the ratio between the ion activity product (IAP) and the solubility product constant (K), serving as an indicator of the equilibrium state between minerals and water [43]. An SI value of 0 signifies that the mineral is in a state of dissolution equilibrium with the aqueous solution. When SI < 0, the mineral is undersaturated and has a tendency to dissolve further; conversely, when SI > 0, the mineral is supersaturated and likely to precipitate. The SI values for groundwater samples from the NBSB were calculated, as depicted in Figure 5. The results indicate that the SI values for calcite, dolomite, and gypsum are all greater than 0, suggesting that these minerals are in a supersaturated state and are prone to precipitation. In contrast, the SI values for halite are all less than 0, indicating an undersaturated state and a tendency for continued dissolution.

4.2. Variation Characteristics of Major Ion Content in Groundwater

Regular testing of groundwater indicators enables the timely detection of water quality trends and provides a comprehensive understanding of groundwater quality, offering a scientific basis for water resource management and protection [7,20,21]. In this study, five monitoring points (BQ01, BQ03, BQ04, BQ06, and BQ07) with the most complete groundwater data in the NBSB area were selected to analyze the temporal dynamics of their major ion contents, as shown in Figure 6 and Figure 7.

Figure 6 presents the temporal variations of major cations in NBSB groundwater. Overall, the cation concentrations at each monitoring point exhibit minimal long-term trends and show some fluctuation. Compared to Ca²⁺ and Mg²⁺, K⁺ and Na⁺ display greater fluctuations, indicating higher instability, which is reflected in their larger CV values (Table 1). The cation concentrations at BQ01 are relatively stable compared to other points, whereas BQ03 shows a significant upward trend in Ca²⁺, Mg²⁺, and Na⁺ concentrations after September 2022. This change in water quality at BQ03 warrants further investigation. Additionally, the cation concentrations at monitoring point BQ07 are notably higher than those at other points, followed by BQ06 (Figure 6).

Figure 7 illustrates the temporal variations in the concentrations of major anions in NBSB groundwater. Compared to cations, anions exhibit more pronounced fluctuations over time. For example, there was a significant decrease in HCO₃⁻ content at BQ07 in September 2022 and in Cl⁻ content in September 2023. The anion concentrations at BQ03 began to show an increasing trend starting in June 2022, particularly for NO₃⁻ (Figure 7d). At BQ06, the SO₄²⁻ concentration decreased significantly in June 2020, then increased, and has since exhibited a fluctuating downward trend. Conversely, the SO₄²⁻ concentration at BQ07 has shown a consistent upward trend (Figure 7b). Additionally, NO₃⁻ concentrations at BQ07 have demonstrated significant fluctuations since 2021, suggesting potential impacts from human activities on groundwater at this location.

4.3. Variation Characteristics of Groundwater Quality

Figure 8 illustrates the temporal variation in EWQI values for each monitoring point. Most points exhibited an increasing trend, except for BQ06, which showed a slight decrease, with BQ03 exhibiting the most significant increase. In terms of mean values, BQ01 demonstrated superior water quality compared to the other points and showed relatively stable changes. The EWQI at BQ03 generally increased annually, aligning with the trends observed in major ions at this location (Figure 6 and Figure 7). The EWQI at BQ04 also showed an upward trend, but with minimal fluctuation. Conversely, EWQI values at BQ06 and BQ07 exhibited more pronounced variability. In summary, while groundwater quality in the NBSB remains high, it is important to note the gradual increase in ion content at certain monitoring points.

4.4. Suggestions for the Management, Utilization, and Protection of Groundwater Resources in NBSB

Groundwater is a vital water supply resource for the NBSB, significantly contributing to daily life, industrial production, and agricultural development. To ensure the effective management, utilization, and protection of groundwater and to achieve sustainable use of these resources, the following measures are recommended:

(1)

Establish Comprehensive Legislation and Policies: Develop robust laws, regulations, and policies to define groundwater management responsibilities and rights clearly. Regulate groundwater development and use, and enhance supervision over groundwater resources.

(2)

Enhance Groundwater Monitoring: Strengthen groundwater monitoring by establishing a comprehensive network to track groundwater levels, water quality, and quantity. This will enable the assessment of dynamic changes in groundwater resources and provide a scientific basis for their rational use.

(3)

Control Groundwater Pollution Sources: Implement stringent measures to prevent the discharge of harmful substances into groundwater. Increase efforts to manage and mitigate groundwater pollution.

(4)

Improve Groundwater Resource Management: Advance the integrated management of groundwater resources by refining management institutions, coordinating resources across stakeholders, and ensuring the rational allocation and use of groundwater.

(5)

Designate Groundwater Protection Zones: Identify and protect groundwater protection zones by restricting development and utilization in sensitive areas. Prevent pollutant infiltration to safeguard groundwater quality and ensure its sustainable use.

(6)

Promote Public Awareness and Education: Enhance public education and awareness about groundwater resources. Encourage water conservation practices and foster a greater understanding of groundwater protection among the public.

In conclusion, groundwater is a crucial resource for human activities and well-being. The rational development and management of groundwater are essential for the sustainable advancement of society. It is imperative for everyone to contribute to the protection of groundwater resources.

5. Conclusions

This study employed long-term dynamic monitoring data of groundwater quality from the NBSB region to ascertain the hydrochemical characteristics and ion sources of groundwater were determined, the temporal changes in the concentration of major ions in groundwater were elucidated, and the quality and evolution trend of groundwater were evaluated. The main conclusions are as follows:

(1)

From 2018 to 2023, the average pH of groundwater in the NBSB was 7.59, indicating a generally weak alkalinity. The average TDS and TH were 665.63 mg/L and 527.80 mg/L, respectively, classifying the water predominantly as hard-fresh. The predominant anions and cations were HCO₃⁻ and Ca²⁺, with mean concentrations of 160.62 mg/L and 329.97 mg/L, respectively, which classifies the groundwater hydrochemical type in the NBSB as HCO₃-Ca.

(2)

The primary ions in the NBSB’s groundwater are derived from mineral dissolution, primarily from carbonate and silicate minerals. There is also dissolution of halite and gypsum. Additionally, anthropogenic activities have influenced the chemical composition of the groundwater, with NO₃⁻ primarily originating from agricultural practices.

(3)

The fluctuation in anion concentrations over time was more pronounced compared to cations. Overall, while the main ions at the BQ03 monitoring point exhibited a significant increasing trend after 2021, other monitoring points did not show notable increases or decreases over time.

(4)

The overall groundwater quality in the NBSB is satisfactory and suitable for use as a water supply source. However, with the exception of the BQ06 monitoring point, the EWQI values at all other points have shown an upward trend over time, particularly at BQ03, which warrants attention. It is recommended to enhance the management, utilization, and protection of groundwater resources in the NBSB to ensure their sustainable use.