Hydrochemical Characteristics and Origin Analysis of Groundwater in Nanling County, Anhui Province

Abstract

Nanling County, situated on the southern bank of the Yangtze River’s middle and lower reaches in China, and has not yet carried out hydrogeochemical geological surveys. This study is pivotal in ensuring the reliability of the drinking water supply, particularly during emergencies. Utilizing an array of analytical methods—statistical analysis, Shularev classification, Piper trilinear diagram, Gibbs diagram, ion ratio method, and mineral saturation index—this research elucidates the hydrogeochemical characteristics and principal water–salt interactions in Nanling’s shallow groundwater. Our findings, derived from the Shularev classification and Piper trilinear diagram, reveal that, in the southern mountainous and river valley plain regions, the primary hydrogeochemical type of groundwater is HCO₃-Ca. Conversely, in the northern area of Sanli Town and the adjoining plain, groundwater predominantly falls under the HCO₃-Na•Ca category, with some regions showing the characteristics of HCO₃•Cl-Ca, HCO₃•Cl-Na•Ca, and, occasionally, HCO₃•SO₄-Na•Ca. According to the Gibbs diagram analysis, the predominant source of groundwater in this region is attributed to water–rock dissolution processes occurring during groundwater runoff. The increase in Na⁺, Ca²⁺, Cl⁻, HCO₃⁻, and SO₄²⁻ concentrations in the water–rock interaction in the study area is mainly due to the dissolution of rock salt, gypsum, calcite, and dolomite, and the alternating cation adsorption occurs during the reaction. Finally, the mineral saturation index points to the ongoing dissolution of gypsum, calcite, and dolomite, until a state of precipitation–dissolution equilibrium is reached. This comprehensive study provides vital insights into the hydrogeochemical dynamics of Nanling County’s groundwater, contributing significantly to our understanding of regional water quality and its management.

1. Introduction

The chemical composition of groundwater is a crucial aspect of the hydrogeochemical cycle within the Earth’s system. As part of the natural water cycle, groundwater, stored within the lithosphere, engages in dynamic interactions with its surrounding environment. These interactions involve the exchange of substances and energy, which significantly influence the chemical composition of the groundwater [1,2,3]. The ion concentration in groundwater not only mirrors processes such as carbonate weathering, silicate weathering, and ion exchange during groundwater flow, but also acts as a vital tracer for deciphering groundwater circulation pathways. It serves as an effective tool for monitoring changes in groundwater quality and understanding their implications [4,5]. Furthermore, the hydrogeochemical characteristics of groundwater are shaped by a combination of factors including atmospheric inputs, rock weathering, human activities, and biogeochemical processes [6,7]. Therefore, conducting a thorough investigation into the chemical characteristics and origins of groundwater is essential. Such research yields valuable insights into groundwater quality assessment and facilitates informed decision-making in the scientific development and management of groundwater resources.

Traditionally, research on groundwater hydrochemistry has primarily focused on investigating its spatial and temporal variations. However, as hydrogeological theory has advanced, there has been a growing interest in studying the hydrogeochemical characteristics and origins of groundwater. Scholars both domestically and internationally have employed various methods, such as hydrochemical types, ion ratios, mineral saturation index (SI), and multivariate statistical analysis, to examine the hydrogeochemical features of regional groundwater [8,9]. The interactions between water and rocks, ion exchange processes, and precipitation–dissolution mechanisms significantly influence the hydrochemical properties of groundwater [10,11]. For instance, in the Baoding Plain, Xu Yiqing et al. utilized the Schuler classification method to categorize the hydrochemical types of groundwater. They also analyzed the patterns of hydrochemical evolution in the region and explored the factors contributing to the changes in hydrochemical characteristics, with a specific focus on groundwater exploitation. Additionally, Romshoo S.A et al. (2017) utilized hydrogeochemical analysis and hierarchical clustering analysis to identify the main sources of nine ions in 207 groundwater samples collected from the Siwalik Plain in India [12]. Similarly, Sun Houyun (2020) employed hydrochemical ion ratio coefficients and SI calculations to determine the driving factors behind the formation and evolution of groundwater hydrochemistry in the Bayingole Basin, located at the southern end of the Daxing’anling Mountains [13]. Assad Ullah (2023) explored the hydrochemical processes and ion sources of groundwater in Nizampur Basin, Pakistan, using a compositional data analytical perspective [14].

Nanling is located in the middle and lower sections of the Yangtze River, where groundwater resources are relatively scarce, and certain areas exhibit poor groundwater quality. However, hydrogeochemical processes in this region remain unanalyzed, and the hydrochemical characteristics of groundwater are unknown. Thus, this study aims to investigate these aspects. By utilizing the theoretical and actual values of ion ratios resulting from the dissolution of primary minerals within the regional geological background, we can identify the principal sources of substances in groundwater as well as other contributing sources [15]. Furthermore, through hydrochemical diagrams, we can elucidate the evolution of major ion concentrations and comprehend the processes pertaining to groundwater flow [16]. Obtaining a comprehensive understanding of the geochemical processes that determine the solute properties of groundwater can provide valuable insights for evaluating groundwater quality and promoting the scientific development as well as the effective utilization of this vital resource.

2. Materials and Methods

2.1. Study Area Overview

The general situation of the research area is shown in Figure 1. Nanling County is located in the south bank of the middle and lower reaches of the Yangtze River, in the southeast of Anhui Province, It is situated at geographic co-ordinates ranging from 117°57′30″ to 118°31′30″ east longitude and 30°35′20″ to 31°10′40″ north latitude, covering an approximate area of 880 km² (Figure 1). This research area involves eight townships, including Xuzhen, Jishan, Gejiang, Jiafa, and Gongshan in Nanling County, and Sanli, Hewan, and Yandun towns in Jing County. The region experiences a subtropical humid monsoon climate, influenced by the Pacific monsoon. The region receives abundant rainfall and possesses ample surface water resources, which serve as the primary water sources for industrial, agricultural, and domestic use. In contrast, groundwater resources are relatively scarce and remain largely untapped. Geographically, the county lies within a transitional zone, bridging the mountainous region of southern Anhui to the plains along the Yangtze River. The southwestern part of the county boasts higher elevations, while the northeastern part is comparatively flat. Based on geomorphic origin, morphology, and surface composition, in relation to agricultural production, the county can be categorized into three geomorphic combinations: low mountains and hills, hilly terrains, and plain areas. The southern portion comprises low mountains primarily composed of exposed limestone layers, where groundwater predominantly exists within fractures of the bedrock. The central region is characterized by hilly terrains covered with residual or colluvial clays from the Quaternary period. Groundwater in this area can be found in pore spaces within Quaternary sediments and concealed fractures of the bedrock. The eastern part is a plain area covered by Quaternary sediments, consisting mainly of quartz and clay minerals. This region possesses relatively abundant to rich pore water resources. Rice is the primary agricultural crop grown in the plain area, and it requires substantial application of chemical fertilizers, herbicides, and pesticides [17]. The study area falls under the Yangtze River water system, where groundwater flows from the highland area in the southwest through the undulating central hilly region, towards the lower eastern plain area. The overall direction of groundwater flow is southwest to northeast. However, the topography influences groundwater flow differently: in the mountainous area, it generally flows from southeast to northwest, while, in the plain area, it flows from south to north. Groundwater table depths generally remain under 4 m (Figure 2). Atmospheric precipitation and lateral runoff primarily recharge groundwater in the study area, while lateral outflow and evaporation serve as the main groundwater discharge processes.

2.2. Sampling and Processing of Samples

Between June and September 2015, a total of 129 shallow groundwater samples were collected in the study area, following the guidelines outlined in the “1:50,000 Hydrogeological Geological Survey Specification (DZT0282-2015)” and the “Specification for Geochemical Evaluation of Land Quality”. The collection process involved selecting sampling sites along the groundwater flow direction based on hydrochemical profiles. Prior to the sample collection, the wellbore was thoroughly cleaned and water was discharged until the volume exceeded three times the initial amount in the wellbore. On-site testing of parameters, including water temperature, conductivity, pH, and TDS, was conducted, and samples were collected when test parameters began to stabilize. To ensure cleanliness, the sampling bottles were washed at least three times. Each bottle was then filled with 90% water, and the collected water samples were stored in either glass or polyethylene plastic bottles. The samples were sent to the Henan Institute of Rocks and Minerals Testing for analysis 2–3 days after collection. Utilizing an inductively coupled plasma emission spectrometer, the concentrations of cations including Ca²⁺, Mg²⁺, K⁺, and Na⁺ were quantified. Meanwhile, an ion chromatograph was employed to determine the ion concentrations of Cl⁻ and SO₄²⁻ in the samples. The concentration of HCO₃⁻ ions was accurately determined through the EDTA titration technique. Moreover, the ion balance error for all samples was ascertained to be within the acceptable range of ±5%. The analytical methods employed met the accuracy and precision requirements outlined in the water sample testing quality management specification.

2.3. Research Methods

2.3.1. Shularev Classification

The Shularev classification method was developed by Soviet scholars to categorize the hydrochemical types of groundwater. This method is based on the presence of six main ions in natural groundwater (K⁺ combined with Na⁺) and the overall concentration of total dissolved solids. By analyzing the milliequivalent percentage of specific anions and cations in the water, the Shularev classification method directly creates a representation for hydrochemical types. This representation is formed by combining the combinations of anions and cations that exceed a threshold of 25% [18].

2.3.2. Hydrogeochemical Processes

Various hydrochemical analysis methods are commonly employed in hydrology research. These methods include hydrochemical statistical analysis, ion ratio coefficient methods, and graphical methods. One widely used technique is the application of Piper diagrams to identify hydrochemical patterns and classify different types of groundwater based on major ion data [19]. Linking lithological characteristics of aquifers to groundwater composition, Gibbs diagrams help discern the influence of factors such as atmospheric precipitation, rock weathering, evaporation, and concentration on hydrochemistry [20]. The chloride–alkali index, as indicated by Alam MF et al. [21], serves to illustrate the ion exchange process between groundwater and its geological environment. Additionally, the ion ratio coefficient diagram is instrumental in determining the sources of major ions in groundwater [22,23]. Qualitatively predicting the tendency of chemical components to precipitate or dissolve in groundwater, the SI compares the actual pH value of water with the theoretically calculated pH value under equilibrium conditions [24,25]. When SI < 0, minerals continue to dissolve in groundwater, while SI > 0 suggests that minerals have reached a state of equilibrium in groundwater.

To extract significant information, multivariate statistical analysis was conducted using SPSS 22.0. Aquachem 3.7 software was utilized to generate a Piper trilinear diagram, illustrating the correlation between anion and cation concentrations in groundwater and facilitating the identification of predominant hydrochemical types in the study area. Excel was employed to create a Gibbs diagram and an ion ratio coefficient diagram. These diagrams aided in analyzing the primary sources of hydrochemical ions in the groundwater of the study area. Furthermore, the process of mineral precipitation and dissolution was evaluated by calculating the mineral SI using Phreeqc 3.1.7.

3. Results and Discussion

3.1. Statistics of Regional Groundwater Hydrochemical Parameters

The pH levels in the groundwater of the study area ranged from 6.42 to 8.49, with a mean value of 7.46. TDS ranged from 126.20 to 843.60, and TH ranged from 43.02 to 527.4 (

Table 1. Statistical characteristics of groundwater hydrochemical parameters.

Statistic	pH	TH	TDS	Ca2+/(mg/L)	Mg2+/(mg/L)	Na+/(mg/L)	K+(mg/L)	Cl−(mg/L)	SO42−(mg/L)	HCO3−(mg/L)
Minimum value	6.42	43.03	126.20	10.06	2.21	2.57	0.34	2.46	0.18	60.41
Maximum value	8.49	527.4	843.60	136.70	45.16	100.20	101.6	246.6	139.6	462.60
Mean value	7.46	213.52	334.79	64.91	12.40	26.46	8.75	31.17	25.44	235.16
Standard deviation	0.37	78.14	136.14	20.68	8.59	19.81	16.98	41.40	25.42	66.82
Variable coefficient	0.05	0.37	0.41	0.32	0.69	0.75	1.94	1.33	1.00	0.28

). In accordance with the “Groundwater Quality Standard” (GB/T 14848-2017) [26], the standard values for TH and TDS in groundwater are 450 and 1000, respectively. The TH levels in other sample points exceeded the standard value, while the TDS index remained below the standard. Regarding cation concentrations, the order in groundwater within the study area was Ca²⁺ > Na⁺ > Mg²⁺ > K⁺, with Ca²⁺ being the predominant cation. In terms of anion concentrations, the order was HCO₃⁻ > Cl⁻ > SO₄²⁻, with HCO₃⁻ being the primary anion. The coefficient of variation for cations in groundwater ranged from 0.28 to 1.94, indicating significant spatial variability. The coefficient of variation for SO₄²⁻, Cl⁻, and K⁺ ions exceeded 1, suggesting pronounced spatial differences and localized enrichment areas. The elevated concentrations of SO₄²⁻ and Cl⁻ may be attributed to different regional geological structures and human activities.

3.2. Chemical Types of Groundwater

The composition of main ions in groundwater provides insight into the hydrogeological and hydrochemical characteristics of the groundwater system. By categorizing the groundwater into hydrochemical types, the fundamental characteristics of hydrogeochemistry in the study area can be understood. Using the Shularev classification based on the content of seven main ions in groundwater, the groundwater in the study area was classified and delineated [27]. Figure 2 highlights the dominance of HCO₃⁻ as the primary anion and Ca²⁺ as the leading cation in the groundwater composition within the study area. The primary hydrochemical type in the study area was the HCO₃-Ca type, which accounted for 65% of the total area, followed by the HCO₃-Ca•Na type, which represented 25% of the area. While the belt section from Gongshan Town to Sanli Town displayed the HCO₃-Ca•Na-type water, the majority of groundwater types in the low mountain and hilly regions were of the HCO₃-Ca type. In the plain area, approximately half of the HCO₃-Ca water was distributed in the southeast, while the other half of HCO₃-Ca•Na water was predominantly found in the northwest. Isolated small areas containing HCO₃•Cl-Ca-type water, HCO₃•Cl-Ca•Na-type water, and HCO₃•SO₄-Ca•Na-type water were embedded within the distribution. The hydrochemistry types indicated that the groundwater chemistry in the study area was influenced by the precipitation of minerals that include carbonates and sodium, namely, rock salt and albite. As summarized in

Table 2. Statistical analyses of chemical parameters (Unit: mg/L [sample number]).

		TH	TDS	K+	Na+	Ca2+	Mg2+	Cl−	HCO3−	SO42−
HCO3-Ca-type water (72)	Min	122.60	173.90	0.34	2.57	42.97	2.21	2.46	138.4	0.18
	Max	340.30	487.50	19.46	41.74	98.11	27.31	58.02	367.1	73.82
	Average	203.25	281.64	4.09	15.23	65.41	9.69	16.63	227.51	18.49
HCO3-Na•Ca-type water (37)	Min	43.03	126.20	0.39	15.62	10.06	4.29	2.46	90.61	0.95
	Max	437.90	697.80	101.60	74.05	135.4	24.62	72.23	462.6	121.1
	Average	207.16	381.90	17.96	38.15	61.83	12.81	29.83	263.33	28.63
HCO3•Cl-Ca-type water (3)	Min	43.03	126.20	0.39	15.14	10.06	4.29	2.46	90.61	0.18
	Max	437.90	697.80	101.60	74.05	135.4	27.31	72.23	462.6	121.1
	Average	203.07	359.98	16.93	33.83	59.64	13.14	26.46	256.24	27.45
HCO3•Cl-Na•Ca-type water (12)	Min	198.70	396.20	0.39	35.23	50.31	17.7	44.66	211.8	3.93
	Max	527.40	843.60	19.94	100.2	136.7	45.16	246.6	340	121.9
	Average	344.15	577.50	4.29	67.40	86.77	30.97	136.478	271.13	48
HCO3•SO4-Na•Ca-type water (5)	Min	78.56	138.80	4.20	9.10	26.01	2.93	9.85	60.41	28.74
	Max	412.30	631.30	43.07	23.22	126.3	23.54	15.12	350.4	139.6
	Average	182.49	313.52	22.89	16.32	53.8	9.39	13.076	146.11	59.3

and Figure 2, as groundwater traversed from upstream to downstream, the concentrations of Ca²⁺ and HCO₃⁻ tended to decrease, while the concentrations of Na⁺ + K⁺, SO₄²⁻, and Cl⁻ tended to increase. Consequently, in the upper low mountain region of the study area, the groundwater primarily comprised HCO₃⁻ and Ca²⁺, whereas, in the lower plain area, mineral dissolution along the groundwater path led to elevated concentrations of Na⁺, SO₄²⁻, and Cl⁻.

The Piper diagram is a widely used method for identifying hydrochemical patterns based on main ion data. In this study, a Piper trigram was constructed to gain a better understanding of the hydrogeochemical processes, benefitting from its immunity to human influences. Figure 3 shows the Piper trigram highlighting the main ion components in the study area. Regarding cations, the majority of the samples were situated within the A and B regions of the delta-type region towards the lower left. This suggested a mixture of calcium-type, sodium-type, and mixed-type water samples. As for anions, most of the samples cluster in the E region of the delta-type area towards the lower left (Figure 3), indicating a prevalence of bicarbonate-type water. However, a few samples were positioned in the E zone of the upper left and the E zone of the delta-type region towards the lower left, suggesting a mixture of bicarbonate-type with either sulfate-type or chloride-type water. In most samples found in the study area, alkaline earth metals exceeded alkali metals, and alkaline soil and weak acid contents surpassed alkaline soil and strong acid contents in groundwater samples. The primary hydrochemical types observed in the study area were HCO₃-Ca and HCO₃-Ca•Na.

3.3. Correlation between Groundwater Indicators

The identification of ion sources and their consistency can be achieved by examining the correlation coefficients among groundwater components. This, in essence, provides a comprehensive grasp of the key hydrogeochemical processes that govern the chemical attributes of water [28].

Table 3. Correlation coefficient matrix of hydrochemical parameters in the study area (unit: mg/L).

	K+	Na+	Ca2+	Mg2+	Cl−	HCO3−	SO42−	TDS
K+	1	0.034	0.174 *	−0.019	0.046	0.079	0.452 **	0.397 **
Na+		1	0.340 **	0.813 **	0.767 **	0.480 **	0.477 **	0.768 **
Ca2+			1	0.621 **	0.526 **	0.635 **	0.620 **	0.782 **
Mg2+				1	0.830 **	0.598 **	0.508 **	0.846 **
Cl−					1	0.259 **	0.432 **	0.775 **
HCO3−						1	0.258 **	0.595 **
SO42−							1	0.758 **
TDS								1

Note: ** indicates significance at the 0.01 level (two-tailed); * indicates significance at the 0.05 level (two-tailed).

displays the Pearson correlation coefficients for various water chemical parameters within the study area. These coefficients revealed a strong positive correlation between TDS and Na⁺, Ca²⁺, Mg²⁺, Cl⁻, HCO₃⁻, and SO₄²⁻ (all exceeding 0.5), indicating the substantial contribution of these ions to TDS. Moreover, Na⁺, Mg²⁺, and Cl⁻ exhibited a significant correlation, suggesting a common source. Similarly, Ca²⁺, Mg²⁺, and HCO₃⁻, as well as Ca²⁺ and SO₄²⁻, were positively correlated, indicating a potential shared substance source. Conversely, the correlation between K⁺ and other ions was weak, implying that K⁺ may originate from different sources.

3.4. Analysis of Groundwater Hydrochemical Origin

3.4.1. Gibbs Analysis

During the weathering of rocks and groundwater flow, chemical elements undergo leaching and become dissolved within the aquifer, contributing to the groundwater’s chemical composition [29]. The interaction between water and rock, as well as the mobility of elements, are the primary factors that control the hydrochemical processes of groundwater. The Gibbs diagram serves as a prevalent tool for elucidating the interplay between the lithology of aquifers and the compositional makeup of groundwater. It has been widely utilized to characterize the correlation between water chemistry and aquifer lithology [20]. Within the Gibbs diagram, the ratios of Na⁺/(Na⁺ + Ca²⁺) and Cl⁻/(Cl⁻ + HCO₃⁻) to TDS values correspond to three distinct regions: precipitation-dominated, evaporation-dominated, and rock-dominated (Figure 4). Based on the figure, it was observed that TDS values were primarily concentrated within the range of 100–1000 mg/L, indicating moderate salinity. The Na⁺/(Na⁺ + Ca²⁺) ratio ranged between 0.03–0.67, while the Cl⁻/(Cl⁻+ HCO₃⁻) ratio ranged between 0.008–0.578. A majority of the samples fell within the region of rock weathering, suggesting that the study area was predominantly shaped by the weathering and dissolution of rocks. The groundwater sample points of HCO₃•Cl-Ca•Na were close to the evaporation crystallization end, which suggested that this type of groundwater sample was enhanced by evaporation.

3.4.2. Analysis of Rock Weathering

Based on a preliminary study using the Gibbs diagram method, it was determined that rock weathering primarily influences the chemical composition of groundwater in Nanling. To further investigate the impact of rock weathering on groundwater evolution, the end-member method was employed to characterize the influence of different rock types based on the ratios of Mg²⁺/Na⁺ to Ca²⁺/Na⁺ and HCO₃⁻/Na⁺ to Ca²⁺/Na⁺. The common types of rock weathering include carbonate, silicate, and evaporative [30]. As depicted in Figure 5, all groundwater samples in the study area were located between the silicate and carbonate rock weathering regions, suggesting that the water chemistry formation of groundwater in the study area was jointly affected by the weathering of these two rock types. The distribution of HCO₃-Ca-type groundwater was most prominent in the upper reaches of the study area, indicating a closer proximity to carbonate rock weathering. Conversely, the distribution of HCO₃-Ca•Na-type groundwater was more pronounced in the lower plain area, indicating a stronger influence of carbonate rock weathering dissolution. This suggested that the dominant rock weathering type controlling groundwater varied along the downstream flow, with a weakening of carbonate weathering dissolution and an enhancement of silicate weathering dissolution and ion exchange. This led to an increase in Na⁺ concentration and a decrease in Ca²⁺ and Mg²⁺ concentrations, resulting in the evolution of HCO₃-Ca•Na-type groundwater.

3.4.3. Analysis of Ion Proportional Coefficient

Figure 6 shows the scale coefficient diagram of groundwater examples in the study area. An analysis of ion proportionality coefficients offers profound insights into ion origins and aids in delineating the factors that shape the chemical profile of groundwater. The ratio of the Ca²⁺ + Mg²⁺ to HCO₃⁻ milligram-equivalent concentration in water served as an indicator of the hydrolysis characteristics of carbonate karst in groundwater. A ratio of 1 indicated that Ca²⁺ and Mg²⁺ primarily originated from the dissolution of calcite, dolomite, and gypsum. Ratios greater than 1 suggested a predominant contribution from carbonate dissolution. Ratios less than 1 indicated that silicate rock weathering and evaporative rock weathering played a more significant role [31,32]. Furthermore, the Ca²⁺ + Mg²⁺ to HCO₃⁻ + SO₄²⁻ ratio enabled the analysis of the sources of Ca²⁺, Mg²⁺, HCO₃⁻, and SO₄²⁻ in groundwater. When samples fell above the theoretical line (1:1) for Ca²⁺ + Mg²⁺ and HCO₃⁻ + SO₄²⁻, it indicated that the chemical composition of groundwater was mainly influenced by carbonic acid weathering. Conversely, when samples fell below the theoretical line (1:1), it was indicated that silicate weathering and ion exchange mechanisms were the dominant factors [33].

The ratio of HCO₃-Ca groundwater sample points (Ca²⁺ + Mg²⁺) to HCO₃⁻ exceeded the 1:1 line, indicating that carbonate dissolution predominantly influenced the upper reaches of the study area. The ratio of (Ca²⁺ + Mg²⁺) to (HCO₃⁻ + SO₄²⁻) remained close to 1, as observed in the comparative analysis. Interestingly, this region displayed a significantly smaller ratio of (Ca²⁺ + Mg²⁺) to (HCO₃⁻+ SO₄²⁻) compared to the ratio of (Ca²⁺ + Mg²⁺) to HCO₃⁻. This further confirmed the presence of weathering dissolution not only of calcite and dolomite, but also gypsum in the upper lower mountains (Figure 6a,b). In the downstream plain area, the ratio of Na·Ca-HCO₃ groundwater sample points (Ca²⁺ + Mg²⁺) to HCO₃⁻ aligned with the equilibrium line of 1:1 and fell between the two equilibrium lines. Moreover, samples of (Ca²⁺ + Mg²⁺) and (HCO₃⁻+ SO₄²⁻) were located below the 1:1 ratio. These findings pointed towards the conclusion that the silicate weathering and ion exchange mechanisms had an impact on the hydrochemical makeup of the region. As groundwater traveled downstream, the dissolution of carbonate weathering weakened while the dissolution of silicate weathering and ion exchange processes intensify. Consequently, the concentration of Na⁺ increased (Figure 6c), leading to calcite precipitation and a reduction in Ca²⁺ and Mg²⁺ concentrations. This evolution resulted in the transformation of HCO₃-Ca-type groundwater in the upper low mountain area into HCO₃-Na•Ca-type groundwater in the lower plain area.

CaCO₃ + H₂O + CO₂ → Ca²⁺ + 2HCO₃⁻

CaMg(CO₃)₂ + 2H₂O + 2CO₂ → Ca²⁺ + Mg²⁺ + 4HCO₃⁻

The scatter diagram depicting the relationship between Na⁺ and HCO₃ revealed that the ratio of Na to HCO₃⁻ ions remained constant at a 1:1 ratio across all samples. However, the ratio differed between the lower mountain area of the upper reaches of the study area and the lower plain area. In the lower mountain area, the ratio was higher compared to the lower plain area. This observation suggested that the plain area inherited the chemical components of the incoming water while also experiencing a recharge from the upper reaches. It is important to note that other hydrogeochemical processes, such as ion exchange, also occurred. Based on the change patterns in the concentrations of Ca²⁺, Mg²⁺, Na⁺, and HCO₃⁻ in the upper and lower reaches of the study area, it was observed that Ca²⁺ and Mg²⁺ concentrations decreased downstream, while Na⁺ and HCO₃⁻ concentrations increased (Figure 6a,b). This finding further supported the presence of weathering dissolution and ion exchange of silicate minerals, such as 2NaAlSi₃O₈ and calcium feldspar (CaO•Al₂O₃•2Si₂O₂), in the plain area. When sodium silicate minerals underwent weathering, Ca²⁺, Mg²⁺, and HCO₃⁻ were introduced simultaneously with Na⁺ production, consequently increasing the concentration of HCO₃⁻. Additionally, precipitation dissolution equilibrium influences the process, causing the precipitation of excess (Ca²⁺ + Mg²⁺) and reducing the ratio of (Ca²⁺ + Mg²⁺) to HCO₃⁻.

2NaAlSi₃O₈ + 3H₂O + 2CO₂→H₄Al₂Si₂O₉ + 2HCO₃⁻ + 2Na⁺ + 4SiO₂

Ca²⁺ + HCO₃⁻ + CaSO₄·2H₂O→Ca²⁺ + SO₄²⁻ + CaCO₃↓ + H⁺ + 2H₂O

The dissolution of silicate and evaporite minerals serves as the principal sources of Na⁺ ions in water. The milliequivalent concentration ratio of Na⁺ to Cl⁻ can serve as an indicator of the main source of Na⁺ ions in water. A Na⁺/Cl⁻ ratio of 1 in groundwater suggests that the main source of Na⁺ ions is the dissolution of rock salt. Conversely, a Na⁺/Cl⁻ ratio greater than 1 suggests the presence of additional sources, such as the dissolution of silicate minerals or cation exchange in groundwater [34]. Within the study area, a majority of sampling points exceed the 1:1 ratio, suggesting that the source of Na⁺ in groundwater is not solely attributed to rock salt dissolution, but also encompasses the dissolution of silicate minerals or ion exchange mechanisms. In the upstream low-mountain area, where HCO₃-Ca-type groundwater samples were collected, the scatter points for Na⁺ and Cl⁻ were located close to the line representing a 1:1 ratio, with minimal variations in their concentrations. This finding suggested that the main source of Na⁺ ions was the dissolution of rock salt. In the plain area, the scatter points for Na⁺ and Cl⁻ in HCO₃-Ca•Na-type groundwater samples exceed the 1:1 line, while the scatter points for HCO₃•Cl-Ca and HCO₃•Cl-Na•Ca-type water samples were distributed on both sides of the line (Figure 6d). The increased Na⁺/Cl⁻ ratios suggest that the source of sodium ions is not limited to rock salt dissolution, but also includes silicate mineral weathering in sandstone as well as ion exchange processes between Na⁺ and Ca²⁺.

3.4.4. Cation Exchange

The process of cation exchange holds a pivotal position in the hydrochemical evolution of groundwater. The Chloride–Alkalinity Index (CAI) is a valuable tool for explaining the ion exchange chemical process between groundwater and its geological environment [35]. A positive CAI value signifies the occurrence of ion exchange, where alkali metal ions (Na⁺ and K⁺) from rock or clay minerals are substituted by alkaline earth metals (Ca²⁺ and Mg²⁺) in groundwater, thereby elevating the concentrations of Na⁺ and K⁺. On the contrary, a negative CAI value suggests an exchange of alkaline earth metals (Ca²⁺ and Mg²⁺) from rock minerals with the presence of alkali metal ions (Na⁺ and K⁺) in groundwater, causing the influx of Ca²⁺ and Mg²⁺ ions into the groundwater system and a subsequent rise in their concentrations. An examination of the distribution characteristics of CAI values in the study area revealed that most sample points exhibited negative CAI values (Figure 7), while only a few HCO₃•Cl-Ca•Na- and HCO₃•Cl-Ca-type groundwater samples had positive CAI values. This suggested that, in most areas of the study site, Ca²⁺ and Mg²⁺ in the aquifer medium were exchanged with Na+ and K+ in groundwater during runoff. The (Ca²⁺ + Mg²⁺-HCO₃⁻-SO₄²⁻) to (Na⁺ + K⁺-Cl⁻) ratio can also serve as an indicator of cation exchange in groundwater. A ratio below 0 implies the occurrence of cation exchange, while a ratio close to 1 indicates a strong cation exchange. In the study area, the slope of the trend line for the sample points was close to −1, indicating a significant cation exchange in the area.

$$\mathrm{C}\mathrm{A}\mathrm{I}-1=\frac{{\mathrm{C}\mathrm{l}}^{-}-({\mathrm{N}\mathrm{a}}^{+}+{\mathrm{K}}^{+})}{{\mathrm{C}\mathrm{l}}^{-}}$$

$$\mathrm{C}\mathrm{A}\mathrm{I}-2=\frac{{\mathrm{C}\mathrm{l}}^{-}-({\mathrm{N}\mathrm{a}}^{+}+{\mathrm{K}}^{+})}{{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{S}\mathrm{O}}_4^{2-}+{\mathrm{C}\mathrm{O}}_3^{2-}+{\mathrm{N}\mathrm{O}}_3^{-}}$$

3.4.5. Precipitation–Dissolution Equilibrium

The SI holds significant importance in hydrochemical studies as it provides insight into the dissolution saturation state of a component in groundwater. When the SI of a mineral is below 0, it suggests that the mineral can further dissolve in groundwater. Conversely, an SI above 0 indicates that the mineral has undergone precipitation in groundwater. An SI value of 0 signifies that the mineral has achieved equilibrium in groundwater.

Regarding the groundwater samples collected from the designated study region, the SI variations of calcite ranged from −1.47 to 1.10, dolomite ranged from −3.43 to 2.18, gypsum ranged from −3.91 to −1.32, and halite ranged from −9.74 to −6.27 (Figure 8). The saturation indices of gypsum and halite in the groundwater samples were both below 0, indicating that they were not saturated and are still undergoing dissolution. On the other hand, the saturation indices of calcite and dolomite were close to 0, suggesting a state of precipitation–dissolution equilibrium. These findings provide further confirmation that the main ions in the aquifer stem from the dissolution of calcite and dolomite in carbonate rocks, as well as gypsum and halite in evaporite rocks.

4. Conclusions

The cation concentration relationship in the study area groundwater is as follows: Ca²⁺ > Na⁺ > Mg²⁺ > K⁺, with Ca²⁺ as the dominant cation. The anion concentration relationship is as follows: HCO₃⁻ > Cl⁻ > SO₄²⁻, with HCO₃⁻ as the dominant anion. Ca²⁺ and Na⁺ are the primary cations, followed by Mg²⁺, while HCO₃⁻ and Cl⁻ are the main anions, followed by SO₄²⁻. Evaluating the Shukarev classification and Piper trigram, the water chemical types in the study area can be divided into five types, with the most significant types being HCO₃-Ca and HCO₃-Ca•Na. The presence of HCO₃•Cl-Ca water, HCO₃•Cl-Ca•Na water, and HCO₃•SO₄-Ca•Na water is sporadic and limited. The main ions in the study area show the highest cation concentrations in the A and B regions of the left lower delta-type region, indicating a mixture of calcium-, sodium-, and mixed-type waters within the samples. The anions predominantly lie in the E region of the lower delta-type region, indicating a predominance of bicarbonate-type water. The Gibbs map reveals that the groundwater in the study area is primarily influenced by rock weathering and ion exchange. The end element method indicates that the distribution of HCO₃-Ca groundwater in the upper reaches is primarily influenced by the dissolution of carbonate rocks, while the distribution of HCO₃-Ca•Na groundwater in the lower plains is mainly affected by the dissolution of silicate rocks. Downstream flow weakens the dissolution of carbonate weathering while enhancing the dissolution of silicate weathering. An ion ratio analysis indicates that Ca²⁺ and Mg²⁺ in groundwater from the upstream low mountain area primarily originate from the dissolution of calcite, dolomite, and gypsum, whereas Na⁺ ions mainly result from the dissolution of silicate minerals and evaporite minerals. The results of the chlor–alkali index demonstrate a strong cation alternation in the study area groundwater. The SI indicates that gypsum and rock salt continue to dissolve, while calcite and dolomite reach precipitation–dissolution equilibrium.