4.1. Ion Concentration Ratio (ICR) Analysis
In order to assess the ion sources more effectively, this study conducted a concentration ratio analysis for 57 water samples, resulting in a groundwater ion concentration ratio diagram for the study area, depicted in
Figure 3.
In the karst aquifer of the study area, the minerals that primarily influence the groundwater chemical composition are calcite, dolomite, gypsum, and anhydrite. The ratio of milliequivalent concentrations of
to
can reflect the degree of carbonate dissolution in the water. As depicted in
Figure 3a, all data points are situated on the side of the 1:1 line, indicating a high proportion of
in the groundwater. This phenomenon suggests that the primary process contributing to the hydrochemical composition of groundwater is the dissolution of carbonate minerals (Hong et al., 2016) [
20], aligning with the fact that the Xiejiadian Water Source Protection Area is predominantly characterized by the wide distribution of carbonate rocks. Furthermore, it is inferred that gypsum, anhydrite, and other sulfate rocks and rock salt have a limited distribution, occurring only in localized regions where they are in contact with carbonate rock formations. Combining the hydrogeological and stratigraphic data from
Section 2.3, it can be observed that blocky gypsum and anhydrite are only present in the aquifer lithological groups of the Sanshanzi Formation (∈
4O
1s), Chaomidian Formation (∈
4O
1), and Beianzhuang Formation (O
2b). These gypsum occurrences are sporadically distributed in the central and low mountainous areas east of the Zihe River, in the western graben zone, and near the main river channel.
The carbonate and sulfate rock dissolution chemical reactions (Equations (1) and (2)) are as follows:
According to Equations (1) and (2), it is understood that if the milliequivalent concentration ratio of
to
is close to 1, then
and
should come from the dissolution of carbonate and sulfate minerals such as calcite, dolomite, and gypsum. If the ratio is greater than 1, it indicates the dissolution of silicate minerals, while a ratio less than 1 suggests that filtration processes are accompanied by ion exchange adsorption processes (Abdelilader R et al., 2012) [
21]. As depicted in
Figure 3b, the concentration ratio of the samples approximately follows a linear distribution, deviating from the 1:1 line. This implies some dissolution of silicate minerals, but the majority of the analysis results are concentrated on both sides of the 1:1 equilibrium line. This indicates that carbonate rocks and sulfate rocks are the primary reactants in groundwater. Furthermore, due to minimal sulfur dioxide in rainwater, the rainwater has little contribution to the
in groundwater. Hence, this study hypothesizes that the current composition of groundwater in the study area is related to atmospheric precipitation, but it is not a controlling factor. Various water–rock interactions occur during groundwater flow (Equations (1) and (2)), and our results indicate that these interactions are the controlling factors influencing the regional groundwater chemical composition.
To investigate the dissolution intensity of dolomite and calcite in the study area, an ion concentration ratio chart of
to
(
Figure 3c) was utilized. The 1:2 and 1:4 ratio lines represent the dissolution equilibria of calcite and dolomite, respectively. Most of the samples are distributed between the 1:4 and 1:2 equilibrium lines, with a tendency towards being in the vicinity of the 1:2 equilibrium line. This suggests that the
and
in karst water primarily originate from the co-dissolution of calcite and dolomite (Wang et al., 2006) [
22].
Ion concentration ratio charts of
to
are commonly used to identify the contribution of gypsum dissolution to the content of calcium ions
in groundwater (Huang, et al., 2010) [
23]. From
Figure 3d, the coefficient of 0.33 was calculated based on the molar ratio of
to
using the reaction equation (Equation (1)). Moreover,
is defined as the
content from non-carbonate rock sources. From
Figure 3d, it can be observed that the collected water samples are mainly distributed above the 1:1 complete gypsum dissolution curve. Some water samples are close to the equilibrium dissolution curve, indicating that the dissolution of gypsum contributes to a certain amount of the
content in the groundwater. However, besides the dissolution of gypsum, there are other calcium-containing substances that also contribute to the
concentration in groundwater.
The cation exchange process Is a common chemical reaction in groundwater systems, which occurs through the electrostatic attraction between cations and the breaking and formation of other chemical bonds (Li, et al., 2019) [
24]. In order to validate the hypothesis that cation exchange is one of the geochemical processes controlling the chemical properties of the groundwater, the relationship between
and
and between
and
was examined. If cation exchange significantly influences the chemical composition of groundwater, the slope of the equation would be −1 (Fisher et al., 1997) [
25]. Observing the ratio of
milliequivalent concentrations in the study area (
Figure 3f), a linear regression equation (Equation (3)) was fitted based on the distribution of all water samples in the figure; the slope of the equation is −1.518. This deviation from the theoretical value of −1 indicates that ion exchange processes are not pronounced within the study area, signifying that they are a weak controlling factor for the groundwater’s chemical characteristics.
The Schoeller Index proposed by Schoeller (Schoeller 1965) [
26] offers a viable indicator of ion exchange reactions between groundwater and the surrounding environment (CAI–I and CAI–II). The calculation formulas for these indices are provided in Equations (4) and (5), with ion measurements expressed in meq/L. A negative CAI value signifies a cation exchange process (as per Equation (6) in the forward direction), characterized by Na
+ desorption and the concurrent absorption of Ca
2+ or Mg
2+. Conversely, a positive CAI value indicates a reverse ion exchange process (as per Equation (6) in the reverse direction) (Wang et al., 2017) [
27].
Figure 3e provides information on the ratio between CAI–I and CAI–II.
Evidently, the water samples from the various aquifers within the study area predominantly fall in the upper-right quadrant of the CAI-I and CAI-II distribution charts, with the majority exhibiting positive values. This indicates that ion exchange reactions in the study area are occurring in the reverse direction, as described by Equation (6), following the fundamental principles of chemical equilibria. According to these principles, a substantial concentration of
ions is required in the groundwater system of the study area to drive the reverse reaction described by Equation (6). Based on the regional groundwater chemical characteristics, the mean
concentration is 12.97 mg/L, while the average
concentration is 117.85 mg/L. This does not meet the conditions for the equation evolution. Furthermore, considering the relative strength of the ion exchange capacity and geological conditions, it further confirms that the cation exchange process in the study area is relatively weak, and its impact on the chemical composition of groundwater is minor.
4.2. Correlation Analysis
Groundwater from the same source and similar flow paths often exhibits similar chemical characteristics. Based on this, a correlation analysis of the chemical components of groundwater can be conducted to reveal the consistency and differences in the sources of major components. This study utilized SPSS 24 version software to calculate the Pearson correlation coefficients among the ten chemical indicators of groundwater in the study area. Two-tailed tests were performed at the 0.05 and 0.01 levels to assess the significance of the correlation coefficients. The resulting correlation matrix is presented in
Table 3. Ions with correlation coefficients (R) greater than 0.7 exhibit a strong correlation, while ions with correlation coefficients between 0.4 and 0.7 show a moderate correlation between them.
From the ratio analysis in
Section 4.1, it is evident that TDS values are primarily influenced by water–rock interactions. The TDS value exhibits strong correlations with
(0.68),
(0.50),
(0.58), and
(0.48), indicating that the dissolution of these ions contributes to the increase in groundwater TDSs. Notably, there is a strong correlation between
and
with
(both are 0.65), suggesting that calcium and magnesium ions mainly originate from the widespread dissolution of carbonate rocks (calcite (
), dolomite
) in the study area. This further validates that HCO
3− is derived from the common dissolution of calcite and dolomite, aligning with the ratio analysis results. However, the correlation coefficient between Ca
2+ and Mg
2+ is 0.23, indicating some differences in their sources.
There is a significant correlation between
(0.68), suggesting that the
ion content is closely associated with the dissolution of gypsum (
). Besides the strong correlations between
and
(0.65) and
(0.68), there is also some correlation between
and
(0.67) and
(0.67), indicating that, apart from water–rock interactions, some of the
might be related to the infiltration and
dissolution of contaminants containing
and
(Zhang et al., 2014) [
28]. This aligns with the conclusion drawn in
Section 4.1 for the ICR analysis that “besides the dissolution of gypsum, other calcium-containing substances also contribute to the Ca
2+ concentration in groundwater”.
in addition to its strong correlation with
(0.65), also has weak correlations with other conventional ions, indicating that its primary source is solely related to the dissolution of dolomite. COD (Chemical Oxygen Demand) is an indicator of organic pollutant levels in water, with the main sources of organic pollution in groundwater being industrial and agricultural pollution, as well as domestic pollution. The correlations between COD and
(0.34),
(0.14),
(0.15), and
(0.10) are not high, suggesting that the sources of these three ions have little relationship with the use of pesticides and fertilizers, domestic and industrial wastewater discharge, and solid waste and industrial waste.