4.2.1 Gibbs
Gibbs diagram is a semi-logarithmic coordinate diagram used to determine the main influencing factors of groundwater chemistry type, and the analysis of the ratio is used to determine which of atmospheric precipitation, evaporation concentration, and water-rock action is responsible for the dominant component in the water body(Gibbs 1970). Groundwater sample Na+/(Na+ + Ca2+), TDS, and Cl−/(Cl− + HCO3−) extended from 0.2 to 1.0, 500 to 900 mg/L, and 0.1 to 0.7, respectively. Most water sample sites were positioned in the central part of the Gibbs plot, suggesting rock weathering to be the dominant factor influencing groundwater chemistry Most of the water sample sites are located in the central part of the Gibbs plot, indicating that the groundwater chemical composition in this study area is mainly influenced by rock weathering. Since Cl− accounts for a relatively small proportion of the anions, the HCO3− concentration is approximately equal to 3 times the Cl− concentration, resulting in small Cl−/(Cl−+HCO3−) values at most of the water sample points, leading to a small shift to the left of the water sample drop. A similar rise in Na+/(Na++Ca2+) suggested that the exchange of cations has an effect on groundwater water chemistry.
4.2.2 Ion-ratio analysis
The Gibbs diagram indicated that weathering of rocks and water-rock interactions are the main factors influencing the chemistry of groundwater in the study area, with a delayed alternating effect of groundwater flow, and no obvious effect of evaporative concentration in the study area. The water-rock action includes dissolution/sedimentation, cation adsorption and exchange, and evaporation and concentration. Hence, the current study further investigated dissolution/sedimentation and cation adsorption and exchange. There can be significant differences in the contents and ratios of water chemistry indicators among different aquifers. Therefore, the ion area ratio diagram can be applied to identify the main hydrogeochemical processes influencing groundwater (Q. Yang et al. 2016; Liu et al. 2018).
Under natural conditions, the rock salt dissolution releases equal quantities of Na+ and Cl−. The Na+: Cl− ratio close to 1 indicated that dissolution filtration of rock salt minerals plays a dominant part in chemistry of groundwater. Dissolved Cl− has a conservative nature, whereas the contents of dissolved Na+ change due to chemical effects, such as adsorption and precipitation. This leads to variation in Na+: Cl− ratio. As demonstrated in Fig. 5a, most groundwater samples (76%) plotted under the y = x line. This result indicated that rock salt dissolution was not the dominant process influencing groundwater. This result also suggested that Na+ contained in groundwater did not solely originate from rock salt dissolution, but likely also from silicate weathering and cation exchange. The Ca2+: SO42− weight ratio should equal 1 when the dominant process is the dissolution of gypsum. As demonstrated in Fig. 5b, most (76%) groundwater samples deviated from the y = x line, indicating the concentration of Ca2+ is too high, which is not conducive to the dissolution of gypsum.
Also, the correlation between groundwater HCO3− and Ca2+ concentrations was investigated. The HCO3−: Ca2+ ratio approximating 1 indicated calcite dissolution, whereas 2 indicated dissolution of dolomite (X. Li et al. 2016). As demonstrated in Fig. 5c, 40% of the water sample points plotted above y = 2x, suggesting the presence of dolomite dissolution, whereas 20% fell between y = x and y = 2x, implying the contribution of carbonate to groundwater hydrochemistry evolution. Water sample points plotting below the y = x line (40%) exhibited high Ca2+ concentrations, implying that the weathering of dolomite and calcite were not dominant processes and the presence of other sources of Ca2+ or cation exchange.
The bivariate plots of (HCO3−+SO42−) and (Ca2++Mg2+) can determine the main sources of Ca2+ and Mg2+(Marghade, Malpe, and Subba Rao 2015). Most of the groundwater sample points are plotted above the y = x line, and the ratio exceeding 1, indicating reduced contents of Ca2+ and Mg2+.Most groundwater sample points plotted above the y = x line and had a ratio exceeding 1, indicating reduced contents of Ca2+ and Mg2+. Therefore, Ca2+ and Mg2+ predominantly originated from the dissolution of silicate and sulfate or the presence of cation exchange (Fig. 5e).
The plot of (Na++K+-Cl−) versus (Ca2++Mg2+-SO42−-HCO3−) can be used to determine the cation exchange process. Variations in these two indices indicate the acquisition or migration of Ca2+, Na+, and Mg2+ from sources other than salt rock, hydrochloride rock, and gypsum dissolution. The two indices will show a linear relationship if cation exchange dominates, with a slope of − 1 (Wu and Qian 2017). Most of the groundwater variety points in the study area mainly plotted near the y=-x line, indicating that cation exchange is an important mechanism for the source of groundwater chemical components(Fig. 5f).
In addition, The study of the relationship between HCO3−/Na+ and Mg2+/Na+, Ca2+/Na+ can qualitatively reflect the influence of dissolution of different rock types on groundwater chemistry (Gaillardet,J. et al. 1999). As shown in Figs. 5g and 5h, there were high contents of silicate in groundwater samples, indicating silicate weathering to be an important hydrochemical process regulating groundwater chemistry, whereas evaporite dissolution is responsible for a certain proportion of groundwater chemistry. This result is consistent with that shown in Fig. 5d. Around 76% of water sample points plotted below the y = 0.5x line, indicating the dominant role of silicate dissolution.
Anthropogenic activities represent the main factor affecting groundwater chemistry in many places. The study area is not industrially developed, with agricultural and domestic wastewater acting as the main contributors to groundwater pollution. NO3− is a special pollutant in groundwater in agricultural areas. Given the conservative nature of Cl−, the relationships among Na+, Cl−, and NO3− can be utilized to isolate the influences of anthropogenic activities on groundwater chemistry. Most groundwater samples plotted within the triangular area, indicating the dominant source of groundwater NO3− to be urban sewage (Fig. 5i).
The chlor-alkaline index is an effective approach for studying ion exchange processes, and CAI-Ⅰ and CAI-Ⅱ have been confirmed to verify cation exchange types. The chloride base index can be used to compare exchange correlations between Na+ and Mg2+ and between Na+ and Ca2+. The strength of aquifer ion exchange can be determined by calculating the chloride base indices CAI-Ⅰ and CAI-Ⅱ. A positive result for both indices is indicates forward cation exchange represented by Eq. (1); if both are negative, reverse cation exchange occurred, represented by Eq. (2).
2Na++(Ca,Mg)X2====(Ca,Mg)2++2NaX (1)
(Ca,Mg)2++2NaX====2Na++(Ca,Mg)X2 (2)
As shown in the Fig. 5j, the CAI-Ⅰ and CAI-Ⅱ values were less than zero for most of the samples. Therefore, it was concluded that reverse cation exchange was identified as a main reason that resulted in decreases in Ca2+ and Mg2+, consistent with the results in Fig. 5e.
4.2.3 Saturation indices (SI)
The saturation index (SI) calculated with the hydrogeochemical software PHREEQC is an important measure of the state of equilibrium of various minerals in the groundwater system (J. Yang et al. 2020). SI > 0, SI = 0, and SI < 0 represent a supersaturated (tendency to precipitate), equilibrium between dissolution and precipitation, and unsaturated state (tendency to dissolve in the solution), respectively. The present study calculated the SI of groundwater hard gypsum, calcite, aragonite, dolomite, gypsum, and rock salt. Anhydrite and gypsum SI ranged from − 3.880 to − 1.594 and − 3.576 to − 1.290, respectively, indicating slight dissolution. The SI of dolomite varied widely from − 13.315 to 0.993, reflecting the large variation in dolomite dissolution precipitation between groundwater sites and its high instability. Most SI values were less than 0, with only those of aragonite, calcite, and dolomite exceeding 0. This indicated that study area’s groundwater system has not reached saturation, and the rocks can continue to dissolve in the groundwater. The SI results were consistent with those of the ion ratio in previous studies. The dominant sources of groundwater Ca2+, Na+, and HCO3− in the groundwater were isolated as dolomite, calcite, gypsum, and rock salt dissolution, whereas the saturation states of aragonite, calcite, and dolomite indicated trends of precipitation for these minerals. Combined with the conclusions drawn earlier in this article, although there is a certain amount of dissolution of calcium and magnesium carbonates, but at the same time, with the reverse cation exchange, so the concentration of Na+ in the water is increasing.