3.3.1. Natural Factors
Gibbs diagrams, which are constructed by plotting ratios of Na
+/Na
+ + Ca
2+ and Cl
−/(Cl
− + HCO
3−) versus TDS [
41], are widely used to identify natural processes, such as precipitation, rock-water interaction, evaporation/ crystallization, governing groundwater geochemistry [
33,
42,
43]. As seen in
Figure 7, all samples of both shallow and deep groundwater for both seasons are plotted in the rock dominance area, indicating that water-rock interaction is the main natural fundamental sources of groundwater chemical constituents in the study area. Evaporation was observed to have less contribution to the dissolved chemical constituents of groundwater during both dry and rainy seasons.
To further understand the details of rock water interaction in the study area, some ratio plots of major ions were conducted (
Figure 8). The relation of Na
+ versus Cl
− of both shallow and deep groundwater for both seasons are demonstrated in
Figure 8a. Generally, if Na
+ in groundwater is originated from the halite dissolution (1), then the molar ratio of Na
+/Cl
− should be approximately equal to 1. While if the ratio is greater than 1, Na
+ is mainly contributed by silicate weathering (2), whereas less than 1 indicates derivation from anthropogenic sources [
43]. In this study, nearly all deep groundwater samples and most of the shallow groundwater samples during both seasons are with the ratio greater than 1, implying that silicate weathering (2) is an important source of sodium in shallow and deep groundwater [
44]. Halite dissolution (1) is also one of the sources of sodium and chloride for both shallow and deep groundwater as some samples are observed with the ratio approximately equal to 1. This is confirmed by the Saturation Index (SI) of halite (
Figure 9). In addition, it is worth to note that part of shallow groundwater samples are with the ratio less than 1, indicating the chemical constituents of shallow groundwater are contributed by anthropogenic sources in some degree.
The relation of Ca
2+ and SO
42− is shown in
Figure 8b. It is observed that Ca
2+ increases with SO
42− in both shallow and deep groundwater, suggesting that these two ions may derive from the dissolution of gypsum and anhydrite (3, 4). The mineral equilibrium calculation results (SI
gypsum < 1, SI
anhydrite < 1) (
Figure 9) indicate that all samples are under-saturated with the respect of gypsum and anhydrite, confirming the potential contributes of these two minerals dissolution. While the phenomenon that samples of both shallow and deep groundwater fall above the 1:1 line (
Figure 8b), implying that Ca
2+ and SO
42− are also included in some other different geochemical processes besides aforementioned processes [
33].
The relationship between Ca
2+ + Mg
2+ and HCO
3− can reveal the source of calcium and magnesium of groundwater [
33]. If the Ca
2+ and Mg
2+ only derive from the dissolution of carbonate, the (Ca
2+ + Mg
2+)/HCO
3− molar ratio would be about 0.5. In this study, nearly all samples have higher ratios (>0.5) (
Figure 8c), signifying the existence of additional sources of Ca
2+ and Mg
2+ and less contribution of Ca
2+ and Mg
2+ by carbonate dissolution [
45]. This confirmed by the SI of calcite and dolomite (
Figure 9), showing that majority of the samples of both shallow and deep groundwater are over-saturated with respect to the calcite and dolomite.
The diagram of (Ca
2+ + Mg
2+) versus (HCO
3− + SO
42−) were constructed to determine the mineralization processes (
Figure 8d). The data that fall along the 1:1 line indicate contribution of dissolution of carbonates and sulfate minerals, and the data lying above and below the 1:1 line signify additional influences of the reverse cation-exchange process (5) and cation-exchange process (6), respectively [
3]. As shown in
Figure 8d, most of the data fall along the 1:1 line suggesting the weathering of carbonates and sulfate minerals. As discussed above, the contribution of carbonates weathering is less, so the dissolution of sulfate minerals (such as gypsum and anhydrite) is the main contribution. In addition, nearly all deep groundwater samples are observed to lie below the 1:1 line, suggesting the effects of cation-exchange process (6) on deep groundwater. However, some shallow groundwater samples fall above the equiline and some lie below the line, showing the contribution of the reverse cation-exchange process (5) and cation-exchange process (6) in the shallow groundwater in different area.
Chloro-alkaline indices, including CAI-1 and CAI-2, were introduced to examine the influences of cation-exchange processes on groundwater chemistry (
Figure 10). These two indices are expressed as Equations (7) and (8) (all ions are expressed in meq/L) [
46]. The aforementioned indices would be negative values if the hydrochemistry is dominantly affected by the normal cation-exchange process (6). On the other hand, if reverse cation-exchange process (5) takes place, these two indices would show positive values. Nearly all deep groundwater samples showed negative CAI (
Figure 9), confirming the results of
Figure 8d that cation-exchange process (6) occurs in deep groundwater. Some shallow groundwater samples are observed with negative CAI and others are with positive CAI, indicating cation-exchange process (6) and reverse cation-exchange process (5), respectively. This is consistent with the conclusion of
Figure 8d.
3.3.2. Human Activities
The contributions of human activities to groundwater chemistry are complex and uncertain, and hard to determine [
9,
45]. Nitrate, which is mainly originated from multiple anthropogenic sources, is widely used to indicate the effects of human activities on groundwater chemistry [
47,
48]. In this study, the concentration of NO
3− in sallow groundwater shows an increasing trend with the increase of Cl
− content (
Figure 11a), indicating the similar sources of these two ions [
49]. Cl
− is commonly derived from anthropogenic inputs as well as halite dissolution. Thus, the positive relation between NO
3− and Cl
− signifies their anthropogenic inputs. This is also confirmed by the positive correlation of TDS with (NO
3− + Cl
−)/HCO
3− (
Figure 11b).
As can be seen in
Figure 4 and
Figure 11, the high NO
3− contents (exceeding geochemical limit 10 mg/L: S5, S6, S7, S16, S26) are only observed in shallow groundwater in the urban area during both seasons, implying that anthropogenic inputs such as domestic wastewater have a certain of influences on the chemistry of shallow groundwater in the urban area. This is also evidenced by the spatial distribution of water type of shallow groundwater (
Figure 6a,b), of which Ca-Mg-Cl type for urban area while Ca-HCO
3 and Ca-Na-HCO
3 type for rural area. Urban anthropogenic wastes inputs are also found responsible for the relative higher NO
3− content for the deep groundwater in the urban area (D6) in contrast with that in the rural area as the result of thin aquitards, but the influence is very limited. In the rural area (mainly agricultural area irrigated with reclaimed water), the nitrite content of shallow groundwater presents a poor correlation with the Cl
− (the Pearson correlation coefficient is 0.118 for dry season and 0.383 for rainy season), suggesting human activities such as reclaimed water irrigation and reclaimed water river leakage have very limited influences on the shallow groundwater quality. This is possibly related to the thick aquitards above the water level. As a result, the NO
3− contents of all shallow groundwater are below the geochemical limit 10 mg/L (
Figure 11a). Deep groundwater in the rural area also have very low nitrite content (all below 10 mg/L). NO
3− contents show nearly no variation with the increase of Cl
− contents (
Figure 11), demonstrating that the chemistry of deep groundwater in the rural area is not affected by anthropogenic pollutions.