*4.4. Hydrogeochemical Simulation and Analysis*

The potential hydrogeochemical processes in the groundwater were investigated through ion ratio relationships. In this section, the minerals saturation index (SI) was calculated by Phreeqc 2.8 [45] to further determine the forms of the various minerals present and the geochemical interactions.

## 4.4.1. Mineral Saturation Index Analysis

The saturation index of minerals can visualize the dissolved equilibrium state of components in groundwater. Three representative water samples were selected for this study: the severely contaminated water sample S54 with hexavalent chromium concentration of 257 mg/L, the moderately contaminated water sample S01 with hexavalent chromium concentration of 60.3 mg/L, and the lightly contaminated water sample S43 with hexavalent chromium concentration of 0.729 mg/L. For the water sample S54, the saturation index of hexavalent chromium minerals was CaCrO4 > Na2CrO4 > MgCrO4 > K2CrO4 > Na2Cr2O7 > K2Cr2O7 > CrO3, and the saturation index was less than 0, which indicated that hexavalent chromium minerals were easily dissolved in the groundwater, and the proportion of chromium oxide was the largest among chromium minerals. The saturation index of trivalent chromium minerals was ordered as Cr2O3 > MgCr2O4 > Cr(OH)3 > CrCl3, and the saturation index of Cr2O3, MgCr2O4 and Cr(OH)3 were greater than 0, while the saturation indices of CrCl3 were less than 0. It can be concluded from the saturation index of trivalent chromium minerals that most of the trivalent chromium minerals were insoluble in groundwater, and CrCl3 made the greatest contribution to the concentration of trivalent chromium ions. For groundwater sample S01, the saturation index of hexavalent chromium minerals and trivalent chromium minerals showed the same variation pattern with S54. For groundwater sample S43, the saturation index of hexavalent chromium minerals was ranked as CaCrO4 > Na2CrO4 > MgCrO4 > K2CrO4 > CrO3 > K2Cr2O7 > Na2Cr2O7, and the saturation index of hexavalent chromium minerals was less than 0, indicating that hexavalent chromium minerals were easy to dissolve into groundwater, but the order of mineral saturation index had changed, and the Na2Cr2O7 component had the largest proportion in chromium-containing minerals. The saturation index of trivalent chromium minerals showed the same change pattern as S54 and S01(Figure 8a).

By comparing the saturation index of chromium-containing minerals at S54, S01 and S43, and it can be concluded that the higher the chromium ion concentration in the aquifer, the greater the saturation index of chromium-containing minerals, the smaller the tendency for hexavalent chromium minerals to dissolve and the greater the tendency for trivalent chromium minerals to precipitate.

Figure 8b showed the simulation results of the saturation index of the main minerals. The results showed that anhydrite, gypsum and halite were under unsaturated state in groundwater, and halite was the most prone to dissolution, and calcite, aragonite and dolomite were under saturated state, and dolomite was the most prone to precipitation.

**Figure 8.** Comparison of saturation index (SI) at three points, (**a**) refers to chromium-containing minerals, including trivalent chromium and hexavalent chromium. (**b**) contains anhydrite, gypsum, halite, aragonite, dolomite and calcite.
