4.3.2. Major Ion Ratio Relationship

In order to further explore the process of site water chemistry, the interrelationship between each major ion was analyzed [42]. As can be seen from Figure 7a, most of the shallow groundwater samples were distributed along the Na/Cl ratio 1:1 line (halite dissolution line), and the Na/Cl ratio of a few samples was greater than 1, indicating that there were other sources of Na<sup>+</sup> in the groundwater in addition to halite dissolution filtration, mainly related to the production process of sodium dichromate, which led to the increase of Na+ concentration in the groundwater around the chromium slag dumps. Most of the deep groundwater samples were distributed along the Na/Cl ratio 1:1 line, and the Na/Cl ratio values of a few samples were greater than 1, mainly because the shallow contaminated groundwater entered the deep aquifer, leading to the Na<sup>+</sup> concentration to increase.

The sources of Ca2+ and SO4 <sup>2</sup><sup>−</sup> are commonly the dissolution of gypsum and anhydrite. As can be seen from Figure 7b, most of the shallow groundwater samples were distributed along the Ca/SO4 ratio 1:1 line (gypsum and anhydrite dissolution line), and a few samples had Ca/SO4 ratio less than 1, mainly because the plant will use a large amount of sulfuric acid in the production process leading to the increase of SO4 <sup>2</sup><sup>−</sup> concentration in groundwater around the chromium slag pile, while the groundwater sample points near the Ca/SO4 ratio 1:1 line showed most samples with Ca/SO4 ratios greater than 1, and it may be alternate cation adsorption with constant Ca2+ release. The deep groundwater samples were all distributed along the Ca/SO4 ratio 1:1 line, but most of the samples presented Ca/SO4 ratios greater than 1, indicating that the dissolution of aragonite, calcite and dolomite was a potential source of Ca2+ in the site groundwater.

From the relationship pattern of (Ca2++Mg2+)/(SO4 <sup>2</sup>−+HCO3 −) [44], it was clear that the majority of water samples were distributed along the 1:1 line, except for groundwater samples with heavily contaminated samples, further indicating that calcite, dolomite, gypsum and anhydrite dissolution were potential sources of major ions in the groundwater mineralization process (Figure 7c).

**Figure 7.** Groundwater main ion relationship diagram. (**a**) represents the ratio of Na+ and Cl−, which generally indicates the dissolution of halite. (**b**) rep-resents the ratios of Ca2+ and SO4 <sup>2</sup>−, which generally indicates the dissolution of gypsum and an-hydrite. (**c**) indicates the dissolution of calcite and dolomite. (**d**) can indicate the presence or ab-sence of ion exchange interaction.

In addition, the relationship between (Ca2++Mg2+)-(SO4 <sup>2</sup>−+HCO3 <sup>−</sup>) and (Na++K+)- Cl− can usually be used to determine whether there is an ion alternate adsorption reaction in the aquifer. If the two are linear and the slope is close to –1, it indicates the existence of ion exchange, where the sample points distributed along the fitted line on the right side of the origin are the cation alternate. The sample points distributed along the fitted line to the left of the origin were the reverse reaction of alternate cation adsorption, and R<sup>2</sup> = 0.94 as shown in Figure 7d. This indicated that ion exchange existed in the groundwater of the site, which further illustrated that the part of groundwater sample points continuously released Ca2+ under the reverse reaction of alternate cation adsorption. It verified the result that the Ca/SO4 ratio was greater than 1.
