4.2.1. Hydrogeochemical Facies

A Piper diagram can help to evaluate the geochemical relationships between different dissolved ions and dominant types of water chemistry in groundwater [38]. The TDS of shallow groundwater was 738.4–2532.6 mg/L [39]. The shallow groundwater was highly variable with HCO3 −, SO4 <sup>2</sup>−, Cl−, NO3 <sup>−</sup> as the main anions and Na+, Ca2+, Mg2+ as the main cations. The NO3 − concentration in the water samples was high and exceeded the quality standard for domestic drinking water, probably due to the inappropriate use of fertilizers making NO3 − enter the groundwater with rainfall and irrigation [40]. The HCO3-Cl-Mg-Ca type, SO4-HCO3-Na type and HCO3-Mg-Ca-Na type characterized the hydrogeochemical composition of shallow groundwater (Figure 3). The groundwater below the chromium slag pile (CSP) was SO4-HCO3-Na type. The reason was related to the production process of chromium salt, and the specific chemical reaction was 2Na2CrO4 + H2SO4 = Na2Cr2O7 + Na2SO4 + H2O, so SO4 <sup>2</sup><sup>−</sup> and Na+ will enter the groundwater under the leaching effect of chromium slag. The downstream was basically uncontaminated, and the groundwater was HCO3-Mg-Ca-Na type. The groundwater of middle reaches and east of the chromium slag piles was HCO3-Cl-Mg-Ca type.

**Figure 3.** Piper diagram for shallow and deep groundwater. CSP represents chromium slag pile.

The TDS of deep groundwater in the site was 504–1198.26 mg/L. The anions were mainly HCO3 −, SO4 <sup>2</sup>−, Cl−, NO3 <sup>−</sup>, and the cations were mainly Na+, Ca2+, Mg2+. The NO3 − concentration in the deep groundwater was lower than that in the shallow groundwater, but still exceeded the quality standard of domestic drinking water. It may be that the shallow groundwater overflowed into the deep groundwater, resulting in abnormal NO3 − concentration in the deep groundwater. The deep hydrogeochemical composition was simpler than the shallow ones, and the groundwater was mainly HCO3-Cl-Mg-Ca type, HCO3-Na-Mg type and HCO3-SO4-Mg-Na-Ca type.
