Spatial Distribution Pattern of Ions in Deep Groundwater

According to the analysis results of deep groundwater samples, it can be concluded that the total hardness of 68% of the tested samples exceeded the three groundwater standards, and the distribution of total hardness was similar to the distribution of Ca2+ and Mg2+ concentrations, with a correlation coefficient of 75% (Figure 5). The over-standard rate of TDS was 16%. For cations, Mg2+ had the highest correlation coefficient with TDS, reaching 0.81, followed by Cr(VI), reaching 0.79, indicating that Mg2+ contributed the most to the increase of TDS, and for anions, SO4 <sup>2</sup><sup>−</sup> had the highest correlation coefficient with TDS, reaching 0.89, indicating that SO4 <sup>2</sup><sup>−</sup> contributed the most to the increase of TDS. Na<sup>+</sup> exceeded the standard by 5%, and the correlation coefficient reached 79% with hexavalent chromium. The over-standard rate of NO3 − was 79%. The over-standard rates of Fe and Mn reached 68% and 74%, respectively, which were comparable to those of Fe and Mn in shallow groundwater. The 58% of Cr(VI) concentration exceeded the standard in deep groundwater, and the highest correlation coefficient between Cr(VI) and SO4 2− reached 0.92. Deep groundwater was less polluted than shallow groundwater, and the ion concentration was less than that of shallow groundwater, and the overall exceedance rate of each component was lower than that of shallow groundwater. The reason for exceeding the standard was mainly due to the shallow groundwater overflowing into deep groundwater.

**Figure 5.** (**a**) Exceedance rate of each component of groundwater, (**b**) pollutant correlation coefficient matrix.
