4.2.2. Spatial Distribution Pattern of Ions

Spatial Distribution Pattern of Ions in Shallow Groundwater

Based on the analytical results of shallow groundwater samples, the spatial distribution pattern of ions was analyzed to study the spatial contamination characteristics of each ion component in groundwater (Figure 4a). The pollutant correlation coefficient matrix was plotted using R (Figure 4b). The Pearson's correlation coefficient was used to characterize the correlation between different pollutants.

**Figure 4.** (**a**) Exceedance rate of each component of groundwater, (**b**) pollutant correlation coefficient matrix.

The TDS of 67% of the shallow groundwater samples exceeded three types of groundwater standards, especially the TDS concentration below the chromium slag pile and near the slag pile was the highest, and the water quality was poor, which was seriously affected by the chromium slag pile. Among the tested samples, the total hardness of 97% of the groundwater samples exceeded three types of groundwater standards, and the distribution of total hardness was similar to that of Ca2+ and Mg2+ concentration distribution, and the correlation coefficient reached 75%, indicating that the total hardness was mainly affected by the Ca2+ and Mg2+. The SO4 <sup>2</sup><sup>−</sup> concentration of 51% of the groundwater samples exceeded three types of groundwater standards, and the distribution of sulfate concentration was extremely similar to that of TDS, and the correlation coefficient reached 95%. Generally, SO4 <sup>2</sup><sup>−</sup> ions came from the dissolution of gypsum or sulfate sedimentary rocks. However, it can be seen that a large amount of sulfuric acid was used in the production of chromium salt, so it can be concluded that the SO4 <sup>2</sup><sup>−</sup> exceeded the standard in groundwater and was closely related to the production of chromium salt, and the correlation coefficient with Cr(VI) reached 83%. The NO3 − concentration of 92% of the groundwater samples exceeded three types of groundwater standard. Na+ concentration distribution was similar to that of TDS and SO4 <sup>2</sup><sup>−</sup> with a correlation coefficient of 96%, and the correlation coefficient between Na<sup>+</sup> and Cr(VI) was 0.72, mainly due to the fact that a large amount of sodium chromate and sodium sulfate will be produced in the chromium salt production process, resulting in the exceedance of Na+ concentration in groundwater. The exceedance of Fe and Mn reached 74% and 70%, respectively. The exceedance of Cr(VI) was 49%, which had a certain contribution rate to TDS, and the correlation coefficient reached 78%. Pb and Hg

concentrations in groundwater samples had low exceedance rates, which may be related to human activities.
