*4.1. Hydrochemistry*

The analysis of groundwater chemistry data can help to determine the geochemical parameters of groundwater and the distribution characteristics of contaminants. The statistics and analysis of the test results of groundwater samples were shown below.

The minimum value of pH of shallow groundwater in the study area was 7.55 and the maximum value was 9.26, which was alkaline overall and favorable to the precipitation and oxidation of trivalent chromium. The average distribution of the main cation concentrations: Na<sup>+</sup> > Ca2+ > Mg2+ > Cr > K+ > Fe > Mn > NH4 <sup>+</sup> > As > Pb, and the average distribution of the main anion concentrations: HCO3 − > SO4 <sup>2</sup><sup>−</sup> > Cl<sup>−</sup> > NO3 − > CO3 <sup>2</sup><sup>−</sup> > NO2 −. The order of triple nitrogen content in groundwater in the study area was NO3 − > NO2 − > NH4 +, which mainly polluted groundwater in the form of nitrate nitrogen [37].

Among the ionic components of shallow groundwater, the coefficients of variation of SO4 <sup>2</sup>−, Cl−, NO3 <sup>−</sup>, Na+, Ca2+, Mg2+, HCO3 − and Fe were relatively small, and the contents of these ions in shallow groundwater were relatively stable. In contrast, the coefficients of variation (C.V.) of NO2 −, Mn, NH4 +, As, Pb, Cr(VI), CO3 <sup>2</sup><sup>−</sup> and K+ all exceeded 100%, which belonged to a strong degree of variation, indicating that the contents of these ions in shallow groundwater were highly variable, and at the same time these ions were sensitive factors that vary with environmental and anthropogenic factors.

The pH of deep groundwater in the study area was 7.8 at minimum and 8.34 at maximum, which was alkaline overall. The distribution of the main cation concentration: Na<sup>+</sup> > Ca2+ > Mg2+ > Cr(VI) > K<sup>+</sup> > Fe > Mn > NH4 +, the distribution of the main anion concentration: HCO3 −> SO4 <sup>2</sup><sup>−</sup> > Cl<sup>−</sup> > NO3 − > NO2 −, and As, Pb, CO3 <sup>2</sup><sup>−</sup> were not detected in the deep aquifer. The distribution of the remaining ions was similar to that of the shallow layer.

As can be seen from Table 1, the coefficients of variation of SO4 <sup>2</sup>−, Cl−, NO3 <sup>−</sup>, Na+, Ca2+, Mg2+, HCO3 <sup>−</sup>, Fe and K<sup>+</sup> in the deep groundwater were relatively small, and the contents of these ions in deep groundwater were relatively stable. The coefficients of variation of NO2 −, Mn, NH4 +, Cr(VI) and CO3 <sup>2</sup><sup>−</sup> all exceeded 100%, which belonged to strong degree of variation.


**Table 1.** Statistical descriptions of chemical parameters.

Note: The units of minimum, maximum and average values in the table are mg/L except for pH. SD: Standard Deviation. C.V.: Coefficient of Variation.
