*3.1. Sample Collection and Analysis*

In groundwater contamination investigation sampling, determining the location of sampling sites was a key factor. This sampling was mainly based on the following principles. Firstly, samples should uniformly along the groundwater flow direction. Secondly, the sampling points are encrypted in the seriously polluted areas. Thirdly, the general shape of the contamination plume can be determined according to the sampling points.

In December 2020, field investigations and sampling were conducted in the study area, and 92 groundwater samples were collected, including 73 shallow groundwater (SGW) samples and 19 deep groundwater (DGW) samples, with the locations of the sampling sites as shown in Figure 1. The total dissolved solids (TDS) and pH were measured in the field using a multiparameter tester. Other chemical indicators (Na+, Ca2+, Mg2+, K+, Cr(VI), HCO3 −, SO4 <sup>2</sup>−, Cl−, NO3 −) were measured in the laboratory of Henan Province Rock and Mineral Testing Center. The charge balance errors of all samples were within 5%, and the accuracy of each indicator met the quality requirements.
