Analysis of Environmental Background Values of Chloride and Sulfate in Shallow Groundwater in Karst Area of Guizhou

This study focuses on the environmental background values in shallow groundwater in karst area of Guizhou. Firstly, the study area is divided into different groundwater environmental units according to the analysis data of 3 699 shallow groundwater chemical samples collected for the Guizhou Drought-Resistant Well-Drilling and Water Prospecting Project from 2007 to 2015, and the analysis of the geological background and hydrogeological conditions. Then, the box plot and the iteration standard deviation method are used to eliminate the abnormal values, and determine the distribution type of water chemical data in each hydrogeological area. Finally the 95th percentile of the data is taken as the upper threshhold of the environmental background values after deleting the outliers. The results show that the shallow groundwater in Guizhou karst area is neutral alkaline water, the cations are mainly Ca²⁺, Mg²⁺, the anions are mainly HCO₃^- and SO₄^2- and the groundwater types are mainly HCO₃-Ca·Mg type and HCO₃-Ca type. The main sources of ions in groundwater are weathering and hydrolysis of rocks and minerals. The distribution types of SO₄^2- and Cl^- ions in groundwater are mainly normal distribution, followed by lognormal distribution and least skewed distribution. The environmental background threshold values of the SO₄^2- ion of the Middle Triassic Guanling Formation and the Permian coal-bearing strata are 68.71-164.32 mg/L, while those of other areas are 19.42-39.05 mg/L. The threshold background values of Cl^- ion range from 3.45 to 6.65 mg/L, and the regional variations are small.