Discussion on the method of searching for safe drinking waterin high-sulfate areas of Guizhou Province

Zhenning area is located in the southwest of Guizhou Province, which is a national key development area in the main functional zone of Guizhou Province. It is a typical karst mountainous areas, where carbonate rocks are widely distributed. The excessive sulfate content in groundwater is one of the main environmental geological problems, which makes the problem of functional water shortage quite prominent, especially drink water for local villagers. This paper discusses how to find safe drinking water in the study area. Our analysis is based on data from hydrogeological and environmental geology survey, geophysical exploration, hydrogeological drilling, water sample collection and testing. It is found that the sulfate in groundwater in this area is mainly from the T2g aquifer formation, and the sulfate content in groundwater reaches 275.42-1,100 mg?L-1, implying the water of Ⅳ-V class. Tracing by 34S isotope in groundwater shows that the sulfate is mainly from gypsum dissolution. The implementation of a " combined exploration and pumping" well ZK3 reveals that the gypsum formation is distributed in the middle and lower part of T2g1a with a thickness of about 1-1.5 m. The stratified pumping test shows that the content of SO42- in groundwater in the test section of the T2g aquifer formation is 720 mg?L-1, evaluated as V class water. The water inflow is 256.61 m3?d-1 when the depth is reduced to 11.50 m. When the groundwater aquifer of the T2g gypsum formation is penetrated and sealed, the content of SO42- in groundwater is 124 mg?L-1, the borehole self-discharge rate is 330.05 m3?d-1, and the depth is reduced to 10.80 m. In this condition the borehole water inflow reaches 628.84 m3?d-1, and the water quality is Ⅲ class. This investigation demonstrates that safe and drinkable groundwater can be found below the gypsum layer by drilling to some depth. Such a method can also be applied to other regions with similar problems of water quality.