Hydrochemical characteristics and hydraulic connection of shallow and mid-layer water in typical mining area: a case study from Sulin mining area in Northern Anhui, China

Due to the large amount of water supply, the water level difference between mid-layer water and shallow water increases. There is no authoritative answer whether there is a hydraulic connection between shallow and mid-layer water. Therefore, a clear understanding of hydrogeochemical characteristics and the water chemical connection of them is indispensable for groundwater protection and the safety of water supply. The results of the statistical analysis showed that shallow and mid-layer water were both neutral water. The content of SO42−, Cl− and Na+ in both exceeded the grade III standard limit in the Standard for Groundwater Quality of China. Piper diagram revealed shallow water was mainly of Na + Mg + HCO3− and Mg + Na + HCO3− type, and mid-layer water was mainly of Na + Mg + HCO3− + SO42− and Na + Mg + HCO3− type. Besides, using the coefficient variations method of comprehensive water quality index (pH, TDS, F−, and NO3−), it is found that except for a local area with thin thickness (less than 5 m) of the first aquiclude, the mid-layer water was not affected by shallow water in Sulin mining area. This study may be helpful for the safety of water supply in the study area and other mining areas in the Huaibei coalfield.