Groundwater Geochemical Variation and Controls in Coal Seams and Overlying Strata in the Shennan Mining Area, Shaanxi, China

Water resource conservation and ecological protection are key coal mining issues in northern Shaanxi Province and the Yellow River Basin. Revealing the characteristics and variation patterns of groundwater quality in the coal series and its overlying aquifers can provide a geological foundation for solving or optimizing these issues. Taking the Zhangjiamao coal mine of the Shennan mining area in northern Shaanxi Province, western China, as an example, water samples were collected for analysis from the: quaternary strata, weathered bedrock, burnt rock, coal series, and coal seam. Test parameters included conventional ion concentrations, total dissolved solids (TDS), and pH. Key water chemistry indicators such as oxidation/reduction index (ORI) and groundwater chemical closure index (GCCI) were used to explain the water quality differences. The Quaternary water, burnt rock water, and weathered bedrock water were dominantly the Ca-HCO3 type, the coal series water (Yan'an Formation) was dominantly Ca-HCO3 and Na-Cl types, and the coal seam water was dominantly Na-Cl type. From the shallow groundwater to coal seam water, dissolution and leaching gradually decrease and degree of retention gradually increases. Coal seam water was characterized by high TDS, high GCCI, and low ORI, reflecting a closed hydrogeochemical environment and moderate sulfate reduction. Leaching, salt accumulation, sulfate reduction, and cation exchange jointly control the groundwater chemical characteristics and evolution of the coal series and its overlying aquifers. Salt accumulation and cation exchange reactions of the stagnant coal seam water in the arid and semiarid climates and shallow buried conditions result in increased mineralization; the water quality is vastly different from that of the overlying aquifers, which are dominated by leaching. Groundwater circulation in the coal series and coal seam are of the infiltration-retention type, and the overlying aquifer of the coal series are of the infiltration-runoff type. A comprehensive hydrogeological model was constructed of the Middle Jurassic coal series and its overlying aquifers in the area. The results of this study have implications for the identification of mine water influx sources in the Shennan mining area, and the understanding of controls on the groundwater geochemical variation in Jurassic coal field of western China.