Hydrochemical characteristics and evolution processes of karst groundwater in Carboniferous Taiyuan formation in the Pingdingshan coalfield

Coal mining may affect the hydrochemical characteristics and evolution processes of groundwater, leading to a deterioration in groundwater quality. This study was conducted to understand the hydrochemical characteristics and determine the hydrochemical processes controlling the karst groundwater hydrogeochemistry in Carboniferous Taiyuan formation in the Pingdingshan coalfield, Henan Province, China. Statistical techniques and conventional methods were utilized to gain a comprehensive understanding of the hydrogeochemical processes of karst groundwater. Results showed that the groundwater was fresh water, and was slightly acidic-to-slightly alkaline. The main cations were Ca2+ and Na+, and the predominant anions were HCO3- and SO42-. Most of the groundwater was predominantly of the Ca-HCO3 type. Principal Component Analysis (PCA), ion correlation analysis, and conventional graphic methods (Piper and Gibbs diagrams) indicated that the dissolution of silicate, gypsum, and carbonate minerals controlled the formation of Na+, Mg2+, Ca2+, SO42-, and HCO3-. Meanwhile, cation exchange was another important process regulating karst groundwater quality. Moreover, the concentrations of SO42- and NO3- were high, suggesting that the karst groundwater in this study area had been affected by anthropogenic activities, such as agriculture and mining. To avoid water inrush and keep mines east of the study area safely operational, the Guodishan faults within the mining area should be grouted to cut off the connection between limestone aquifers in the east and west study areas. In addition, it is necessary to strengthen the scientific research on fertilizers that allow plants to absorb nitrogen rapidly and control the amount of fertilizer used to protect the groundwater from pollution. The results of this study are helpful for the prevention and control of water disasters, and are beneficial for groundwater resource protection in the Pingdingshan mining areas. They will also provide a reference for similar studies by other mining hydrogeologists across the world.