Hydrogeochemical evolution induced by long-term mining activities in a multi-aquifer system in the mining area br

The hydrogeochemical evolution of groundwater is related to and affected by long-term mining activities, which maydeteriorate the quality of groundwater. The Fengfeng mine in Handan, North China has a 30-y history of coal miningwith long-term mining activities and complex geological conditions, resulting in a complex hydrogeochemical envi-ronment in the mining region. In this study, the hydrogeochemical evolution mechanism of groundwater in a multi-aquifer system in the Fengfeng Mining Area was investigated using machine learning (self-organizing maps combinedwith K-means clustering) and sulfur and oxygen isotopes (delta 34SSO4and delta 18OSO4). The hydrogeochemical characteristicsof different aquifers in the mining area changed to different degrees after mining compared with the characteristicsbefore mining. The spatiotemporal variations in groundwater components were found to be controlled by pyrite oxi-dation, gypsum dissolution, and carbonate dissolution, which are affected by mining activities. Pyrite oxidation pri-marily occurred in the Carboniferous thin-layer limestone aquifer (CLA) and Permian sandstone aquifer (PSA). Thehydrogeochemical evolution in the Ordovician limestone aquifer (OLA), the main aquifer in the study area, was af-fected by leakage recharge from CLA and PSA caused by mining activities. The results showed that owing to the effectsof long-term mining, the altered groundwaterflow system affected the evolution of groundwater components in eachaquifer, particularly the sulfate concentration. This study reveals a distinct hydrogeochemical evolution induced by mining activities, which can provide a basis for groundwater resource management in mining areas