Study on the Impact of Closed Coal Mines on Groundwater in the Panlong River Basin (Shangdong Province, China) Based on Sulfur and Oxygen Isotopes

To deeply investigate the impact of closed coal mines on groundwater sulfate contamination in the Panlong River basin, Zaozhuang, Shandong Province, China, the hydrochemical characteristics and controlling factors of groundwater were analyzed based on 64 surface water and groundwater samples. The sources of sulfate contamination were identified by sulfur and oxygen isotope analysis. The results indicate that the sulfate content in the pore groundwater exceeds the Standard for Groundwater Quality (China) Category III (250 mg/L), with a maximum content of 666.2 mg/L. Specifically, the exceedance rate of sulfate in pore water near the western boundary of the Taozao coal field is directly related to mine water. The exceedance rate of sulfate in karst groundwater is 28%, peaking at 1131 mg/L, and is mainly distributed outside the western boundary of the Taozao coalfield and near the Dingzhuang water source in the southeast of the coalfield, indicating a significant influence from high-sulfate mine water in the coal-bearing strata. The sulfur and oxygen isotope differences are significant between surface water, mine water, karst water, and pore water. Through Bayesian end-member analysis, it was determined that 40–83% of sulfate in the downstream pore water of the Taozao coal field originated from mine water, while 48–86% of the sulfate in the karst water originated from mine water, which greatly affects the groundwater in the western and southeastern parts of the Taozao coalfield. Therefore, measures should be taken as soon as possible to control the risk of sulfate pollution of neighboring groundwater, especially karst groundwater, by mine water from closed coal mines.