Identification of characteristic pollutants in surge water of closed coal mines in Junlian mining area, Sichuan

The water environment governance and restoration in closed coal - mine areas serves as a vital foundation for the sustainable development of the mining area’s ecological environment. The key lies in the identification of characteristic pollutants within the groundwater environment of the mining area. In the Junlian Mining Area on the southern margin of the Sichuan Basin, the Longtan Coal - measure is exploited. The overlying Triassic and underlying Permian strata are both distributed with relatively thick carbonate rocks. The discharge of pollutants in the water gushing from the Longtan Coal - measure strata of closed mines has exerted an impact on the karst groundwater environment. A characteristic pollutant identification system for closed coal - mine areas is established. Through hydrochemical characteristic analysis methods, hydrogen - oxygen isotope methods, and principal component analysis methods, the source - indicating properties of pollution indicators in the water gushing from the Longtan Coal - measure strata are analyzed and quantified. The pollution attributes are quantified using the analytic hierarchy process, and the pollution degree is quantified by the sub - item pollution index method. The quantification results are weighted and integrated by the subjective - objective comprehensive weighting method for comparative analysis, thereby enabling the identification of characteristic pollutants among the pollution indicators of the water gushing from closed mines. The results indicate that the pollution indicators in the water gushing from closed mines mainly originate from the water - rock interaction resulting from the weathering and dissolution of carbonate rocks, silicate rocks, and their ore - bearing components, as well as the residues of mining activities. In the study area, the pollution indicators iron and manganese possess attribute characteristics such as strong toxicity, non - degradability, and weak mobility. Their enrichment is likely to have an impact on the local water environment, while COD_Cr and sulfate with relatively high migration intensity are prone to polluting the regional water environment. Among the pollution indicators, iron, manganese, COD_Cr, and sulfate exhibit significant pollution degrees. Ultimately, iron, manganese, COD_Cr, and sulfate, which have a relatively large proportion of the comprehensive weight, are identified as the characteristic pollutants in the water gushing from closed mines in the study area. They require key monitoring and treatment. Meanwhile, by combining the identification results with the karst hydrogeological conditions, four genetic models of characteristic pollutants are proposed, namely the “coal gangue leaching and infiltration type”, the “goaf water - rock interaction type”, the “oxidation of mining facilities type”, and the “composite type”. This research plays a guiding role in the groundwater pollution treatment in the Junlian mining area on the southern margin of the Sichuan Basin and also provides ideas for the identification of characteristic pollutants in groundwater - polluted areas of similar mining regions.