Geochemical process and water quality evaluation of karst groundwater in the Baiquan spring area under exploitation of coal and iron resources and inflow of southern water into Hebei Province

Under the new water situation of the south water into Hebei, the groundwater environment of Baiquan Spring area has been changed, and the geochemical process of karst groundwater needs to be clarified.In this research, numerical simulation, machine learning (self-organized clustering) and isotope (δD and δ¹⁸O) methods were used to systematically reveal the geochemical processes of karst groundwater in the Baiquan Spring area under the mining activities and the inflow of water from the south into Hebei Province, and the water quality was evaluated on the basis of entropy-variable weighted water quality index(EWQI). After the entry of south water into Hebei, the karst groundwater level in the Baiquan spring area was raised as a whole. Temporally, it presented both inter-annual dynamic changes and intra-annual seasonal changes. Spatially, the recharge area displayed a drastic change type, while the runoff and discharge areas showed a slow change type.The groundwater dropping funnel was mainly distributed in the coal and iron ore intensive area in the southeast of the spring area. The karst groundwater in the spring area was weakly alkaline, and the hydrochemical types were dominated by Ca−HCO₃ and Ca−SO₄ types. The major anion and cation concentrations followed the order of ρ(\mathrmHCO_3^- ) > ρ(\mathrmSO_4^2- ) > ρ(Cl⁻) and ρ(Ca²⁺) > ρ(Mg²⁺) > ρ(Na⁺) > ρ(K⁺). The ions showed a gradually increasing distribution pattern along the runoff path.Karst groundwater chemistry was dominated by weathering dissolution of rocks (calcite, dolomite and gypsum) and reverse cation exchange.The contents of \mathrmSO_4^2- and \mathrmNO_3^- in karst groundwater system in spring were affected by anthropogenic activities to some extent.Karst groundwater was derived from atmospheric precipitation and underwent secondary evaporation prior to infiltration, with the deuterium surplus value being reduced during runoff.The results of the water quality evaluation showed that the quality of karst groundwater was overall better than quaternary groundwater. Approximately 50% of the karst water and 37.5% of the quaternary water samples were suitable for drinking purposes, respectively. TDS, ρ(\mathrmSO_4^2- ) and ρ(\mathrmNO_3^- ) are crucial indicators affecting groundwater quality in the spring area.Potential anthropogenic activities causing water quality degradation in the karst groundwater system of the springs mainly include mine drainage, agricultural irrigation, and urban sewage discharge.Groundwater environmental protection measures for the spring area were proposed through water quality grading evaluation.The research results will be helpful to provide reference for the safety of water supply and the protection of groundwater environment in Baiquanspring area.