Hydrochemical responses and pollution source apportionment driven by cross-aquifer migration in multi-aquifer systems under intensive coal mining

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Intensive coal mining disrupts multi-aquifer systems, triggering cross-layer contaminant migration that endangers groundwater safety. This study investigates hydrochemical evolution mechanisms and quantifies the spatiotemporal heterogeneity of pollution in multi-aquifer systems under mining disturbance, using an integrated approach that combines multivariate statistics, hydrochemical diagrams, the entropy-weighted water quality index (EWQI), and principal component analysis (PCA). Eighty-four groundwater samples from Quaternary groundwater (QG), coal series groundwater (CG), Ordovician groundwater (OG), and mine water (MW) in Pingshuo mining area were collected during flood and dry seasons and analyzed for 11 hydrochemical indicators. The results indicated that groundwater was generally weakly alkaline. Seasonal variation was statistically significant only for pH, whereas ion concentrations (except HCO 3 − ), total dissolved solids (TDS), and electrical conductivity (EC) differed markedly among groundwater types. QG exhibited extreme spatiotemporal heterogeneity (Cl − and NO 3 − variation coefficients > 122%), indicating high sensitivity to external inputs. MW showed significantly lower pH and higher TDS, EC, and ion concentrations, notably SO 4 2− , which were 5.30–8.05 times higher than those in other groundwater types. Dominant hydrochemical types were HCO 3 -Ca·Mg (QG, CG and OG) and SO 4 ·Cl-Ca·Mg (MW), primarily controlled by water-rock interactions but significantly superimposed by anthropogenic activities. The EWQI indicated that MW was entirely undrinkable (Rank 3–5). QG and CG showed partial undrinkability (42.86%-57.14% and 16.67%-33.33%, respectively), while OG exhibited optimal quality (only 6.25% undrinkable) with high seasonal stability. The PCA identified three pollution sources: water-rock interactions driven by mining (PC1, 66.82%–77.77%), inputs from industrial, agricultural, and domestic sewage (PC2, 13.56%–15.53%), and acidity-alkalinity factor (PC3, 6.68%–7.32%). QG experienced triple-source influence; MW was dominated by PC1 and PC3; OG was primarily controlled by PC3. PC1 contribution increased in dry seasons, while PC2 in flood seasons. Therefore, the findings offer a scientific basis for adaptive, stratified groundwater management under mining pressures.