Hydrogeochemical evolution characteristics of groundwater associated with closed coal mine in karst mountainous area and the impact on irrigation and human health

Abstract

Acid mine drainage (AMD) leakage represents a serious concern due to its potential to contaminate surface water and groundwater, thereby posing a significant threat to the well-being of residents inhabiting water-stressed karst mountainous regions. This study examines the hydrochemical characteristics and controlling factors of groundwater and surface water around a closed coal mine in Southwest China. Hydrochemical analysis and hydrogen-oxygen isotopes were applied to water samples to assess the impacts of mine drainage on water quality, irrigation suitability, and human health. Results show that the leaked mine water has a pH below 6 and elevated concentrations of SO₄^2-, TDS, Fe, and Al. Groundwater is classified as SO₄·HCO₃-Ca type, whereas surface water is predominantly SO₄-Ca type. The slopes of δD-δ¹⁸O relationship are 6.89 for groundwater and 5.89 for surface water, with corresponding d-excess values of −1.16 ‰ and −2.88 ‰. Hydrochemical evolution is mainly controlled by sulfuric acid weathering of silicate and carbonate rocks (driven by AMD), along with water-rock interaction, evaporation, cation exchange, and anthropogenic inputs. All samples show negative chloro-alkaline indices (CAI < 0). Surface water quality is moderate to poor, whereas groundwater quality is generally medium or better. Principal component analysis identifies total hardness (TH), SO₄^2-, TDS and Na⁺ as the dominant factors affecting water quality, with component loadings > 0.8 and correlation coefficients of 0.84, 0.83, 0.80, and 0.60, respectively. The permeability index (PI) is below 25 % for all surface water samples and for 53.85 % of groundwater samples, indicating that irrigation use may reduce soil permeability. The annual hazard quotient (HQ) for children reaches 3.20 a⁻¹, 1.62 times that for adults, suggesting that direct ingestion of water in study area may pose a significant non-carcinogenic health risk. These findings provide important insights for managing water resources in karst terrains affected by closed mining operations.