Hydrochemistry evolution mechanisms of groundwater in Hancheng mining area over the past 52 years: Induced by hydrodynamic changes

Long-term over-exploitation of groundwater cause significantly water table decline and water quality degradation, understanding the hydrochemistry evolution mechanism of groundwater in mining areas is significant for the rational exploitation and protection of groundwater. A combination of statistical analysis and hydrogeochemical simulation was applied to study the temporal variation of hydrochemistry of Ordovician limestone water (OLW) in Hancheng mining area, and reveal the influence of hydrodynamic changes. Statistical analysis shows that the TDS of OLW in the of Hancheng mining area had increased about 212% during 1971 to 2022, of which most was contributed by Na ⁺ , Cl ⁻ , and SO ₄ ²⁻ . Combining with the dynamic change of the flow field, the increased mixing proportion of the deep buried OLW contributes to the increment of TDS. The mixing modeling of PHREEQC software were used to calculate the mixing ratio of the surface water and the deep OLW. The result showed that the OLW was mainly recharged by surface water in phase I, which accounts for 94% for the southern zone and 62% for the northern zone; while it declined to 37% and 55% in phase II, 27% and 36% in phase III. Accordingly, the recharge ratio of deep buried OLW is continuously increase, which accounts for 6% for the southern zone and 38% for the northern zone in phase I, and increased to 63% and 45% in phase II, 73% and 64% in phase III. The results of the mixing simulation verified that the increasing recharge proportion of deep groundwater is the key reason for the water quality deterioration in Hancheng mine area. Inverse modeling was conducted to recognize the main hydrogeochemical processes, the dissolution of dolomite, gypsum, halite, the precipitation of calcite and cation-exchange dominated the evolution of major-ion concentration in the OLW along SE-NW direction. This study will contribute to the future management of groundwater in mining area.