Spatial and Temporal hydrogeochemical variations induced by long-term mining activities in karst aquifers, southwestern China

Long-term mining activities reshape the hydraulic and hydrochemical field, and threaten the safe use of groundwater resources and ecosystem balance. A representative lead-zinc deposit in southwestern China, has generated huge depression cone coupled with massive drainage, decreased water-level, changed flowpath and direct hydraulic connection across the aquiclude after concentrated mining lasting for nearly three decades. Meanwhile, there were remarkable spatial and temporal hydrochemical variations but distinct evolution patterns in the Carboniferous and Devonian karst aquifer especially in terms of sulfate. SO₄^2- increased slightly in the Carboniferous aquifer, maintained the hydrochemical type of HCO₃ and HCO₃·SO₄. While, SO₄^2- in the Devonian aquifer decreased drastically, and the hydrochemical type was converted from SO₄ and SO₄·HCO₃ to HCO₃ and HCO₃·SO₄. Before concentrated mining, pyrite oxidation and dissolution of carbonate minerals mutually promoted each other in sluggish groundwater flow, then generated abnormally high concentrations of SO₄^2-, Ca²⁺ and Mg²⁺ in Devonian karst aquifer. At present, SO₄^2- is mainly derived from pyrite oxidation indicated by sulfur isotope except deep groundwater contributed by gypsum dissolution. Groundwater quality in the Devonian aquifer was improved together by the dilution of northern shallow groundwater from the external Carboniferous aquifer and upward recharge of southern deep groundwater itself. Qualitatively speaking, the disturbing degree of hydrodynamic field was consistent with the variation degree of hydrochemical field. Principal component analysis verified that changes in hydrogeological conditions such as different recharge and mixing process resulted from continuous mining activities, were the driving forces of hydrochemical evolution. This research sheds light upon these hydrochemical variations induced by intensive mining, which was the basis of future work related with reactive tranport modeling.