Hydrogeochemical Response of a Variably Saturated Sulfide-Bearing Mine Waste-Rock Pile to Precipitation: A Field-Scale Study in the Discontinuous Permafrost Region of Northern Canada

Abstract

Globally, drainage from sulfide-bearing waste-rock piles, containing high concentrations of toxic metal(loid)s, can severely degrade surrounding ecosystems. Waste-rock piles are typically deposited as unsaturated porous media. The complex physical and chemical heterogeneity of waste rock poses significant challenges to the evaluation of internal hydrological, (bio)geochemical, and mineralogical controls on the magnitude and duration of environmental impacts. An operational-scale waste-rock pile located in the discontinuous permafrost region of Northern Canada was well-instrumented and monitored to examine hydrological processes (including precipitation, evaporation, and infiltration), hydrological and thermal responses to freeze-thaw and dry-wet cycles, and concentration-discharge (cQ) relationships in drainage-water quantity and quality. The net infiltration (subtraction of evaporation from precipitation) into the waste-rock pile occurred from May to November, resulting in rainfall-dominated recharge to pore water, surface seepage, and groundwater. Pronounced variations in water content and temperature in response to freeze-thaw and dry-wet cycles and variations in surface topography were observed in regions of the waste-rock pile impacted by preferential flow pathways. In contrast, distinct variations in water content and temperature were not observed in regions of matrix-dominated flow pathways. The cQ relationships show chemodynamically controlled dilution behavior for most dissolved constituents (e.g., SO4, Fe, Pb, Zn, Cu, Ca, Mn) in the drainage. Therefore, hydrological processes and (bio)geochemical and mineralogical reactions both play prominent roles in determining drainage-water chemistry. This work presents an integrated approach to site-specific monitoring and characterization to evaluate remediation and reclamation options for operational-scale waste-rock piles for long-term ecosystem preservation.