Understanding water flow through variably saturated waste‐rock dumps is important for determining the extent of sulfide‐mineral oxidation, contaminant loadings, and impacts of waste‐rock effluent on groundwater and surface‐water quality. To better understand water flow within full‐scale waste‐rock dumps in the continental subarctic region of Northern Canada, a field experiment was undertaken at the Main and Intermediate Dumps at the Faro Mine Complex, Yukon Territory. Here we present results from an investigation of the hydrological behavior and quantification of the factors controlling water flow through unsaturated waste‐rock dumps and the impacts on long‐term drainage water quality. The results suggest that flow through the fine matrix materials was the dominant flow mechanism, with possible preferential flow through macropores and ponding/runoff during intense infiltration events (i.e., snowmelt and intense rainfall). Cross δ¹⁸O‐δ²H plots of pore water collected from near‐surface waste‐rock samples suggested that evaporation at the surface of the dumps occurred during precipitation‐free periods in the summer. Depth profiles of δ¹⁸O and δ²H of pore water extracted from core samples provided indications of internal evaporation within the waste‐rock dumps and pore‐water displacement mainly in response to summer rainfall events (rather than snowmelt). Mixing calculations using δ¹⁸O and δ²H show that 76–95% of pore water present in the waste‐rock matrix was derived from summer rainfall, leading to lower concentrations of dissolved constituents in the summer effluent, and vice versa in winter. The results will inform cover design and remediation options for the waste‐rock dumps at the Faro Mine Complex.

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.

One of the large-scale field waste rock experiments (test piles) conducted as part of the Diavik Waste Rock Project was deconstructed, providing a spatially located set of geochemical, mineralogical, and particle-size distribution samples. Geostatistical analyses were conducted for sulfur and carbon content and saturated hydraulic conductivity, which affect the geochemical evolution of waste rock, to investigate the spatial dependence of these parameters. Analyses included population statistics, experimental semi-variogram estimation, and theoretical semi-variogram fitting. Population statistics were calculated for additional data sets from samples collected during the construction of the test piles. The population statistical analyses indicated that log-normal distribution provided the best fit for all investigated data sets. Experimental semi-variograms were estimated for the spatially located data set (test pile deconstruction) using the classical estimator, and theoretical semi-variograms were fitted. This investigation showed that the spatial distribution of sulfur, carbon, and hydraulic conductivity within the core of the test-pile experiments can be approximated using a log-normal distribution with a mean and standard deviation calculated using the samples collected during construction of the piles, and that little to no spatial relationship was present for these parameters at the scale of sampling. That the saturated hydraulic conductivity of the matrix material can be represented by the same statistical distribution throughout the test pile is significant because water flow, as well as mineral surface area and reactivity are dominantly controlled within the matrix portion of the test pile. Reactive transport simulations are included to demonstrate the influence of the matrix material on effluent geochemistry.

The Giant Mine operated from 1948 to 1999 to recover Au from arsenopyrite-hosted ore and is undergoing one of the largest remediation projects in Canadian history. Mine-waste containing flotation tailings and As-bearing roaster-generated Fe oxides was deposited in the Northwest Tailings Containment Area (NW-TCA) from 1988 to 1999. Since 2017, a longitudinal study has been undertaken at the NW-TCA to examine hydrogeological and geochemical controls. A comprehensive conceptual model was derived from integrating 8 years of field measurements (2017-2024) through an 18-m depth profile (GM7). Tailings exhibited circumneutral pH porewater, excess neutralization capacity, and elevated dissolved As concentrations (>25 mg L^-1), predominantly in the saturated tailings. Currently, As-rich tailings porewaters at GM7 infiltrate to the base of the NW-TCA and discharge through the underlying fractured bedrock. Field measurements of the tailings hydrogeology, isotope geochemistry, mineralogy and microbiology, coupled with multicomponent reactive transport modelling, indicate that: (i) reductive dissolution of As-bearing Fe oxides controls As mobility in the NW-TCA, (ii) As released from oxidative dissolution of arsenopyrite is primarily attenuated by Fe-arsenate phases at the near-surface, and (iii) adsorption of arsenite onto existing Fe oxides is a likely As attenuation mechanism in the saturated tailings. This has profound implications for the assessment of the Giant Mine tailings, where Fe oxides represent the most abundant As-bearing phase and thus constitute a long-term source of As, driven by microbial activity and available C sources in the tailings. These findings provide direction to optimize remediation efforts of As-bearing roaster residues and sulfidic flotation tailings.