Faro Waste Rock Project: Characterizing Variably Saturated Flow Behavior Through Full‐Scale Waste‐Rock Dumps in the Continental Subarctic Region of Northern Canada Using Field Measurements and Stable Isotopes of Water

Abstract

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.

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

“…Two test pits were excavated in the upper surface in the Main Dump East to a maximum depth of 4 m and maximum areal dimensions of 9 × 9 m for sample collection and installation of tensiometers and ECH 2 O probes (Bao, Blowes, et al., 2020). Three co‐located tensiometers and ECH 2 O probes were installed at 0.3 m below ground surface (mbgs), 0.6 mbgs, and 0.9 mbgs adjacent to the two test pits (UW17‐TP1, and UW17‐TP2) to monitor dynamics of matrix potential and water content in response to dry and wet cycles in near‐surface waste rock.…”

“…From May 2019–May 2021, rainfall and snowmelt samples were collected to define the local meteoric water line (LMWL) at the FT following the procedure described in Bao, Blowes, et al. (2020).…”

Mine Waste Rock: Insights for Sustainable Hydrogeochemical Management

“…The physical and hydrological conditions of waste-rock piles (size, particle size, hydraulic retention time; discussed in Section 3) have a strong impact on the geochemical water-rock interaction and therefore occurrence of secondary minerals. Secondary minerals often occur as distinct rims or coatings on weathered host particles, but their occurrence is not necessarily related to the waste-rock composition in their direct vicinity: mixing of different drainage types within heterogeneous waste-rock piles [162] (Section 3.1) or generally rapid infiltration rates compared to slow precipitation rates may facilitate secondary mineral precipitation seemingly unrelated to the local waste-rock composition [163]. While certain secondary phases readily precipitate (e.g., Fe-(hydroxy)oxides [2,16]), others may take decades to precipitate and crystalize to detectable levels [115].…”

“…Point measurement techniques provide precise measurements of hydrogeological properties but are challenging to extrapolate to larger areas because of the waste-rock heterogeneities. Previous work on the hydrogeological properties of waste-rock piles have used point measurements of water content, for instance using time-domain reflectometry (TDR) sensors [16,279,280], frequency-domain (FDR) sensors [281], and soil water potential sensors [282,283], or by oven-drying discrete waste-rock samples taken at several depths in boreholes [162,163]. Remote sensing techniques offer larger scales of investigation, but often at the expense of lower resolutions that might be inadequate to discriminate the internal fine structure of waste-rock piles.…”

“…Remote sensing techniques offer larger scales of investigation, but often at the expense of lower resolutions that might be inadequate to discriminate the internal fine structure of waste-rock piles. Nevertheless, electrical resistivity techniques (ERT; [275,281]), ground penetrating radar (GPR; [284][285][286]), electromagnetic induction (EMI; [284,287]), fibre optic distributed temperature sensing (FO-DTS; [288]), or oxygen/hydrogen isotopic signatures [162] have all been applied successfully to investigate hydrogeological dynamics in waste-rock piles. In addition, basal lysimeters are often used to record internal mixing and drainage at the bottom of waste-rock piles [115,280,282].…”