Influence of Mine Dewatering‐Effluent Cycling on Arsenic Loading in a Gold Mine Tailings Containment Area

The Giant Mine (1948–1999) generated 16 Mt of Au‐bearing mill tailings (2800 mg kg −1 ) originating from a mixture of flotation tailings (84.8 wt%), calcine residues (14.4 wt.%), and arsenic trioxide roaster waste (0.8 wt.%). A water treatment system for high As mine dewatering effluent has operated since the end of mine operations, with the intermediate storage area being the Northwest Tailings Containment Area (NW‐TCA). The tailings porewater contains elevated concentrations of dissolved As, Sb, Zn, and other metals. A multi‐year water balance supported by an examination of unsaturated and saturated flow conditions and based on isotope and geochemical analysis was conducted to understand the NW‐TCA hydrological system. Hydraulic gradients indicate persistent downward flow in the south end of NW‐TCA. Water balance calculations indicate an average of 384,000 m 3 y −1 of water from the NW‐TCA entered the groundwater system during the study period (2017–2022). Stable water isotope and water chemistry measurements indicate porewater in areas of high‐water table is influenced by mine dewatering effluent. Isotope mass balance indicates 60% of the mine dewatering effluent, which contains high concentrations of As, is sourced from water cycled through the NW‐TCA. The impact of the mine dewatering effluent is minimal in areas with a deep vadose zone, containing lower concentrations of As. Hydrological simulations indicate groundwater flow rates into the deep groundwater flow system from the NW‐TCA would be modest in the absence of the mine dewater pond because the system is near net evaporative in the absence of mine dewatering effluent.