As construction activities at an open-pit gold mining project in Ontario approached completion, adjustments in site water management and treatment were essential. The impact of these activities on nearby water bodies resulted in elevated concentrations of ammonia, copper, and nitrate in the receiving environment. The water balance model was meticulously updated to accommodate these changes, identifying contamination sources, and loading from the open pit, processing plant, and mine rock and tailings facilities. A comprehensive assessment of various scenarios was conducted to determine optimal water management strategies and treatment requirements. Following a BAT study, a treatment process for excess impacted water was developed, incorporating metals precipitation, ammonia removal, biological denitrification, and reactive media polishing filtration. These measures not only ensure regulatory compliance but also minimize environmental impact, optimizing overall water management and treatment efficiency at the site.