Elevated nitrate and nitrite concentrations in the water stored in the Gibraltar Mine Tailings Storage Facility (TSF) prompted Taseko Mines Ltd. to seek out innovative water treatment technologies. Drawing from our extensive in-situ biological treatment experience, Ensero developed and implemented an in-situ treatment approach, drawing from our various areas of expertise including geochemistry, hydrodynamic modelling, engineering, treatability testing, microbiology, regulatory permitting, and water treatment. Within 60 day of in-situ biological treatment, TSF supernatant water met discharge requirements. This approach resulted in 60% reduction in costs over comparable ex-situ water treatment costs, with zero investment in permanent infrastructure, and treatment of significantly larger volumes of water on a highly expedited timescale. This in-situ biological treatment approach was the first of its kind for a TSF in British Columbia.