Field and laboratory data combined with computational modelling have been used to predict the pit water quality for the Highway Reward mine. Open pit mining of the Highway Reward copper-gold deposit has produced a final mining void with a diameter of 600 m and a depth of 280 m. This void will be left to fill with ground and surface water once pumping of pit water ceases. Several leaching tables were constructed to simulate weathering reactions and surface water run-off in the major pit wall units during a 200-day leaching experiment. To date, about three-quarters through the experiment, combined run-off from these cells has reached a pH <4, corresponding to actual present day pit water pH values and chemistries. While sulfide oxidation in the pitwalls leads to an acid, metal-rich leachate, bicarbonate-rich groundwater inflow during the dry season acts as a buffer. Acid generating salts appear to have only a small impact on the overall pit water chemistry. The kinetic test data have been combined with measured pit water chemistry data, measured surface and seasonal groundwater inflows, climatic data, and calculated pit lake surface area and volume to produce a high-resolution pit water quality model. This high-resolution pit water quality model will aid in the mine decommissioning process.