An increasingly common practice for metallurgical plants is to discard their wastes by combining them with mine tailings prior to disposing the blended material to a containment facility. This practice has occurred since 1985 at the Kidd Creek tailings impoundment where natrojarosite, a waste produced from the adjacent Zn refinery, is combined with mine tailings and is deposited in a single impoundment. To assess the environmental impact of the co-disposal, a laboratory column experiment was conducted. The column material was flotation tailings from the Kidd Creek site containing 3 wt % natrojarosite residue. Dilute sulfuric acid was passed through the column to simulate the acid generated in the unsaturated zone of the tailings impoundment. The results of this experiment were compared to the results of a previous experiment conducted on unamended flotation tailings. The results showed that the effluent from the column containing the natrojarosite-bearing mixture had a faster decrease in pH, earlier increases in the concentrations of dissolved metals such as Pb and Cd, and a greater persistence in effluent metal concentrations such as Pb, Zn and Ni. To prevent the observed enhanced release of dissolved metals from mine waste disposal areas, natrojarosite should not be co-disposed with tailings.