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Mixing of potentially acid-generating (PAG) waste rock with non-PAG rock can mitigate acid generation at hard rock mines. Studies have been carried out to evaluate viable PAG/non-PAG mixing ratios at the Gold Acres gold mine (Nevada, USA). The mine model for the Gold Acres Complex is used to determine the most likely and worst-case PAG:non-PAG ratios. Humidity cells (HCTs) were run for PAG (144 weeks) and non-PAG (67 weeks) end members, together with mixtures at ratios of 2:98 (the most likely case; 47 weeks), 4:96 (reasonable worst-case; 136 weeks) and 10:90 (an unreasonable case; 47 weeks) to evaluate leachate quality; all of the mixtures remained pH-neutral. Incremental mineralogic changes over the course of the HCTs could not be ascertained because the samples had undergone 37 years of weathering in the open pit. However, a mass balance demonstrated that lower solute concentrations than expected were released, based on the end-member leachate chemistry. Brief discussion is presented of results from the Kutcho Creek, Samatosum, and Stratmat mines in British Columbia (Canada), where their test data was found to mischaracterise the actual potential for acid rock drainage., Mixing of potentially acid-generating (PAG) waste rock with non-PAG rock can mitigate acid generation at hard rock mines. Studies have been carried out to evaluate viable PAG/non-PAG mixing ratios at the Gold Acres gold mine (Nevada, USA). The mine model for the Gold Acres Complex is used to determine the most likely and worst-case PAG:non-PAG ratios. Humidity cells (HCTs) were run for PAG (144 weeks) and non-PAG (67 weeks) end members, together with mixtures at ratios of 2:98 (the most likely case; 47 weeks), 4:96 (reasonable worst-case; 136 weeks) and 10:90 (an unreasonable case; 47 weeks) to evaluate leachate quality; all of the mixtures remained pH-neutral. Incremental mineralogic changes over the course of the HCTs could not be ascertained because the samples had undergone 37 years of weathering in the open pit. However, a mass balance demonstrated that lower solute concentrations than expected were released, based on the end-member leachate chemistry. Brief discussion is presented of results from the Kutcho Creek, Samatosum, and Stratmat mines in British Columbia (Canada), where their test data was found to mischaracterise the actual potential for acid rock drainage.

An investigation was undertaken to predict the post-closure water chemistry of the Cortez Hills Underground Deep South Extension where the mine workings will be backfilled with cemented waste rock. The objective was to determine the potential effects of the tunnel water on adjacent groundwater once the groundwater table recovers, hydraulic conditions stabilise, and ambient groundwater flow is reestablished. The average background groundwater chemistry from three wells representing discharge to the tunnel area during closure, and the worst-case groundwater dissolution chemistry of tunnel floor/unconsolidated backfill, shotcrete (sprayed concrete), cemented backfill and country wallrock/cemented backfill reactions were used to calculate their contribution to the total mass loading during tunnel flooding. The volume of groundwater flow through the tunnel area was coupled with these releases to calculate a total tunnel water quality. All analytes met Nevada environmental standards, except for As, Sb, and Pb, due to their naturally-elevated background groundwater levels. The dissolved tunnel water chemistry was predicted to be pH=8.2, with an alkalinity of 124 mg/L CaCO3 and with all solutes meeting regulations except for As and Sb. The predicted water chemistry was dominated by the influent groundwater, which comprised around 97% of the mass loading and was similar to two existing underground seeps., An investigation was undertaken to predict the post-closure water chemistry of the Cortez Hills Underground Deep South Extension where the mine workings will be backfilled with cemented waste rock. The objective was to determine the potential effects of the tunnel water on adjacent groundwater once the groundwater table recovers, hydraulic conditions stabilise, and ambient groundwater flow is reestablished. The average background groundwater chemistry from three wells representing discharge to the tunnel area during closure, and the worst-case groundwater dissolution chemistry of tunnel floor/unconsolidated backfill, shotcrete (sprayed concrete), cemented backfill and country wallrock/cemented backfill reactions were used to calculate their contribution to the total mass loading during tunnel flooding. The volume of groundwater flow through the tunnel area was coupled with these releases to calculate a total tunnel water quality. All analytes met Nevada environmental standards, except for As, Sb, and Pb, due to their naturally-elevated background groundwater levels. The dissolved tunnel water chemistry was predicted to be pH=8.2, with an alkalinity of 124 mg/L CaCO3 and with all solutes meeting regulations except for As and Sb. The predicted water chemistry was dominated by the influent groundwater, which comprised around 97% of the mass loading and was similar to two existing underground seeps.