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Roasting arsenopyrite-bearing gold ore for more than fifty years has resulted in nearly 300 000 tons of arsenic trioxide waste at the Giant mine, Yellowknife, NWT, Canada, mostly stored in underground chambers sealed with concrete bulkheads. Water samples from underground seepages are pH-neutral due to carbonate-bearing wall-rock and concrete. Some underground mine waters and surface soil pore waters at the Giant mine contain extremely high concentrations of As which have interacted with arsenic trioxide. These concentrations, several orders of magnitude higher than guidelines, occur in the absence of acid rock drainage. The planned remediation at Giant will reduce but not eliminate As release from the site. Specifically, the total As released (mostly in the form of dissolved As) will decrease from the current level of approximately 500 kg per year to less than 200 kg per year. Without this remediation, assuming that Giant mine were allowed to flood as is the case for most underground mines after closure, As release could increase to many thousands of kilograms per year. Interestingly, the very high cost of the proposed remediation plan is still outweighed by the monetary value of the gold extracted over the lifetime of Giant mine., Roasting arsenopyrite-bearing gold ore for more than fifty years has resulted in nearly 300 000 tons of arsenic trioxide waste at the Giant mine, Yellowknife, NWT, Canada, mostly stored in underground chambers sealed with concrete bulkheads. Water samples from underground seepages are pH-neutral due to carbonate-bearing wall-rock and concrete. Some underground mine waters and surface soil pore waters at the Giant mine contain extremely high concentrations of As which have interacted with arsenic trioxide. These concentrations, several orders of magnitude higher than guidelines, occur in the absence of acid rock drainage. The planned remediation at Giant will reduce but not eliminate As release from the site. Specifically, the total As released (mostly in the form of dissolved As) will decrease from the current level of approximately 500 kg per year to less than 200 kg per year. Without this remediation, assuming that Giant mine were allowed to flood as is the case for most underground mines after closure, As release could increase to many thousands of kilograms per year. Interestingly, the very high cost of the proposed remediation plan is still outweighed by the monetary value of the gold extracted over the lifetime of Giant mine.