THE LEGACY OF ARSENIC TRIOXIDE AT GIANT MINE, YELLOWKNIFE

Large mines that operated before the 1970’s leave complex environmental legacies. A remarkable example is the Giant Mine near Yellowknife, Northwest Territories, Canada where roasting arsenopyrite-bearing gold ore for more than fifty years has resulted in nearly 300,000 tons of arsenic trioxide solid waste. Approximately 500 kg of arsenic (mostly dissolved) is released from the site each year to Great Slave Lake, and remediation costs estimated at $1billion.

Giant Mine represents one of the largest concentrations of arsenite at the earth’s surface, and ground waters interacting with the arsenic trioxide stored in underground chambers have extreme compositions. Seepages from underground drillholes and fractures contain up to 4000 mg As L^-1 and 17 mg Sb L^-1. Approximately 70% of the total is As(III). The dominant dissolved constituents are As, Ca, Mg, Na and SO₄ with low Fe. Biofilms developed at seepage points contain psychrotolerant arsenite oxidisers which flourish at temperatures less than 10ºC. Water samples are pH-neutral but an unfiltered sample exhibited a decrease in pH over several weeks, likely due to microbially-driven As(III) oxidation, which lowers pH through the following reaction: 2H₃AsO₃ + O₂ → HAsO₄^2- + H₂AsO₄^- + 3H⁺. The low pH values are not observed at the underground seepage points because of rapid neutralization by the dissolution of carbonate-bearing wallrock and concrete.

Approximately 20,000 tons of arsenic trioxide were released through the roaster stack from 1949-1999, mostly during the first few years of operation. Significant amounts of arsenic trioxide, identified as arsenolite by micro-diffraction, persist in near-surface soils. Total As concentrations in soils on the mine site are as high as 5000 mg kg^-1, similar to concentrations in tailings. The Sb and Au content of the arsenolite grains suggest that most of the particles were deposited prior to 1964. Dissolution textures are very rare and natural weathering rates are likely too slow to effectively remove arsenic trioxide from soil. The percentage of As that is soluble in simulated body fluids was measured for three soil samples and found to be 34% for gastric fluids and 18% for lung fluids.