The characterization, mobility, and persistence of roaster-derived arsenic in soils at Giant Mine, NWT

- Roaster-derived As2O3 persists in outcrop soils; some is slowly weathering.
- Arsenic concentrations are usually greatest in near-surface soils.
- Outcrop soil arsenic concentrations can rival or surpass those in tailings.
- Most roaster-derived arsenic in soils at Giant was likely deposited before 1964.
- Topography and climate help concentrate arsenic and promote As2O3 persistence.

Approximately 20,000 tonnes of arsenic (As)-bearing emissions from roasting gold (Au)-bearing arsenopyrite ore were aerially released from 1949 to 1999 at Giant Mine, near Yellowknife, Canada. Soil samples collected within 4 km of the former roaster from sites undisturbed by mining or other human activity contain up to 7700 mg/kg total As. Total As concentrations are highest within a few cm of the surface, and particularly enriched in soil pockets on rock outcrops. Scanning electron microscopy and synchrotron microanalysis show that roaster-derived arsenic trioxide (As2O3) has persisted in shallow soils in the area. Roaster-generated maghemite and hematite are also present. These anthropogenic forms of As are much more common in near-surface soils than natural As-bearing minerals. Comparison of the proportions of As, Sb, and Au concentrations in outcrop soil samples and historic As2O3-rich dust captured by emission controls suggest most of the roaster-derived As in soils at Giant was likely deposited before 1964. Topographic restriction by rock outcrops and a dry, cold climate likely contribute to the persistence of As2O3 in outcrop soils.Limited post-depositional mobility of roaster-generated As is evident from textural relationships in thin section, the presence of As in weathering products, the variation of As:Sb with depth, and the elevated As concentrations in soil pore waters sampled by suction lysimeters. Synthetic gastric and lung fluids extracted an average of 34% and 18% of the total As in three adjacent soil samples. Given that As2O3 is one of the most toxic and bioaccessible forms of As, the persistence of roaster-generated As in shallow soils may be a human health risk at this site.