ARSENIC GEOCHEMISTRY OF A LAKE IMPACTED BY HISTORICAL GOLD MINING

Roasting of gold-bearing arsenopyrite (FeAsS) ore resulted in the release of approximately 20,000 tonnes of arsenic (As) trioxide (As₂O₃) to the atmosphere as stack emissions from Giant Mine, Yellowknife, NT, Canada, a gold mine that was in operation from 1949-1999. Arsenic trioxide, which is considered to be the most bioaccessible As compound, has potentially accumulated in the sediments of surrounding lakes, some of which are used for fishing and recreation. Past sampling of surficial materials in the region has also suggested that As concentrations may be naturally elevated. Therefore, understanding the speciation and mobility of As in the sediment-water system is essential for risk assessment.

Long Lake is located approximately 5 km from Giant Mine. In July 2015, sediment cores were extracted from shallow- and deep-water sites to capture vertical variations in sediment chemistry. Dialysis arrays (peepers) were also installed at the shallow-water site to capture vertical variations in porewater chemistry. Select sediment samples were analyzed using scanning electron microscopy coupled with mineral liberation analysis (SEM-MLA), electron microprobe analysis (EMPA), and synchrotron-based microanalyses. The deep-water core was dated using ²¹⁰Pb methods.

At the shallow-water site, the maximum sediment As concentration (90 mg kg^-1) occurs in a narrow band directly below the sediment-water interface (SWI), where the dominant host of As is Fe-oxyhydroxide (which has a mean As content of 4 wt.%). Congruent porewater profiles for As and Fe indicate that the remobilization of As is governed by the reductive dissolution of As-bearing Fe-oxyhydroxide. At the deep-water site, the maximum sediment As concentration (1500 mg kg^-1) occurs 18.5 cm below the SWI. The sediments from this interval are enriched in As₂O₃ (as identified by SEM-MLA) likely originating from roaster emissions; ²¹⁰Pb dating indicates that this horizon corresponds to the period of maximum emissions from the Giant roaster. The presence of As-bearing sulfides, however, suggests that the dissolution of As₂O₃ results in the formation of less bioaccessible phases where reduced sulfate is available. The differences observed between the shallow- and deep-water sites are likely a result of sediment focusing.