Spatiotemporal patterns of mining-associated metals in sub-arctic lake sedimentsThursday, November 21, 2019 - 8:40am to 9:00am Theatre Two
Gold found on the shores of Great Slave Lake in 1935 has led to a legacy of pollution in Canada’s North. Mining operations at two major gold mines, Giant and Con, released a fine, toxic dust into the atmosphere during the extraction process – arsenic trioxide (As2O3). A lack of emission controls resulted in the release of over 20,000 tonnes of As2O3 into the atmosphere which dispersed onto the landscape surrounding Yellowknife. Most of it was discharged from Giant Mine in the 1950s and has potentially created repositories in the many lakes, rivers, and soils. In the absence of long-term environmental monitoring prior to and during resource development, identifying the extent of pollution is challenging but important for assessing risks to ecosystem health. Measurements of arsenic concentrations in lakewater and surficial sediment surveys have supported conclusions that no potential ecosystem health effects exist beyond a 30-km radius. However, paleolimnological studies at distances well beyond 100-km have identified elevated arsenic concentrations aligning with the timing of peak emissions. Findings suggest that the size of the area that received pollution from Giant and Con Mines remains poorly understood and further research is needed to fully characterize the emissions footprint. To address this need, spatial and temporal patterns of metal deposition are being reconstructed from the analyses of lake sediment cores along an 80-km transect following the prevailing wind direction (NW). Preliminary results are consistent with other previous paleolimnological studies and instill confidence in the use of sediment cores as natural archives. Arsenic concentrations are well above the CCME Probable Effects Level of 17 mg/g in lake sediment records obtained as much as 80 km from the mines during peak emissions. Distinct differences in metal deposition have been observed along the transect. Substantial increases in arsenic and antimony concentrations towards the sediment surface occur at lakes from 10 to 40 km (near-field). In contrast, lakes located from 50 to 80 km (far-field) have well-preserved Giant and Con mine signals, with arsenic and antimony concentrations returning to approximately pre-industrial levels towards the surface. Stratigraphic variations in arsenic concentrations and arsenic inventories, which represent the total amount of excess arsenic in lake sediment relative to pre-industrial background concentrations, identify pollution has travelled farther than previously believed. Perhaps most interestingly, concentrations do not entirely decrease with increasing distance from the mines. Differences in mining-associated metal loading and profile preservation may be due to diagenetic processes, a continued supply of metals from legacy stores in the surrounding catchment, or differences in bedrock geology between lakes. Ongoing analyses including radiometric dating will aid in determining the relative roles of sedimentation rates, natural processes such as forest fires surrounding individual catchments, and how varying landcover types may influence sediment metal profiles.