An assessment of environmental geochemical baseline for Matthews Lake, Northwest Territories

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The Seabridge Gold Inc. Courageous Lake exploration project (SCL) is located 240 km north Yellowknife in the central NT. Mining took place within the SCL claim at the Tundra (1964-1968) and Salmita (1983-1987) mines and ore was processed onsite. Levels of Arsenic (As) in the lake have consistently exceeded Canadian Council of the Ministers of the Environment guidelines. Elevated concentrations of As and other metals of concern (e.g., Al, Cd, Cu, Hg) in the lake could be the result of natural weathering, seepage from past mining activities, or a combination of both. A long-term sediment quality assessment of Matthews Lake is lacking and baseline pre-mining activity geochemistry for this region is not known. With two previous mine sites within the SCL site it is critical that geochemical baseline and past land-use impacts on the sediment quality of Matthews Lake be defined. It is nearly impossible to differentiate between baseline, past, and future land-use impacts on Matthews Lake using technologies currently employed for environmental monitoring due to the incredibly slow sedimentation rates within northern lakes. For example, Ekman grabs and Glew corers, commonly used to obtain lake sediments for analysis during site characterization and environmental monitoring, can produce samples that may homogenize up to 200 years of sediment accumulation, effectively mixing sediments deposited prior, during, and post mining. We propose using a technique where sediments are frozen in situ with a freeze corer and extracted, thus reducing or eliminating disturbance of lake sediments caused by traditional coring devices. We will slice frozen sediment cores at mm-scale with a custom designed microtome to achieve a temporal sampling resolution equivalent to a sample every 2-10 years. Our research is expected to provide site-specific environmental monitoring with specific emphasis on determining the proportions of anthropogenic and non-anthropogenic As. If this preliminary research is successful more detailed future research on multiple cores from around the lake may be used to account for sources of natural variability to be used to measure future chemical change and land-use impact on sediment quality.