Sä Dena Hes Mine site : teasing-apart mining-related versus naturally elevated metals concentrations in enviromental media : implications for risk management Baker, Randy F.; Mackintosh, C.; Unger, M. L; Bertrand, V.
The Sä Dena Hes Mine is a former lead-zinc mine near Watson Lake, Yukon Territory operated for only 16 months in the early 1990s. After operations ceased, the property was acquired by the Sä Dena Hes Operating Company, of which Teck Resources Limited was the operating partner. Between 2013 and 2015, site characterization and ecological and human health risk assessments were undertaken to support permanent closure of the site. During the course of sampling, highly elevated concentrations of cadmium, lead and zinc were observed in inorganic soil and even more so in organic soils (2 – 4x higher than inorganic) downgradient of waste rock areas, especially in the ‘1380 Gully’. This gully is downgradient of the 1380 adit, directly adjacent to the main mineralized zone. Similarly, lead and zinc were highly elevated in willow and alder twigs (10 – 60 ppm dw vs < 1 in reference), ground invertebrates (50 – 125 ppm vs < 2 in reference) and small mammals (10 – 45 ppm vs < 1 in reference) in this area, posing elevated ecological risks. Human health risks were also identified in this area from elevated metal concentrations in soils (direct contact) and plants/small animals (consumption). Initially it was assumed that undisturbed areas adjacent to metals-contaminated areas were contaminated via aeolian and/or aquatic pathways from mine sources. Subsequent step out sampling revealed that this trend was also observed in well forested, undisturbed areas beyond the gully. Geochemical investigations confirmed that concentrations and ratios of Pb:Zn in inorganic soils of these areas were characteristic of mineralized zones. Enriched concentrations and different Pb:Zn ratios of organic soils by cationic metals (Cd, Cu, Pb, Zn) was the result of uptake by plants and adsorption into A horizon soils that likely occurred over centuries. These lines of evidence concluded that while mining-related activity may have been a minor contributor, it was not primarily responsible for elevated metal concentrations in the 1380 Gully area. This has significant implications for risk managers when directing remedial efforts and communicating residual risks to regulators, First Nations and other stakeholders.
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