Existing mine pit and natural lakes in northern Minnesota as predictors of total mercury concentration of a mine pit at closure

摘要

Open pit mining oftaconite ore has been ongoing in northern Minnesota since the early 1900s and mine pit lakes are a common feature on the Mesabi Iron Range. The approximate western half of the Iron Range is within the Mississippi River watershed and the approximate eastern half is within the Lake Superior Basin watershed. In the Lake Superior Basin the outflow water from pit lakes is required to meet a standard of 1.3 nanograms per litre (ng/L) for total mercury concentration. The average concentration of mercury in precipitation in northern Minnesota is approximately 12 to 13 ng/L (1996-2009 time period). Available surface water chemistry data indicate that a number of existing mine pit lakes on the eastern end of Minnesota's Iron Range and several remote lakes in Voyageurs National Park (located about 50 km to the north of the Iron Range) have annual average total mercury concentration in the water column or outflow water that is less than 1.3 ng/L. Characteristics of pit lakes and natural lakes set in bedrock, when compared to other lake-specific research findings in the Upper Midwest, suggest that atmospheric mercury deposited to these water bodies is likely removed from the water column by volatilisation back to the atmosphere and by sedimentation and burial in the bottom sediments.