Mercury contamination, particularly in aquatic food webs, is an ongoing concern. The methylated forms of mercury, those most likely to be encountered by the consumption of fish and other aquatic species, are among the most toxic and bioaccumulative compounds known.

Mercury in the free metal form volatilizes at low temperatures; this unique property allows mercury to be far more mobile in the environment than other metals and thus travel far from its point of origin. Mercury continually cycles between the free metal, ionic, and organically-bound forms; therefore, metallic mercury discharged in one location may be responsible for methylmercury contamination in areas far removed from the original source.

The prevailing assertion is that mercury loadings to Lake Superior sediments originate predominately from long-distance atmospheric deposition. However, our recent studies reveal that approximately from 18-30% of mercury loadings to Lake Superior sediments are attributable to atmospheric sources, with the remainder originating from terrigenous sources.

For the past 150 years, numerous copper, silver, and gold mines have exploited the rich mineral resources of the Lake Superior basin. Tailings piles from these mining activities are located around 3/4 of the Lake Superior shoreline. Although some non-mining industries discharge mercury and mercury amalgamation was utilized to recover gold and silver from native ores around the Lake Superior basin, neither of these sources completely explains elevated mercury levels found in areas throughout the nearshore environment. For example, in Michigan's Keweenaw Peninsula, a region where mercury amalgamation was not used, stamp sands and poor rock from copper mining operations were found to contain mercury. Analysis of samples of nearly pure native copper and silver from Keweenaw Peninsula mines revealed that the mercury is associated with the native metals rather than the parent rock, suggesting the presence of naturally occurring mercury-silver or mercury-copper amalgams.
Significant mercury:copper and mercury:silver correlations in Keweenaw Waterway and nearshore Lake Superior sediment cores suggest a terrestrial rather than an atmospheric source of mercury to the sediments. Comparison of the locations of Lake Superior sediments that show indications of perturbation from copper milling discharges with SeaWiFS images indicate that mercury-rich slime clays are being transported along the Keweenaw Current. This is a heretofore unconsidered and perhaps significant source of mercury to Lake Superior sediments.