Legacy mercury releases during copper mining near Lake Superior

Abstract

To examine issues of mercury contamination in lake sediments and fish, we require insight into historic sources of mercury and details of watershed methyl mercury (MeHg) cycling. Modern-day National Atmospheric Deposition Program (NADP) estimates of atmospheric mercury deposition in the upper Midwest region range from 4-10 μg/m2/y (wet only) to 5-30 μg/m2/y (gross deposition). Sedimentary records from scattered Michigan lakes, removed from mining sites, record around 5-24 μg/m2/y modern THg deposition. However, these values are not representative of historic deposition near mining sites. On the Keweenaw Peninsula, mercury occurs naturally in copper ores and was discharged by smelting and stamp mill (tailings) operations. Here we examine mercury fluxes into two lakes (Portage and Torch Lake, portions of the Keweenaw Waterway) off Lake Superior, part of the previous Torch Lake Superfund site. Total mercury fluxes document greatly enhanced mercury loading (mean ca. 1590 μg/m2/y; peaks of 5120 to 21,300 μg/m2/y) during the height of copper mining (1880-1930), followed by a rapid decline once activities ceased. Methylmercury profiles appear to document both current methylation and historic methylation during mining operations. Time differences in MeHg and THg profiles may relate to watershed delivery time lags, toxic effects of copper on methylating bacteria, or to stratigraphic mobility. Whereas rapid sedimentation and lowered copper flux are promoting ecosystem recovery in Portage Lake, slower burial by organic-rich sediments is enhancing metal concentrations in Torch Lake sediments.