HOUGHTON, MI--Researchers at Michigan Technological University have found that most metal contaminants in Lake Superior sediments come from native ore deposits, not from the atmosphere as scientists had previously supposed.
Doctoral candidate Sandra Harting and Professor W. Charles Kerfoot of the Lake Superior Ecosystem Research Center at Michigan Tech found that while organic contaminants such as polychlorinated biphenyls (PCBs) may originate primarily from atmospheric deposition, the natural occurrence of metals in ore deposits and the history of mining discharges along the Lake Superior shoreline suggest substantial heavy metal loadings to the lake.
The most significant metal contaminant found during the Michigan Tech study is mercury. "Currently suspected sources of mercury include chlor-alkali plants, paper mills, and municipal discharges," said Harting. However, discharges from past mining activities were distributed around 75 percent of the Lake Superior shoreline. On Michigan's Keweenaw Peninsula, native copper and silver was historically recovered from the parent rock by stamping--crushing the rock using steam-driven stamp heads followed by sluicing the crushed rock to separate out the native metals and to remove the waste rock or stamp sands. Approximately a half billion tons of stamp sands were either discharged directly into Lake Superior or into its tributaries from Keweenaw Peninsula milling operations alone.
During their study, Harting and Kerfoot and their colleagues examined sediment cores from both offshore and nearshore environments of Lake Superior.
"We found mercury at high concentrations relative to background,
increasing towards shoreline regions," explained Kerfoot. "Anthropogenic
(human-caused) mercury levels were highly correlated with copper and silver
deposits among sediment core samples from nearshore Lake Superior, L'Anse Bay,
and the Keweenaw Waterway in a wide range of levels. Th
Contact: Charles Kerfoot
Michigan Technological University