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. The high correlation between silver and mercury suggests a native metal source for the latter, as silver is generally not cycled atmospherically. Subsequently, the historic pattern of copper, silver, and mercury discharges was traced back to stamp mill sources."
Harting said that mercury contamination associated with mining of this type could originate from two principal sources: 1) imported mercury used in metal extraction and assay procedures, or 2) from mercury as a naturally occurring amalgam in silver-rich copper and gold deposits. "Historic manufacturing and supply records show no use of mercury for extraction purposes in Keweenaw Peninsula native copper mining operations," she said. "However, direct analysis of samples from shoreline stamp sands, tailing piles, and copper and silver ores revealed that the mercury is associated with native metal ores as a naturally occurring amalgam."
The Michigan Tech team examined ores from 13 Keweenaw mines, two Arizona mines, and one site in New Zealand. All disclosed high mercury concentrations, suggesting that the natural amalgam incidence is widespread.
These findings are important because the finest fractions of stamp sands, the so-called slime clays, are simultaneously the most susceptible to water transport and the most enriched in mercury. When this material comes into contact with wetlands, wetland discharges, or similar environments, mercury methylation may result. Methylmercury, said Kerfoot, is the most toxic and bioaccumulative form of mercury.
Kerfoot said he doesn't know the extent of methylation in Lake Superior, but that this aspect is being investigated by scientists under a 5-year NOAA/National Science Foundation study grant called KITES (Keweenaw Interdisciplinary Transport Experimental Study).
For more information, contact Charles Kerfoot at 906-487-2769 or via email: wkerfoot@mtu.edu .