Sediment Toxicity from Copper in the Torch Lake (MI) Great Lakes Area of Concern

Abstract

Torch Lake (MI) is a Federal Superfund Site and a Great Lakes Area of Concern. Torch Lake was impacted by over 200 million tons of copper rich mine tailings that were deposited in and along the shores of the lake. Twenty percent of the volume of the lake was displaced and the sediments have high concentrations of copper (1,000 mg/kg dry weight on average). Pore water from four sediment cores was analyzed at incremental depths for copper, total organic carbon, and toxicity using the Microtox ® 90% Comparison Test. Cores were also analyzed for copper and organic matter in the dry sediments. Statistical evaluation of data indicated that the upper sediments compared to the deeper sediments: 1) were less toxic (49% light loss versus 68%); 2) contained less pore water copper (0.59 mg/L vs. 0.81 mg/L); 3) had a higher percent organic matter (2.2% vs. 1.6%); and, 4) had no difference in the solid phase copper concentrations. Further evaluation of the sediment toxicity through direct comparison to copper chloride standards demonstrated that all pore water samples had reduced toxicity. The reduced toxicity of the pore water samples was reproduced by adding synthetic organic carbon to the copper chloride standards. These findings have implications for the EPA's No Action alternative for the sediments of Torch Lake. In making their recommendation, the EPA cites that preliminary data (believed to be obtained from nearby Portage Lake) shows that the sediments of Torch Lake are being covered and detoxified by natural sedimentation. However, total copper concentrations in the sediments from the south basin of Torch Lake do not indicate that the sediments are being covered and diluted by natural organic matter laden particles. Also, detoxification may be difficult to demonstrate without a baseline of sedimentation readings for comparison. In fact, solid phase copper concentrations may remain high due to scouring and erosion of surrounding stamp sand beaches or the steep side walls of the lake. This instability of particles has important implications for future restoration and monitoring activities in Torch Lake.