Copper profiles in the sediments of a mining-impacted lake

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Purpose: The sediments of Torch Lake, located in Houghton County, MI, USA, have been impacted by copper mining activity. Remediation of the site has focused on immobilizing shoreline tailing deposits. However, a large amount of copper remains in the mining-sourced sediment, and high copper concentrations persist in surface sediments. This study analyzes sediment core and sediment trap data to determine the source of copper in the surface sediments. Materials and methods: Sediment cores were retrieved from the lake, sectioned into 0. 5-cm to 1. 0-cm slices, and analyzed for Cu (atomic absorption spectrophotometry), bulk density, and organic matter content (loss on ignition). Sedimentation rates were determined using radioisotope dating. Sediment traps were deployed throughout the lake, and the collected material was analyzed for Cu and other metals (ICP-MS). Results and discussion: Radioisotope- and sediment trap-derived sedimentation rates agree well. Copper concentrations in the post-mining sediment are significantly (approximately 1. 5 to 2 times) greater than those in the underlying mine tailings. Current copper deposition rates, calculated from sediment trap data, are lower (∼1. 5 to 2 times) than accumulation rates measured in the surface sediment of the cores. Conclusions: Physical sediment properties and copper concentrations indicate the shoreline tailings piles are probably not a significant source of copper to the post-mining sediment, suggesting the excess copper is originating either in the water column or in the underlying tailings. Copper deposition rates measured in the sediment traps suggest a combination of these mechanisms. © Springer-Verlag 2010.

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Journal of Soils and Sediments