Environmental chemistry of copper in torch lake, Michigan

Abstract

A study of Keweenaw Peninsula and adjacent Lake Superior waters was undertaken to evaluate the significance of Cu tailings deposits as a potential source of Cu and the controlling Cu concentrations in these waters. Metal concentrations were determined by atomic absorption techniques. Copper concentrations, especially in areas affected by discharge of large quantities of Cu mine tailings for over 100 yr, were relatively high. Concentrations of apparently dissolved Cu of up to 100 μg 1⁻¹ were found in Torch Lake in the summer of 1972. These high Cu levels may be partially explained by industrial spills of copper wastes that occurred around that time. However, relatively high concentrations appeared to persist in Torch Lake waters throughout the annual cycle. The vast quantities of crushed Cu-bearing ore that comprise the bottom sediments and line the shores show a Cu content ranging from 1300 to 3800 mg 1⁻¹. These materials act as a reservoir of Cu providing a continual supply of Cu to these waters. Laboratory leaching studies of these materials demonstrate that they can release potentially significant amounts of Cu when suspended in lake water.

Although the Cu levels found in Torch Lake exceed US EPA-recommended maximum allowable levels for these waters, there are reports of substantial fish and algal populations. Equilibrium calculations indicate the predominance of various soluble Cu species in the following order of abundance: Cu(OH)⁺ > Cu⁺⁺ > CuCO₃. However, Cu in these waters may not be controlled by solubility relationships of Cu compounds but rather by sorption onto surfaces of Fe and Mg hydrous oxides resulting in the occurrence of Cu in relatively non-toxic forms. Any additional mining or reclamation operations pose a potential hazard to aquatic ecosystems because of the wide-spread Cu contamination that already exists in the waters of this area.