Release of Copper from Mine Tailings on the Keweenaw Peninsula

Abstract

Over 500 million tons of copper-rich mine tailings were dumped into lakes, rivers, wetlands, and along the shore of Lake Superior between 1850 and 1968. Metals leaching from mine residues have impacted ecosystems throughout the Keweenaw Peninsula as well as Lake Superior. The objective of this study was to elucidate the chemical processes that release Cu from mine tailings into the water. Copper in mining residues from three contrasting environments (lake sediments, wetland stamp stands, and exposed lakeshore tailings piles) was fractionated with a sequential extraction technique (SET) to identify and quantify the labile pools of copper. The SET revealed that the carbonate and oxide fractions were the largest pools of Cu (ca. 50 ∼ 80%) in lakeshore and wetland stamp sands whereas the organic matter fraction was the largest reservoir (ca. 32%) in the lake sediments. X-ray diffraction and SEM confirmed the presence of the copper-bearing minerals cuprite, tenorite, malachite, and elemental Cu. Size fractionation studies suggested that weathering of native (elemental) Cu results in enrichment of particle surfaces with Cu oxides and carbonates; fine particles also are enriched in these phases. Both laboratory titrations and computer modeling suggested that aqueous Cu concentrations are limited by mineral (malachite and copper(II) oxides) dissolution and precipitation reactions. Concentrations of DOC and pH depressions caused by microbial activity strongly affect the dissolved Cu concentrations. At some sites, aqueous concentrations of copper approach equilibrium with a Cu oxyhydroxide that has a solubility intermediate between that of cupric hydroxide (Cu(OH) 2) and tenorite (CuO).