EVALUATION OF COPPER-DISSOLVED ORGANIC MATTER BINDING IN SURFACE WATERS AT THE PROPOSED PEBBLE MINE IN ALASKA

The Pebble Mine project is a world-class proposed copper and gold mine in southwestern Alaska that is upstream of one of the most productive salmon fisheries in the world. Understanding the bioavailability and toxicity of copper to salmonids is critical for evaluating the potential environmental effects of the development of this mine. Calcium, pH and dissolved organic matter (DOM) exert the primary controls on copper toxicity through geochemical processes related to competition for fish gill uptake sites and the formation of less toxic complexed copper species (free copper is the most toxic). The strength and abundance of DOM’s metal binding sites can influence copper toxicity.

To understand and quantify copper-DOM binding in waters at the proposed Pebble Mine site, we isolated DOM from three rivers on the site – the North Fork Koktuli, Upper Talarik, and South Fork Koktuli Rivers – and measured the strength of copper binding by DOM using two methods. The three rivers had low dissolved organic carbon concentrations (2.6-4.4 mg C/L) and low alkalinity. Hydrophobic acids (humic and fulvic acids) were isolated from the three river waters for the copper binding studies, which were conducted at pH 6.0. An ion-selective electrode was used to measure free copper for copper titrations that spanned a Cu:DOM ratio of 0.003-0.3. For the copper titrations, conditional binding constants were determined for a two-ligand model using FITEQL. Similar binding constants were found for the three river waters: a log K₁ of 6.9-7.2 and a log K₂ of 4.7-5.4. For Cu:DOM ratios closer to baseline conditions (0.0001 to 0.01), competitive ligand exchange-solid phase extraction (CLE-SPE) experiments were conducted to measure conditional binding constants. The strength of copper binding to the DOM isolates increased as the ratio of Cu:DOM decreased. The conditional binding constants were used to model copper bioavailability to resident fish populations under baseline and potential operational conditions for the proposed Pebble Mine project.