Food chain analysis of exposures and risks to wildlife at a metals-contaminated wetland

A food chain analysis of risks to wetland receptors was performed in support of a baseline ecological risk assessment at the Milltown Reservoir Sediments Superfund site in Montana. The study area consisted of over 450 acres of primarily palustrine wetland contaminated with metals from mining wastes transported from upstream sources (average of 465 mg/kg for Cu in sediments, and 585 mg/kg in soils). The food chain analysis focused on several species of terrestrial and semi-aquatic animals indigenous to montane wetlands of the northern Rocky Mountains. Receptors consisted of mice, voles, muskrats, beaver, various waterfowl species, osprey, bald eagles, and deer. Samples of aquatic and terrestrial invertebrates, small mammal tissues, fish tissue, aquatic and terrestrial vegetation, soils, sediment, and surface water were collected and analyzed for As, Cd, Cu, and Zn. A linear multimedia food-chain model was constructed to estimate daily intakes of the metals for each receptor, with assumed values for ingestion of aquatic and terrestrial food items, ingestion of local surface water, and incidental ingestion of soils and/or sediments. Evaluation of health risks to the receptors was performed by comparison of exposures expressed as daily intakes to a suite of toxicity values. The range of values consisted of the lower end of chronic toxicity data found in toxicology databases or the literature for the same or similar species, modified to account for extrapolation uncertainties. Daily intakes of chemicals of concern were below or within the range of toxicity values for all receptors. The weight of evidence from the food chain analysis and earlier bioassessment and ecological studies suggest that the health of the wetland receptors is at minimal risk due to the presence of elevated metals in sediments, upland soils, water, or food items at the site.