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Toxicity of Metals in Water and Sediment to Aquatic Biota

We evaluated the toxicity of metals to aquatic biota of the upper Animas River and the lower reaches of Cement and Mineral Creeks based on toxicity tests with field-collected stream water and sediment, laboratory tests with selected metals of concern, and site-specific models of seasonal metal concentrations and metal toxicity thresholds. Stream water and sediment from several sites were toxic to amphipods, Hyalella azteca. However, stream water was less toxic than sediment, and the toxicity of stream water was reduced in the presence of sediment, presumably due to sorption of dissolved metals. The toxicity of stream water to amphipods and fathead minnows, Pimephales promelas, differed seasonally: toxicity was greater during late winter and early spring (before snowmelt) than in late spring or summer. Toxicity of stream water was associated with elevated concentrations of several dissolved metals, especially zinc and copper. Laboratory tests demonstrated that dissolved zinc was highly toxic to amphipods, less toxic to fathead minnows, and least toxic to brook trout, Salvelinus fontinalis, the predominant fish species in the Animas River watershed study area. Significant toxic effects of zinc on early life stages of brook trout occurred at concentrations of 960 μg/L or greater. In contrast, both brook trout and fathead minnows were highly sensitive to copper; significant reductions in growth occurred at concentrations less than 10 μg/L. Models of the seasonal toxicity of zinc and copper in stream water were developed for three gauging stations near Silverton, Colorado. These models predicted extended periods of zinc toxicity to amphipods at all three sites and seasonal toxicity of copper to fathead minnows and brook trout in lower Mineral Creek (year-round) and in the Animas River downstream of Silverton (in winter). Predictions of these models were consistent with results of toxicity tests with stream water and with surveys of stream biotic communities, which suggests that dissolved copper concentrations in the study area are a significant limiting factor for brook trout and that dissolved zinc concentrations may limit populations of sensitive invertebrate taxa.