Modeling Frequency of Occurrence of Toxic Concentrations of Zinc and Copper in the Upper Animas River

Scientists participating in the USGS Abandoned Minelands Initiative have quantified metal concentrations and loadings from mining-related and natural background sources in the upper Animas River of southwestern Colorado, with the goal of guiding remediation decisions by federal landmanagement agencies. We have compared site-specific toxicity thresholds with frequencies of dissolved metal concentrations in stream water to evaluate the contributions of zinc and copper to toxic effects in fish and aquatic invertebrates in the upper Animas. Median lethal concentrations (LC50s) of zinc and copper were determined for fathead minnows, Pimephales promelas, and amphipods, Hyalella azteca, from seven-day toxicity tests under water quality conditions typical of the upper Animas. Frequency analysis based on hysteresis models was used to predict seasonal occurrence and daily probabilities of dissolved zinc and copper concentrations at two gaging stations near Silverton, Colorado. Results of these analyses indicate that dissolved zinc concentrations at both locations frequently exceed 7-day LC50s for amphipods, and occasionally exceed zinc LC50s for fathead minnows. In contrast, copper concentrations rarely approach lethal levels for either species. Model results are consistent with recent on-site toxicity tests with these two species. Comparison of modeled zinc and copper concentrations with published toxicity thresholds for brook trout, Salvelinus fontinalis, suggest that both zinc and copper contribute to chronic toxicity in resident trout in the upper Animas.