Integrated laboratory and field approach for assessing impacts of heavy metals at the Arkansas River, Colorado

This research employed an integrated laboratory and field approach to assess effects of heavy metals at the Arkansas River, a Colorado stream impacted by historic mining operations. Ambient metal levels, chronic toxicity, metal bioaccu-mulation by benthic organisms, and benthic community structure were examined at stations located upstream and downstream from California Gulch (CG), a U.S. Environmental Protection Agency Superfund site near Leadville, Colorado. Although each approach demonstrated effects of metals from CG, important differences among approaches were observed. Ambient metal concentrations and chronic toxicity tests conducted with Ceriodaphma dubia indicated reduced water quality at upstream and downstream stations. Because of temporal variability in metal levels, water samples collected for chronic toxicity tests did not reflect average exposure conditions. Instream monitoring of benthic communities was necessary to characterize metal impacts at the Arkansas River, but this approach had limitations. Levels of metals in periphyton and benthic invertebrates were significantly elevated at stations downstream from CG. Variation in metal levels among taxa and the absence of some taxa from some stations may limit the use of bioaccumulation studies for monitoring metal impacts. Benthic community structure was altered downstream from CG, but some metrics (e.g., species richness, abundance) were not affected by heavy metals because of the replacement of sensitive taxa by tolerant taxa. Because each of the approaches employed in this study provided different information on the degree of metal impacts, we recommend an integrated approach for assessing effects of metals on streams.