Many surface waters are impacted by elevated concentrations of multiple metals resulting from acid mine drainage and industrial inputs. Because these elevated concentrations of metal mixtures sometimes can be toxic to aquatic life, there is a need to better understand and predict the toxicity of metal mixtures in the environment. Laboratory toxicity tests are important for providing data to develop multi-metal toxicity models. However, many laboratory toxicity tests do not closely mimic the aqueous geochemistry at contaminated sites. Therefore, it is difficult to accurately predict the toxicity of metal mixtures in the field. We investigated differences in the toxicity of Cd, Cu, Ni, and Zn to neonates of a freshwater invertebrate (Daphnia magna) in laboratory waters and in waters from the Clear Creek Superfund Site, a Colorado stream impacted by acid mine drainage. Aquatic chemistry characteristics such as water hardness, alkalinity, and the concentration and composition of dissolved organic matter exert important controls on aquatic toxicity, and our preliminary results suggest that the toxicity of metal mixtures in many routinely-used laboratory waters does not resemble the toxicity observed in natural waters that are contaminated with the same metals. Median lethal concentrations (LC50 values) of Cd, Cu, Ni, and Zn in laboratory toxicity tests performed in USEPA moderately hard reconstituted water differed substantially from LC50 values in toxicity tests conducted in site-simulated waters and in waters collected from the field. For example, toxicity to D. magna is considerably less-than-additive in binary-metal tests with Cd and Zn in USEPA moderately hard reconstituted water. We also observed less-than-additive toxicity of Cd and Zn in site-simulated and field-collected waters, but the effect is significantly dampened due to the atypical chemistry in the lab water. This study highlights the importance of water chemistry when extrapolating results of metal-mixture toxicity tests from the laboratory to the field, and it emphasizes the need for predictive models that incorporate geochemical influences on the toxicity of metal mixtures.

D. Magna metal toxicity in a mining impacted stream: Comparing results in simulated and field-collected waters

249th National Meeting (2015)

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