John Wiley & Sons, Ltd.

Development of a regression model to predict copper toxicity to Daphnia magna and site‐specific copper criteria across multiple surface‐water drainages in an arid landscape

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The water effect ratio (WER) procedure developed by the U.S. Environmental Protection Agency is commonly used to derive site‐specific criteria (SSC) for point‐source metal discharges into perennial waters. However, experience is limited with this method in ephemeral and intermittent systems typical of arid climates. This study presents a regression model to develop WER‐based SSC for a network of ephemeral and intermittent streams influenced by nonpoint sources of Cu in the southwestern USA. Acute (48‐h) Cu toxicity tests were performed concurrently with Daphnia magna in site‐water samples and hardness‐matched laboratory waters. Median effect concentrations (EC50 values) for Cu in site‐water samples (n = 17) varied by more than 12‐fold, and the range of calculated WER values was similar. Statistically significant (α = 0.05) univariate predictors of site‐specific Cu toxicity included (in sequence of decreasing significance): DOC, hardness/alkalinity ratio, alkalinity, K, and total dissolved solids. A multiple‐regression model developed from a combination of DOC and alkalinity explained 85% of the toxicity variability in site‐water samples, providing a strong predictive tool that can be used in the WER framework when deriving SSC values. The biotic ligand model (BLM) under‐predicted toxicity in site waters by more than 2‐fold. Adjustments to the default BLM parameters improved the model's performance but did not provide a better predictive tool compared to the regression model developed from DOC and alkalinity. Environ Toxicol Chem © 2014 SETAC

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