Evaluation of water-effect ratio methodology for establishing site-specific water quality criteria

One approach outlined by the U.S. Environmental Protection Agency (U.S. EPA) for derivation of site-specific water quality criteria for metals in natural surface waters involves the development of water-effect ratios (WERs). This approach entails multiplying national water quality criteria by an experimentally derived WER, where the WER is defined as the ratio of the toxicity of the metal in the site water to the toxicity of the same metal in standard laboratory water. We discuss technical issues associated with test methods described in the U.S. EPA WER guidance document that may lead to inappropriate WERs. Critical issues include accounting for differences in calcium and magnesium concentrations (Ca:Mg ratios), alkalinity, and pH between site and laboratory waters; ensuring appropriate fish acclimation; and accounting for interspecies variability, multiple metals interactions, end-point variability, and temporal and spatial variability in the derivation of the WER. Failure to address these issues may have the unintended effect of deriving site-specific water quality criteria that are underprotective of aquatic life. We recommend that WER testing and future regulatory guidance for derivation of site-specific water quality criteria incorporate consideration of these potential confounding variables so that site-specific criteria can be established with greater confidence.