Water Effect Ratio and Ambient Water Quality Criteria

The water effect ratio (WER) is defined as the ratio of the toxicity of a chemical in site water to the toxicity of the same chemical in standard laboratory water. Because standard laboratory water would have been used to generate toxicity data used to calculate State or Federal Water Quality Criterion, a WER which is greater than or less than 1 would infer that the chemical would be more or less toxic in site water. Therefore, the ambient water quality standard might be adjusted to meet the same aquatic life protection goals. The water effect ratio is developed to compensate for site-specific biogeochemical factors such as hardness, alkalinity, organic carbon, etc. which can influence the bioavailability and toxicity of chemical.

In practice, WER are often used to generate site-specific water quality standards that are higher than State or Federal standards.

The process of generating and using WER in the NPDES permitting process requires close coordination with the permitting agency. Work to prepare acceptable WER may require water quality monitoring and laboratory toxicity tests. NPDES permits issued using a WER may also include additional receiving water monitoring requirements.


Caltha LLP provides specialized expertise to clients nationwide in the evaluation and use of WER, and site-specific water quality standards. Caltha staff have prepared National Ambient Water Quality Criteria for US EPA and provide an expert resource for permittees and/or their technical consultants as they address State and Federal water quality standards.

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For further information contact Caltha LLP at
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