Water Effects Ratio (WER)
According to Rule 2.508, toxic substances criteria may be adjusted by a site specific Water Effects Ratio (WER) as defined in 40 C.F.R. Part 131.36 (c).
The WER approach compares bioavailability and toxicity of a specific pollutant in receiving water and in laboratory test water. It involves running toxicity tests for at least two species, measuring LC50 for the pollutant using the local receiving water collected from the site where the criterion is being implemented, and laboratory toxicity testing water made comparable to the site water in terms of chemical hardness. The ratio between site water and lab water LC50 is used to adjust the state acute and chronic criteria to site specific values.
For more details refer to the State Of Arkansas Continuing Planning Process (CPP).
WERs are not appropriate for site-specific criteria development for copper, cadmium, nickel, lead, silver, or zinc. The Biotic Ligand Model (BLM) should be used for these parameters.