Regulating Whole Effluent Toxicity Using “percent Effect” as the Test Endpoint

Traditionally, the No-Observed-Effect-Concentration (NOEC) or Inhibition Concentration (IC-25) are used to report the level of toxicity in effluent samples. These endpoints, and the methods used to calculate them, have several well known limitations that interfere with reasonable potential calculations, discharge limit development and permit compliance determinations. An alternative approach using “percent effect” as the test endpoint provides a more useful and intuitive method for implementing whole effluent toxicity limits in NPDES permits. The alternative procedure has been successfully applied in South Carolina.