It is time for changes in the analysis of whole effluent toxicity data.

The whole effluent toxicity (WET) program in the United States, Canada, and other countries typically requires multi concentration testing of effluents. While multiconcentration testing of chemicals is desirable for regulatory and scientific reasons, we believe this requirement is not as efficient for evaluating effluent compliance in a WET program. The key regulatory question of concern is whether an effluent is toxic or not, which is best answered statistically using a hypothesis approach, not a point estimate approach. However, the traditional hypothesis approach currently recommended does not reward high within-test precision. This report describes the need for 3 specific changes in the analysis of WET compliance data that we believe would yield a more robust WET regulatory program: (1) restate the null hypothesis so that test power is associated with demonstrating that the effluent is not toxic, (2) use USEPA's Test of Significant Toxicity (based on the noninferiority approach) to identify unacceptable toxicity as well as acceptable effects with a high probability, and (3) evaluate only the test control and the critical concentration of concern (e.g., instream waste concentration). We demonstrate that instituting these 3 changes would provide: Positive incentives for permittees to produce high-quality WET data, a transparent analysis approach in which the permittee could have greater control over regulatory decisions based on test results, and potentially a less expensive testing program because fewer effluent concentrations need to be examined within a test. As a result, WET test frequency could be increased for the same cost as current testing programs while providing greater representativeness of effluent quality.