ARCHIVE SITE - Last updated Jan. 19, 2017. Please visit for the latest NACWA information.


NACWA White Paper on Whole Effluent Toxicity Permit Testing and Limitations for Clean Water Agencies


EPA’s National Pollutant Discharge Elimination System (NPDES) program regulations require "whole effluent toxicity" (WET) effluent testing by publicly owned wastewater treatment facilities (POTWs) during each permit cycle. The regulations also require (with certain exceptions) that NPDES permits include limitations for WET, if WET data demonstrate that the facility has “reasonable potential” (RP) to cause or contribute to a receiving water excursion above either (1) numeric state water quality standards for WET or (2) the state’s narrative (“no toxics in toxics amounts”) water quality standard. These requirements are of substantial concern for clean water agencies because (1) WET permit limitations often pose difficult compliance challenges, (2) it is difficult and expensive to determine and/or correct the underlying cause of WET exceedances, (3) testing is expensive, and (4) like any permit requirement, once imposed, limitations are difficult to ever remove.

The National Association of Clean Water Agencies (NACWA), along with several state POTW organizations and industrial groups, challenged in federal court EPA’s 2002 repromulgation of WET test methods. For strategic reasons the appeal focused on the most problematic sublethal endpoints within the chronic methods. NACWA believes particularly that for those methods and endpoints, the test methods are simply not accurate or precise enough for the NPDES program uses called for by EPA and state regulations, and they do not accurately predict receiving water conditions or “toxicity.” This is especially so where there are relatively low levels of “toxicity,” as is often the case with POTW effluents. Although the appeal was aggressively pursued, the Court of Appeals for the District of Columbia Circuit upheld the test methods in December, 2004. Nonetheless, the decision supported only the methods themselves and specifically did not support any particular NPDES permitting use of WET.

Based on the WET Court Decision (Appendix A) and the experience that NACWA members have gained in dealing with WET permitting among the states and EPA Regions, NACWA has assembled this White Paper. The White Paper includes guidance for working with NPDES permit agencies in developing (1) WET testing conditions, (2) endpoints for judging results, and (3) permit conditions that provide for (based on WET results) moving to less frequent routine testing, toxicity determination procedures, and numeric permit limits when justified. This White Paper also provides suggested NPDES permit language (in Appendices B-D) taken from permit language currently used by the states.

  • Appendices (all in icon-pdf)
    • Appendix A 
      • December 2004 Court of Appeals Decision
    • Appendix B 
      • Region 3 Monitoring
      • Region 4 Monitoring
      • Region 6 Monitoring
      • Region 9 Monitoring
    • Appendix C 
      • Region 2 TRE
      • Region 4 TRE
      • Region 5 TRE
      • Region 6 TRE
      • Region 7 TRE
    • Appendix D 
      • Region 4 Limits
      • Region 5 Limits
      • Region 9 Limits
Sign In ▼

Upcoming Events

Winter Conference
Next Generation Compliance …Where Affordability & Innovation Intersect
February 4 – 7, 2017
Tampa Marriott Waterside Hotel
Tampa, FL