Whole Effluent Toxicity

Whole Effluent Toxicity (WET) is the aggregate toxic effect of an effluent measured directly with a WET test. A WET test (WETT) is a test that includes the use of aquatic, bacterial or vertebrate species to measure acute or chronic toxicity, and any biological or chemical measure of bioaccumulation, bioconcentration or impact on established aquatic and biological communities.

The advantage of WET testing is that it evaluates the integrated effects of all chemicals in an aqueous sample. The Department of Environmental Protection (DEP) began implementing WETT in the 1990s through its NPDES permitting program, and developed an interim implementation policy. In November 2012, DEP developed a WET Standard Operating Procedure (SOP) that provides new direction, and is based on EPA's latest advancements and DEP's implementation experience with the objective of promoting statewide consistency on decision-making.

As NPDES permits are reissued to permittees that require WET testing, DEP will establish permit conditions that implement the SOP. The SOP utilizes EPA's 2010 guidance, "National Pollutant Discharge Elimination System Test of Significant Toxicity Implementation Document (PDF)" (EPA 833-R-10-003), as its method for determining toxicity. When NPDES permits have the permit language referred to in the SOP, permittees and their accredited laboratories will need to use DEP's WET Analysis Spreadsheet to determine whether endpoints in a WET test "pass" or "fail." Nothing has changed in terms of the methods that must be used or the minimum requirements for accredited laboratories. The data analysis of WET test results will be changing, as well as decision-making during the permit term while tests are being conducted.

Once the new permit language has been established, permittees will need to submit a printout of the WET Analysis Spreadsheet and the WET Summary Report (3800-FM-BCW0485) as an attachment to the main WETT report. For permit renewal applications, permittees will need to submit a printout of the WET Analysis Spreadsheet that include all results for the latest four consecutive tests to facilitate DEP's review.

NOTE: If the permit pre-dates the SOP (November 2012), the determination of toxicity will be made based on the current permit language, i.e., comparison of the in-stream waste concentration to a point estimate derived from a statistical analysis of test results (NOEC, LC50). Permittees should always refer to their existing permit for the appropriate WET standards and requirements.

Permittees with questions on WET should contact the DEP regional office that issued the permit or DEP's Division of NPDES Permitting at 717-787-5017 or RA-EPNPDES_Permits@pa.gov. Laboratories with questions on WET and permittees interested in a list of laboratories accredited for WET testing in Pennsylvania should contact DEP's Bureau of Laboratories, Laboratory Accreditation Program, at 717-346-8209.