Update on Whole Effluent Toxicity and Reasonable Potential Determination

1. Update on Whole Effluent Toxicity and Reasonable Potential Determination

2. Reasonable Potential • 40 CFR § 122.44(d)(1)(v) “(W)hen the permitting authority determines, using the procedures in paragraph (d)(1)(ii) of this section, toxicity testing data . . . that a discharge causes, has the reasonable potential to cause . . . an in-stream excursion above a narrative criterion within an applicable State water quality standard, the permit must contain limits for (WET).”

3. ReasonablePotential Number of failures • Zero • One or two • Three or more

4. Reasonable Potential • Originally, RP was not determined for WET during the application process • WET was a monitoring requirement • A TRE was required for persistent significant lethality • At the end of a TRE a WET limit could be included in the permit

5. Sublethal Toxicity • In 2007 EPA began to objecting to permits being issued with no additional requirements for permittees that had a history of sublethal failures • Added a “trigger” for a sublethal TRE • Eventually lead to SL WET limits

6. RP for WQBELs • A single sample approaching or in excess of the calculated limit does not automatically require a limit • Samples may be averaged • Different than federal methodology

7. RP for WET • Zero failures, standard 5 year permit • One or two failures, 3 year permit, monthly testing after a failure • Three or more failures, 5 year permit with a WET limit, optional compliance period

8. SUMMARY • EPA and TCEQ signed letters on December 28, 2015, agreeing to an RP policy • One or two failures will lead to 3-year permits that will require increased testing for any additional failures. • A WET limit will be included in the next permit if three or more failures occur during the 3-year permit term. • Michael Pfeil • (512) 239-4592