The key change to note is how toxicity results will be handled under the new provisions. Toxicity test data will be analyzed with one single statistical test, the Test of Significant Toxicity (TST), for the purpose of determining permit compliance. The incorporation of the TST is meant to simplify and standardize the answer to a toxicity test – whether or not a discharge from a water treatment plant going into the receiving environment is toxic to the organisms living in the environment. Of course, in some cases, the answer is not that simple….

As we discussed in our previous blog post, toxicity tests are biological tests based on methods that vary depending on region, species, and test endpoints measured. The TST functions best with datasets where the variability is low, thus rewarding discharges who contract laboratories with a high level of precision. However, some biological endpoints are more inherently variable than others. Therefore, dischargers should work with their laboratory or environmental toxicology expert to understand the implications of the TST on their individual circumstances and use test designs that reduce their chance of producing a false-positive result.