Toxicity to Fathead Minnows from Naturally-Occurring Pathogens

The fathead minnow (Pimephales promelas) is the most common fish species used in whole effluent toxicity (WET) tests in North America. Along with the water flea, Ceriodaphnia dubia, the fish is included in hundreds of effluent permits as a tool for meeting discharge standards and complying with the Clean Water Act goal of no discharge of toxic pollutants in toxic amounts. The South Fort Polk Wastewater Treatment Plant (SFWWTP) at the Fort Polk Joint Readiness Training Center experienced significant toxicity (lethality) to fathead minnows (Pimephales promelas) in their chronic whole effluent toxicity (WET) tests. Per the facility’s permit, a Toxicity Reduction Evaluation (TRE) was required. An important aspect of the TRE was a Toxicity Identification Evaluation (TIE), which was conducted at AECOM’s Environmental Toxicology Laboratory in Fort Collins, Colorado. Initial studies, as well as historical data, indicated a highly seasonal pattern of toxicity, with the WET permit violations occurring only in the spring. Two 2007 TIE studies were inconclusive regarding the cause of the observed mortality. A third TIE in the spring of 2008 confirmed that certain manipulations that are typically effective in removing pathogens eliminated significant mortality to fathead minnows in the SFWWTP effluent. Tests using effluent from the chlorine contact basin confirmed that the cause of mortality was not originating from the treatment plant itself, but was entering the wastewater downstream. Phase II and Phase III studies found that toxicant could be recovered and reintroduced into clean water. Microbiological analysis confirmed the presence of several microbial pathogens that could be causing the observed fish mortality.  The cause of the mortality was identified as “Sporadic Mortality Phenomenon” (SMP) and AECOM recommended allowing the use of an altered test configuration for the fathead minnow WET test.