Toxicological Effects of Fluctuating Contaminant Exposures in Relation to Water Quality Criteria and NPDES Permit Limits

Water quality criteria for protecting aquatic life are based primarily on laboratory testing that used constant exposure concentrations. However, actual impacts of toxics vary depending on chemical and flow characteristics of the waterbody and effluent. Typical effluent and nonpoint source exposure concentrations fluctuate in frequency, magnitude, and duration, often resulting in different toxicological impacts than experienced in constant concentration tests. There is a critical need to improve our understanding of the link between exposure and effects in real-world fluctuating exposure scenarios in order to improve the predictive accuracy of pollutant limit calculations and enable more appropriate, site-specific permit limits. Tetra Tech has currently reviewed over 30 toxicological studies that used either time-dose response or pulsed/intermittent exposure designs representing over 15 contaminants and 10 species. Most of these studies were conducted using freshwater species and acute exposures. Our analyses of these studies shows that pulsed or fluctuating exposures are often less toxic than continuous exposures, given the same peak contaminant concentration, assuming that the contaminant is accumulated internally by the organism or that damage to the organism is “accumulated” at a faster rate than tissue repair. Current models for dealing with acute exposures appear to be fairly robust under fluctuating exposures, particularly if kinetic constants are based on normalized time and effects data. However, many fast-acting pollutants, that can cause immediate damage to gills and other organs, may have under-protective criteria if based on an average concentration over a 2–4 day exposure. To test this possibility we examined daily ammonia data and multiple chronic Ceriodaphnia WET tests for two California wastewater facilities. We observed that peak ammonia concentration was related to Ceriodaphnia chronic endpoints, consistent with the fastacting nature of this contaminant. Experimental chronic tests conducted using fathead minnows and either nitric acid or sodium chloride, also exhibited results consistent with fast-acting substances and little lag effect on mortality. In general, effects were observed within 48 hours of pulse exposure and chronic or sublethal effects were not observed. Our results suggest that chronic effects of fluctuating exposures of fast-acting contaminants can be predicted based on current acute toxicological models.