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

摘要

Water quality criteria for protecting aquatic life are based primarily on laboratory testing thatused constant exposure concentrations. However, actual impacts of toxics vary depending onchemical and flow characteristics of the waterbody and effluent. Typical effluent and nonpointsource exposure concentrations fluctuate in frequency, magnitude, and duration, often resultingin different toxicological impacts than experienced in constant concentration tests. There is acritical need to improve our understanding of the link between exposure and effects in real-worldfluctuating exposure scenarios in order to improve the predictive accuracy of pollutant limitcalculations and enable more appropriate, site-specific permit limits. Tetra Tech has currentlyreviewed over 30 toxicological studies that used either time-dose response or pulsed/intermittentexposure designs representing over 15 contaminants and 10 species. Most of these studies wereconducted using freshwater species and acute exposures. Our analyses of these studies showsthat pulsed or fluctuating exposures are often less toxic than continuous exposures, given thesame peak contaminant concentration, assuming that the contaminant is accumulated internallyby the organism or that damage to the organism is “accumulated” at a faster rate than tissuerepair. Current models for dealing with acute exposures appear to be fairly robust underfluctuating exposures, particularly if kinetic constants are based on normalized time and effectsdata. However, many fast-acting pollutants, that can cause immediate damage to gills and otherorgans, may have under-protective criteria if based on an average concentration over a 2-4 dayexposure. To test this possibility we examined daily ammonia data and multiple chronicCeriodaphnia WET tests for two California wastewater facilities. We observed that peakammonia concentration was related to Ceriodaphnia chronic endpoints, consistent with the fastactingnature of this contaminant. Experimental chronic tests conducted using fathead minnowsand either nitric acid or sodium chloride, also exhibited results consistent with fast-actingsubstances and little lag effect on mortality. In general, effects were observed within 48 hours ofpulse exposure and chronic or sublethal effects were not observed. Our results suggest thatchronic effects of fluctuating exposures of fast-acting contaminants can be predicted based oncurrent acute toxicological models.