Effluent Limits for Fluctuating Discharges: Phase II - Alternate Frameworks for Water Quality Criteria
This research evaluated appropriate methodologies to calculate water quality criteria for exposure concentrations that vary in frequency, magnitude and duration. We 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. Many workers demonstrated a relationship between organism response and internal contaminant concentration, independent of the exposure regime. Internal contaminant concentration has been used to predict organism responses in various models. Results of many laboratory studies indicate that for brief, episodic or high magnitude pollutant events, higher acute toxicity (or lower LC50s), is likely as compared to that predicted based on the averaged or long-term concentration. Our literature search, and evaluation of five case studies, revealed some cautions in using chemical benchmark (criteria) values and simple worst case dilution analysis to evaluate effluent compliance.
Our results indicate that magnitude or peak concentration, not duration above a certain threshold, was most important in predicting chronic toxicity to Ceriodaphnia. The relatively infrequent pollutant monitoring in National Pollutant Discharge Elimination System (NPDES) permits, and the general lack of instream contaminant data, means that chemical exceedance duration, or a time-integration approach to permit compliance, is unlikely if not infeasible.
Analysis of case studies illustrated the difficulties in using real world data to test alternate criteria approaches. To help address this problem, we designed and conducted several pulsed exposure chronic toxicity tests (based on standard EPA protocols), using Ceriodaphnia and fathead minnow. Testing used copper, acid (nitric), cadmium, sodium chloride, and Chlorpyrifos, all of which occur in wastewater effluents. For many of these chemicals, we observed little difference in acute and chronic toxicity thresholds, as measured by standard toxicity tests on sensitive species. This means that permit exceedences with a magnitude <5 times the chronic limit, may not yield chronic biological effects if relatively short in duration (<2 days).
Furthermore, multiple exceedences of a chronic limit may not result in discernable biological effects or biological effects will be predictable based upon expected effects of each exceedance individually. Simulation modeling indicated that both mortality and biomass results from chronic fathead minnow tests and cadmium were well described by an extended Mancini-Breck model, which was developed for acute toxicity effects.
Thus, for a number of common wastewater pollutants, a kinetics-based approach to criteria may be a reasonable and worthwhile goal. However, fluctuations close to the chronic limit may not have systematic effects because exposures are within the error limits of chronic test methods. Our research suggests that wastewater plants may have much to gain by implementing front-end equalization because: (1) it will reduce final effluent chemical fluctuations and therefore, permit exceedences; and (2) a more consistent quality effluent means a lower coefficient of variation for chemical concentration over time, which could, using a statistical approach to deriving permit limits, translate to a higher effluent limit.
This title belongs to WERF Research Report Series
ISBN: 9781843396499 (Print)
ISBN: 9781780403205 (eBook)
Download citation file: