Phase 2: alternate frameworks for water quality criteria
incorporating chemical frequency, magnitude and duration 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.
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