Jerome Diamond | PubMed Health

PubMed2005

Effects of pulsed contaminant exposures on early life stages of the fathead minnow.

Water quality standards for protecting aquatic life are based primarily on laboratory tests that use constant exposure concentrations. Typical effluent and nonpoint source exposure concentrations fluctuate in frequency, magnitude, and duration, which may result in different toxicological impacts. Current information indicates that pulsed or fluctuating exposures are generally more toxic than continuous exposures, when averaged over the applicable time period. However, few studies have evaluated chronic or sublethal effects of pulsed exposures, particularly those applicable to wastewater discharge situations. To address this issue, several pulsed exposure toxicity tests were conducted using modified fathead minnow (Pimephales promelas) early life stage (7 d) tests and several chemicals representative of those commonly encountered in wastewater effluents including copper, nitric acid, cadmium, and sodium chloride. Results suggest that survival and/or growth effects depend on the combination of frequency, magnitude, and duration, as well as the type of chemical. Nitric acid and sodium chloride pulsed treatments did not exhibit growth effects independent of survival effects, but both metals did elicit only growth effects in some treatments. Growth effects were related to pulse frequency and duration for copper and pulse duration and magnitude for cadmium. A 12-h exposure of approximately five times the 7-d continuous exposure IC(25) concentration of either metal quickly elicited mortality responses. Prolonged (>24 h) lag effects on survival were not observed in any of the experiments, regardless of the contaminant tested. Our results suggest that current water quality criteria may be underprotective if based on an average concentration over a 2-4 d exposure. For these contaminants, a short-term increase in concentration may elicit effects even though the average concentration is within nontoxic ranges for the organism.

Animals↗

PubMed2005

Improving the TMDL process using watershed risk assessment principles.

Ecological risk assessment (ERA) evaluates potential causal relationships between multiple sources and stressors and impacts on valued ecosystem components. ERAs applied at the watershed scale have many similarities to the place-based analyses that are undertaken to develop Total Maximum Daily Loads (TMDLs), in which linkages are established between stressors, sources, and water quality standards, including support of designated uses. TMDLs focus on achieving water quality standards associated with attainment of designated uses. In attempting to attain the water quality standard, many TMDLs focus on the stressor of concern rather than the ecological endpoint or indicators of the designated use that the standard is meant to protect. A watershed ecological risk assessment (WERA), at least in theory, examines effects of most likely stressors, as well as their probable sources in the watershed, to prioritize management options that will most likely result in meeting environmental goals or uses. Useful WERA principles that can be applied to TMDL development include: development and use of comprehensive conceptual models in the Problem Identification step of TMDLs; use of a transparent process for selecting Numeric Targets for TMDLs based on assessment endpoints derived from the management goal or designated use under consideration; analysis of co-occurring stressors likely to cause beneficial use impairment based on the conceptual model; use of explicit uncertainty analyses in the Linkage Analysis step of TMDL development; and frequent stakeholder interactions throughout the process. WERA principles are currently most applicable to those TMDLs in which there is no numeric standard and, therefore, indicators and targets need to be developed, such as many nutrient or sediment TMDLs. WERA methods can also be useful in determining TMDL targets in situations where simply targeting the water quality standard may re-attain the numeric criterion but not the broader designated use. Better incorporation of problem formulation principles from WERA into the TMDL development process would be helpful in improving the scientific rigor of TMDLs.

Ecology↗