Section 7. 
Implementation Plan for Toxic Chemicals Research 
Problem 
Effective management of toxic chemicals in aquatic ecosystems requires a capability to 
quantitatively predict the ecological effects that can be expected from different levels of chemical 
contamination of water, sediments, and food chains. Procedures for deriving aquatic life WQC 
have existed for many years (EPA 1973, 1980, 1991, 1994, 1995a; Stephan et al. 1985) and have 
been useful for managing toxic chemical inputs to aquatic systems. However, these procedures 
are based on simplifying assumptions and a relatively narrow framework that limit their use in 
fully assessing the risk of a wide range of toxic chemicals to both aquatic life and aquatic- 
dependent wildlife. Sediment guidelines developed more recently (EPA 2000a,b,c,d) have many 
of the same limitations as WQC. To address some of these concerns, NHEERL has prepared a 
draft wildlife research strategy for assessing risks of multiple stressors to populations of 
amphibians, birds, and mammals (EPA 2000e). 
Criteria derivation and application require extrapolations of toxicological effects observed in the 
laboratory to field conditions, which can result in significant uncertainties. Differences in water 
characteristics, chemical partitioning, routes of exposure, organism habits, and exposure time- 
series can greatly affect the relationship between exposure concentrations and a chemical’s 
toxicity, and thus affect the applicability of criteria to natural ecosystems. This is particularly 
true for PBTs, for which effects often depend on tissue residues accumulated in tissues over long 
times as a result of multiple exposure routes. Moreover, criteria often do not address the 
combined effects of multiple chemicals and other stressors, and can lack information for 
potentially sensitive life stages of test species. 
Other uncertainties in criteria arise from the use of organismal-level toxicity to set concentrations 
protective of aquatic populations and communities. The relationship of toxic effects on 
individual organisms to population responses is not well established. Important taxa and 
endpoints can be missing from the sets of tests used to develop criteria and sediment guidelines. 
Indirect effects of chemicals on organisms (effect on food sources, competition, predation, and 
shelter) generally are not considered. 
Criteria are also limited in that they address only specific water concentrations, rather than 
complete dose-response relationships, thus limiting how well risks can be characterized. 
Seasonal issues and the significance of the spatial extent of exposures are incompletely 
addressed. Current criteria procedures also do not include uncertainty analyses or address how 
well risk can be assessed in the presence of limited data. 
Because of these limitations, efforts are needed to develop methods to better characterize risks to 
aquatic life and aquatic-dependent wildlife populations and communities, and to apply these risks 
to criteria development. Assessment endpoints should be better defined and an analytical 
firework developed for linking available data to a more complete and accurate description of 
risks for these endpoints. This assessment framework should describe a range of responses, be 
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