3. Because PBTs tend to distribute widely, if not uniformly, in aquatic habitats, uncertainty 
exists for the extent to which spatially explicit population models are requisite for problem 
formulations. 
4. A national WQC methodology for different classes of PBTs needs definition, through use of a 
generic population model, of the species characteristics, life stages, and toxicity effects that are 
most likely to determine risks to populations, regardless of site conditions. This information will 
provide the basis for determining site-specific model and data requirements for application of the 
criteria. 
Research Projects 
The research and development needs presented in the critical path constitute a very large effort 
encompassing integrated assessment methodology, generic assessment components applicable to 
many chemicals, and data or model needs for specific chemicals. Formulating a Goal 2 toxic 
chemicals research program for NHEERL requires consideration of what can be done with 
available resources to produce the most beneficial improvements in assessment methodology 
over the next several years, while also considering what research will be done in other goals and 
by other parties. The research presented here was selected to provide both general methods 
development and reduction of specific uncertainties. 
Overvietv of Projects-Nonbioaccumulative Toxicants 
At the most general level, there is a need to describe the overall framework and methods that 
should be used to improve aquatic risk characterizations and criteria development/application. 
This includes a short-term need, as represented by the initial step in Figure 9, to describe general 
approaches for improving assessments for nonbioaccumulative toxicants and the use of currently- 
available methods and knowledge, thus providing a starting point for criteria development and 
further research. There also is a longer-term need to periodically update the recommended 
framework and methodologies as research in various areas produces useful results. Project N1 
{Improved Risk Characterization Methods for Developing Aquatic Life Criteria for 
Nonbioaccumulative Toxicants) will address these needs and serve as a focal point for 
developing research projects, and applying and integrating their results. Initial products from this 
project will address APG 2, but efforts will continue under this project to synthesize results from 
other research, describe their application to assessments, and address, in whole or part, other 
APGs. This continuing work will be in large part in collaboration with OW efforts to revise their 
guidelines for WQC derivation. The nature and time line of additional products will be defined 
as these collaborative efforts develop during FY03. 
As discussed in the Critical Path subsection, more meaningful aquatic risk assessments require 
improved approaches and knowledge in various areas. Prominent among these are models that 
address the significance of toxicological responses at the population-level (APG 3). Projects that 
we will pursue in this area will only involve bioaccumulative toxicants (see below), but the 
results of such work will have relevance to nonbioaccumulative toxicants and will be part of 
continuing efforts in project Nl. Proposed research projects involving nonbioaccumulative 
toxicants involve the general topic area identified in APG 4 (extrapolation of toxicity among 
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