based criteria for chemicals will be the generation of new data for several steps on the critical 
path. For example, risk-based criteria will require data on dose-response relationships for 
various chemicals exposed to aquatic life and wildlife, but very few existing chronic exposure 
toxicity tests generate dose-response relationships, and we are not proposing new testing. Also, 
we are developing the basis for PBTK modeling that will be applied to a demonstration project 
for extrapolating toxicity estimates to other species, but more will be needed for generating 
physiological and metabolic information for broader applications of PBTK models. Finally, our 
demonstrations projects will use existing life history information, such as survival and fecundity 
rates, for setting parameters in population models, but scientifically-defensible population 
models for most aquatic and wildlife species will require new life history data that will not be the 
focus of this research program. These needs for new data for various models will be discussed 
further below. 
Specific gaps are related to key components of the conceptual models for nonbioaccumulative 
and bioaccumulative toxicants (Figures 10 and 11). They are organized under the following 
headings: bioavailability, dosimetry and bioaccumulation, toxicity, and population models. 
Bioavailability 
One gap relates to the need to better understand exposure, which is outside of NHEERL 's 
mission: 
1. Exposure models are needed that can estimate physical transport and fate of chemicals. This 
includes persistence/degradation, partitioning, and especially chemical forms reaching aquatic 
organisms. Improved models will be needed for predicting chemical forms of metals and 
activities of organic chemicals in water, bulk sediments, and associated pore waters. 
Two gaps relate to the needs for extending our modeling approaches to other chemicals and 
exposure scenarios: 
2. Although this plan will, initially, develop models for ammonia and metals, gaps will remain 
for applying these models to other nonbioaccumulating chemicals. Organic chemicals with low 
bioaccumulation potential are an important class of chemicals which pose ecological risks 
through a variety of mechanisms of toxicity including disruption of endocrine functions. Present 
and future research in and outside of NHEERL on the effects of these chemicals requires models 
and data for exposure and bioavailability appropriate for linking the toxicology research results 
into criteria development and ecological risk assessment procedures. 
3. Seasonal and spatial variability in bioavailabilities of chemicals, coupled with life stage 
changes, population movements, and mechanisms for avoidance of exposure, produce 
complications for determination of species vulnerabilities. This gap is presently being addressed 
only in the proposed research for loons (project B3). 
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