range of effects, including their uncertainties, and sufficiently account for important site 
characteristics. The resultant risk descriptions should allow risk managers to assess the 
implications of different exposure scenarios and, once acceptable risks are specified, to back- 
calculate acceptable loadings (dotted line on Figure 8). 
Critical research paths also should recognize the need to implement progressive improvements in 
the assessment methodology. Figure 9 shows four steps in such improvement, which largely 
parallel the APGs described in the preceding Goals subsection. The first step will demonstrate 
how criteria can reflect more comprehensive risk characterizations, based on more complete 
descriptions of organism-level dose-response relationships and how they vary with exposure 
conditions. The second step will involve the development and application of methods to address 
population-level risks. The third step will involve the development of extrapolation methods so 
that the risks can be assessed for chemicals or situations for which limited data is available. The 
fourth step will include further enhancement of these techniques and emphasize their 
incorporation into multistressor assessments at various spatial scales. 
Demonstrate methods for improved criteria 
at the individual level based on 
improved characterization of risks 
- 4 ^ 
Develop methods to link individual-level data 
to population-level endpoints 
4 . 
Develop methods to support risk assessments 
for chemicals with limited data 
4 . 
Develop methods to evaluate risks on 
populations at various spatial scales, 
in the context of other stressors 
Figure 9. Critical path for developing site-specific methodologies for establishing the risks of 
toxic chemicals to aquatic life and aquatic dependent wildlife. 
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