exposure conditions and biological endpoints). This encompasses many important issues, and 
three specific areas were selected which address major uncertainties or gaps for criteria, which 
are feasible with available resources, and which are not being resolved to our knowledge in other 
research programs. 
Methods which can extrapolate toxicity data across different endpoints, species, and life-stages 
can benefit criteria development and application in two major ways. First, the importance of 
endpoints or organisms missing from toxicological data sets used for criteria development can be 
estimated (e.g., endangered species). Second, criteria for some chemicals could be developed 
from more limited data sets than generally required. Project N2 {Methods for Extrapolating 
Chemical Toxicity Data Across Endpoints, Life Stages, and Species Which Can Sipport 
Assessment of Risks to Aquatic Life for Chemicals with Limited Data) will address certain 
methods for conducting such extrapolations. 
The toxicities of some contaminants are particularly sensitive to exposure conditions, sometimes 
varying by orders of magnitude. Even with a better risk assessment framework and other tools, 
good risk assessments of such contaminants are not possible without resolution of the effects of 
various exposure parameters on toxicity. A particularly noteworthy uncertainty is the 
bioavailability of metals. The Office of Water is cumently supporting development of BLM, 
which addresses the effects of various chemical constituents on metal toxicity and is based in part 
on previous work in this area by NHEERL. Because of these efforts and various industry-funded 
research projects, further work on water-borne metal bioavailability is not being proposed here. 
Rather, work is proposed in areas that represent critical knowledge gaps that are receiving less 
attention. 
Toxicological responses of organisms in or on sediment are affected by temporal and spatial 
variations in chemical concentrations and speciation in the sediment/water boundary zone. This 
is true in laboratory test systems and even more so under field conditions, making the 
interpretation and application of toxicity data highly uncertain. This area of concern will be 
addressed in project N3 {Assessing the Significance of Non-equilibrium Conditions on Aquatic 
Guidelines to Better Predict Field Effects). (Note : Because of resource reductions after the initial 
preparation of this plan, this project will not be pursued at this time.) 
Aquatic life criteria for metals have generally assumed that exposure is predominantly via water, 
with dietary exposure being negligible; however, some work has indicated that this assumption is 
not true and that risk might be substantially underestimated. Project N4 {Risks of Heavy Metals 
to Aquatic Organisms from Dietary Exposures) will address this area of uncertainty for metals 
risk assessment and should also provide results and insights that can help address this issue for 
other nonbioaccumulative toxicants. 
Overview of Projects-Bioaccumulative Toxicants 
The residue-based toxicity approach for PBTs embodied in the conceptual model for ecological 
risk assessments and criteria development (Figure 11) is the foundation for advancing methods 
that will effectively link PBT loadings to aquatic ecosystems, or watersheds to risks for adverse 
population changes in aquatic life and wildlife associated with aquatic food webs. Therefore, 
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