important factor for decreased bioaccumulation potential. Bioaccumulation for other PBTs, such 
as organometallic compounds like methyl mercury or organic chemicals with mechanisms for 
bioaccumulation different than hydrophobicity, must be assessed on the basis of the mechanism 
for their bioaccumulation. 
The first residue-based WQC for PBTs (PCBs, DDT and metabolites, mercury, and 2,3,7,8- 
tetrachlorodibenzo-p-dioxin [TCDD]), were developed for the Great Lakes under the Great Lakes 
Water Quality Guidance (EPA 1995a). Criteria were developed for risks to human health and 
wildlife, but not aquatic life. A similar procedure with BAFs was used to promulgate the 
National methodology for deriving human health criteria (EPA 2000f). 
Conceptual Model 
The Great Lakes Water Quality Guidance (EPA 1995a) for PBTs is consistent with a conceptual 
model for risk based criteria development for determination of safe loadings to aquatic systems 
or remedial actions in cases where unsafe loadings have created contaminated sediment 
problems. This conceptual model (Figure 11) illustrates a number of fectors that are relevant to 
development of risk assessment methods and water quality criteria for PBTs in aquatic 
ecosystems. The double arrows represent models and relationships for transforming and linking 
the data and conditions rejx^sented by the boxes in the conceptual model. Concepts therein 
which provide an initial basis for development of projects for aquatic PBT research are: 
1. Wildlife, aquatic life, and human health risk assessment methodologies for PBTs follow 
parallel tracks with common elements such as toxicity models, bioaccumulation factors, and 
chemical fate and transport models. 
2. Residue-based WQC for PBTs should incorporate all of the elements of the risk assessment 
paradigm (problem formulation, effects/hazards analysis, exposure analysis, risk characterization, 
and risk management). Setting water and sediment quality standards, in accordance with the 
PBT conceptual model (Figure 11), requires a left to right logic for acquiring data and 
assembling models. 
3. Retrospective risk assessments (e.g., what are/were the ecological risks associated with the 
mass of chemical X in lake Y?) most often occur in response to a chemical stressor diagnosis and 
utilize the models and data in a right to left direction. 
4. While prospective and retrospective risk assessments tend to flow from left to right and from 
right to left, respectively (Figure 11), in reality, site sjjecific assessments are expected to involve 
iterations with reverse flows in data colleaion and modeling. For example, in a prospective risk 
assessment differences in exposure for species in a particular ecosystem may alter assumptions of 
the species at risk and require extrapolation of toxicity data (return to effects analysis) to 
complete a risk characterization. 
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