Analysis of Sources, 



Transport, and Fate of 



Contaminants 



problems, even assuming undetected contaminants are present at 

 concentrations just below their respective detection limits. 



The choice of contaminant concentration values to use in subsequent 

 calculations to estimate exposure (and ultimately risk) is partly a risk 

 management decision. Exposure estimates are commonly based on 

 arithmetic mean concentrations of contaminants in edible tissue offish 

 or shellfish. Use of the upper 90 or 95 percent confidence limit in place 

 of the mean would provide a conservatively high estimate of exposure. 

 Calculation of conservative estimates for exposure is an appropriate 

 step in uncertainty analysis. However, U.S. EPA (1986b) guidelines on 

 exposure assessment discourage the use of worst-case assessments. 

 Use of upper confidence limits for chemical concentrations in com- 

 bination with a plausible-upper-limit estimate for the Carcinogenic 

 Potency Factor may lead to an unrealistic (i.e., highly unlikely) estimate 

 of upper-bound risk, especially if a conservatively high estimate of fish 

 consumption is also adopted. In most cases, the best estimate of 

 exposure based on mean contaminant concentrations should be used 

 to develop risk estimates. If upper confidence limits for chemical 

 concentrations are used to develop risk estimates, the effects of 

 compounding conservative assumptions should be evaluated. 



Exposure pathways and routes are potential mechanisms for transfer 

 of contaminants from a source to a target human population or sub- 

 population. The sources, transport, and fate of chemicals in the en- 

 vironment are analyzed to evaluate exposure pathways and routes. To 

 compensate for a limited database, this analysis often includes mathe- 

 matical modeling of contaminant transport and fate. The modeling of 

 exposure pathways focuses on transfer of contaminants from source to 

 target fishery species, since the transfer step from fishery to humans 

 can be based on knowledge of fishery harvest activities (see below, 

 Exposed Population Analysis). When extensive data on contamination 

 of a fishery is available and source-tracing is not an objective, modeling 

 of chemical transport and fate may be unnecessary. 



Although the specific uses of modeling in exposure assessment are 

 diverse, several broad objectives may be outlined as follows: 



• Estimate the spatial and temporal distribution of concentra- 

 tions of chemical contaminants in edible tissues of fish and 

 shellfish 



• Identify potential sources of contaminants 



• Evaluate alternative source controls or remedial actions. 



Estimation of contaminant concentrations in fish and shellfish by 

 mathematical modeling is especially useful when available data on 

 tissue contaminants are limited. If the distribution of contaminants in 

 sediments or water can be estimated from available data or model 

 predictions, estimates of chemical residues in fishery species can be 

 based on relationships of tissue contamination to environmental con- 



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