Guidance for Conducting Risk Assessments and Related Risk ...

individual. This evaluation results in the generation of exposure scenarios. Exposure scenarios represent
the combination (if applicable) of exposure pathways for an individual based on his/her activity patterns.
4.2.3 Quantification of Exposure
Although exposure concentrations are based on the measured environmental concentrations for both
current and future land use considerations, they often display variability between different sites within a
watershed. It is important to understand that medium- and site-specific considerations influence the
derivation of the exposure concentration. These considerations include the size of the site, land use
designation, techniques used for data aggregation, and selection of applicable spatial statistics. Detailed
guidance for determining the area of exposure units, exposure concentrations, and related issues is
provided in Appendix G.
It should be noted that fate and transport modeling may be required to estimate exposure
concentrations in the future. The project team should determine if fate and transport modeling would
benefit the overall project and the associated risk assessment. Potential site-specific modeling decisions
should be made in association with Data Quality Objective (DQO) decisions to ensure that models will be
supplemented by sampling data and effectively support risk assessment activities. Appendices D-F
provide guidelines on air dispersion modeling, groundwater modeling, and food chain models for use in
risk assessment.
Exposure concentrations serve as one input variable in the equations that are used to calculate
pathway-specific intakes for each receptor. As previously mentioned, many of the exposure equations
that support the land uses on the ORR and at Paducah and Portsmouth are available on the RAIS along
with default parameter values. These equations are updated as new technical information becomes
available.
4.3 TOXICITY ASSESSMENT
The purpose of a toxicity assessment is to weigh available evidence regarding the potential for a
chemical to cause adverse effects in exposed individuals and to provide, where possible, an estimate of
the relationship between the extent of exposure and the increased likelihood and/or severity of adverse
effects. (EPA 1989) Toxicity values for carcinogenic and noncarcinogenic chemicals and radionuclides
are available for use by all subcontractors performing work for the DOE-ORO via the RAIS. Toxicity
profiles for a select set of chemicals are also available via the RAIS.
4.3.1 Toxicity Values
A database of chemical-specific toxicity values is maintained on the RAIS. The toxicity values
database contains information obtained from the EPA's Integrated Risk Information System (IRIS), the
Health Effects Assessment Summary Tables (HEAST), and other information sources; all information
contained in the database is referenced. In addition, the database contains supplemental information
clarifying some issues.
The toxicity values contained in the RAIS database were developed for human health risk
evaluations and assessments utilizing methods presented in part A of Risk Assessment Guidance for
Superfund: Volume 1-Human Health Evaluation Manual (EPA 1989). The toxicity values database
contains footnoted entries for toxicity values that have been withdrawn from IRIS, are provisional, or
have been derived from other information. These footnoted values have been approved (by EPA Region
IV) for use in the risk assessment and evaluation of areas on the ORR and Paducah. In addition, the
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