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3.1 Developing a Conceptual Site Model . The primary condition for use of PRGs is that exposure pathways of concern and conditions at the site match those taken into account by the PRG framework. Thus, it is always necessary to develop a conceptual site model (CSM) to identify likely contaminant source areas, exposure pathways, and potential receptors. This information can be used to determine the applicability of PRGs at the site and the need for additional information. For those pathways not covered by PRGs, a risk assessment specific to these additional pathways may be necessary. Nonetheless, the PRG lookup values will still be useful in such situations for focusing further investigative efforts on the exposure pathways not addressed_ To develop a site-specific CSM, perform an extensive records search and compile existing data (e.g. available site sampling data, historical records, aerial photographs, and hydrogeologic information). Once this information is obtained, CSM worksheets such as those provided in ASTM's Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites (1995) can be used to tailor the generic worksheet model to a site- specific CSM. The final CSM diagram represents linkages among contaminant sources, release mechanisms, exposure pathways and routes and receptors. It summarizes our understanding of the contamination problem. As a final check, the CSM should answer the following questions: • Are there potential ecological concerns? Is there potential for land use other than those covered by the PRGs (that is, • residential and industrial)? / Are there other likely human exposur were not considered in R/ development of the PRGs (e.g. impact to groundwater, local fish consumption; raising beef, dairy, or other livestock)? • Are there unusual site conditions (e.g. large areas of contamination, high fugitive dust levels, potential for indoor air contamination)? If any of these four conditions exist, the PRG may need to be adjusted to reflect this new information. Suggested references for evaluating pathways not currently evaluated by Region 9 PRUs are presented in Exhibit 3-1.