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r� SECTION®NIE remain as a Caltrans maintenance yard The predominant structures located on site include a shop building, a storage building, and an office building There are plans for the construction of a shop building in the center of the site, near the former UST location The approach used for the analysis of the risks posed by the concentrations of the chemicals in soil and groundwater is based on EPA guidance for the evaluation of human health risks at Superfund sites Specifically, this risk assessment is consistent with referenced EPA guidance provided in the following EPA documents E osure Factors Handbook (EPA, 1989a), Risk Assessment QWdance for Su eifund Volume ,I Human Health Evaluation Manual (EPA, 1989b), Superfund Exposure Assessment Manual (EPA, 1988c), Health Effects Assessment Summary Tables (FAST) (EPA, 1994) The following California specific risk assessment guidance developed under the Department of Toxic Substances (DTSC) was used DTSC Supplemental Guidance for Human Health Multimedia Risk Assessments of Hazardous Waste Sites and Permitted Facilities (DTSC, 1993), Guidelines for Planning, Conducting, and Reviewing Human and Ecological Risk Assessments (DTSC, 1995), and California Cancer Potency Factors (DTSC, 1996) Additional risk assessment guidance followed included American Society for Testing and Materials (ASTM) Risk Based Corrective Action (RBCA) guidance (ASTM, 1995) ASTM RBCA software developed by Groundwater Services Inc (GSI)was used for calculating potential nsk In addition, soil concentrations were compared to EPA Region IX Preliminary Remediation Goals (PRGs) to assess potential risk from soil exposures (EPA Region IX, 1998) The Integrated Risk Information System (MIS), a computer database containing up-to-date human health risk and EPA regulatory information for numerous chemicals was used to obtain toxicity values EPA cautions that these documents are intended to provide guidance only in the conduct of health risk assessments and that considerable professional judgment must be used in their application (EPA, 1989b) jFate and transport modeling included in the ASTM RBCA software was conducted to estimate the potential concentrations in groundwater that could result from the existing residual soil contamination Unsaturated zone modeling was used to estunate loading rates of chemicals to groundwater The unsaturated zone modeling also calculated air emission rates because the chemicals may volatilize (evaporate) into the air posing a potential human health risk to workers on site and hypothetical residents off site 9.4 REPORT ORGANIZATION The organization of this risk assessment is outlined below Section 2 0 describes the Caltrans Shop 10 site and historical use, climate, geology, hydrogeology, and potential human receptor populations W0Qd w G T 11987W73000NAMOCKTOMREPORT SHORM-Cn {SNA 1-5