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GEOMATRIX • Mr. Robert Mihalovich Chevron Environmental Management Company April 3, 2002 Page 4 Screening-Level Health Risk Assessment In 1998, Geomatrix conducted a Screening-Level Health Risk Assessment (HRA) using historical soil and groundwater data collected at the site. Petroleum constituents (BTEX and PAHs)were not detected in groundwater, therefore soil data was evaluated with respect to risk based criteria. Soil data were compared to residential PRGs for industrial site use. The results of the Screening- Level HRA indicate that concentrations of petroleum constituents are not sufficient to pose an unacceptable risk under a industrial land use scenario. Furthermore, it is possible that the con- centrations of these constituents are not sufficient to pose an unacceptable risk under a residential land use scenario. A quantitative evaluation of the data with respect to residential land use was not possible due to the elevated detection limits of the PAH data. WORK PLAN TO ADDRESS DATA NEEDS Based on a review of previously conducted investigations, additional data is needed to fulfill the requirements of the 2001 CTA and current RWQCB requests. Specifically, soil data regarding the presence of residual petroleum at depth is required to better define the vertical extent of petro- leum-affected soil in the area of MW-1. Additionally, soil data regarding the presence of PAHs in shallow soil is also required to evaluate the potential health risks associated with residential development of the property. Groundwater quality regarding the presence of TPHd above WQOs is based on two sampling events, therefore, additional monitoring is required to evaluate the consistency of groundwater data at the site. Additionally, data regarding the downgradient extent of groundwater exhibiting concentrations of TPHd as detected without silica gel preparation is required. To address these data needs, we recommend additional soil and groundwater sampling at the site. Outlined below is a description of these activities. Proposed soil and groundwater sampling loca- tions are shown on Figure 4. Soil Sampling and Groundwater Sampling Activities To better define the vertical extent of residual petroleum, one boring (SB-9, Figure 3)will be drilled between SB-2, SB-8, and MW-1 to a depth below the evidence of residual petroleum in soil, which is estimated to be between 60 to 70 feet bgs. A soil sample will be collected from the terminus of boring SB-9 and analyzed for TPH quantified as crude (TPHc), BTEX, and PAHs to evaluate the vertical extent of petroleum constituents. Three additional borings (SB-10 through SB-12; Figure 3)will be advanced to a depth of 10 feet bgs within the area of petroleum affected soil to assess the presence of PAHs in shallow soil. Soil samples will be collected from SB-9 through SB-12 from near surface (i.e., 1 to 2 feet bgs) and shallow soil (i.e., 5 and 10 feet bgs)to evaluate the presence of BTEX and PAHs for risk assess- ment purposes. Samples of shallow soil also will be analyzed for TPHc for informational pur- poses.