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Site Assessment Report, Site Conceptual Model,and Site Investigation Work Plan R& L Diesel Services; 2417 West Lane, Stockton, California, 95205 2.2.3 Plume Characteristics The plume is primarily composed of petroleum hydrocarbons (TPH-D) lower levels of fuel oxygenates (BTEX) and the chlorinated hydrocarbon (PCE) derived from used oil. The petroleum hydrocarbon constituents appear to be weathered, with benzene detected in only one sample from the site. No fuel oxygenates are present in the plume. The highest levels of TPH-D, BTEX compounds and chlorinated hydrocarbons (PCE) are found north of the former UST and directly under the former UST. Maximum concentrations, detected in B1 at 14 feet bgs, of TPH-D, benzene, toluene, ethylbenzene, total xylenes and PCE were 7810 mg/kg, 2.55 ug/kg, 200 ug/kg, 97.3 ug/kg, 407 ug/kg, and 85.5 ug/kg, respectively. 2.3 Potential Pathway Analysis 2.3.1 Utility Corridors Utility corridors which may act as potential preferential pathways have not been fully evaluated at the site. It is anticipated that, with the preparation of sensitive receptor survey, that additional information will be collected to identify potential pathways associated with utilities at the site. 2.3.2 Geologic Pathways_ The silty sandy soils and limited clay identified during excavation activities at a nearby site (Pacific Pride, approximately 0.5 miles north on the east side of West Lane) do not provide a significant barrier to petroleum hydrocarbons constituent migration, although some natural attenuation will occur within the unsaturated zone as the petroleum hydrocarbons disperse in the soil. Although not logged at the time of the geoprobe investigation at the R&L Diesel site, a tight clay layer encountered at approximately 40 feet below grade may act as an impediment to downward migration of petroleum hydrocarbons and solvents to groundwater. 2.4 Potential Exposure Analysis BTEX constituents and chlorinated hydrocarbons can create an exposure risk in certain circumstances through volatilization and vapor intrusion from shallow soils and groundwater. Benzene is generally the constituent of greatest concern in these circumstances; however, levels of benzene present in the samples collected from the spill site and the identified uses of the spill site are not significant enough to indicate a volatilization risk. Additionally, potential vapor intrusion into off-site buildings is unlikely due to the low density of structures surrounding the site and the location of the impacted soil with respect to the nearest structures that are consistent occupied by people. 2.4.1 Sensitive Receptors A sensitive receptor survey has not been conducted for this site. A check of the State of California Geotracker system lists six public water supply wells nearby but, due to security concerns, specific location data is not available online for these wells. The scope of work proposed in this work plan includes the completion of a sensitive receptor survey in accordance with Section 2654(b)(2), Article, Chapter 16, Division 3, Title 23 of the California Code of Regulations. 4