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REMEDIAL ACTION PLAN 1605 South EI Dorado Street.Stocidon California 3.0 SITE CHARACTERIZATION The following sections summarize the characterization of soil and groundwater 3.1 Soil Soil has been characterized during four phases of work at this site, three phases of drilling and one phase of UST removal These phases of investigation have assessed the magnitude and extent of petroleum hydrocarbon impact in soils beneath the site The locations of soil borings, groundwater monitoring wells, and vapor extraction wells are shown on Figures 4, 5, and 7 Analytical results from the soil boring samples are included in Tables 1 through 4 Figure 6 is a cross-section location map Interpretation of the subsurface is shown on geologic cross-section A-A' (Figure 7) The generalized cross-section indicates the soils beneath the site consist mostly of silty clay and clayey silts, with a relatively continuous, 5 to 10-foot thick silty to clayey sand layer extending from approximately 25-30 to 35 feet bgs The cross-sections include analytical results from samples collected during installation of the borings and the screened intervals of the vapor extraction wells and the groundwater monitoring wells As shown on Figures 4, 5, and 7, petroleum hydrocarbon impact under the site is primarily beneath the former UST near VEW-1 and VEW-2 at depths from 10 5 to 35 5 feet bgs 3 2 Groundwater There are currently five groundwater monitoring wells on-site (MW-1 through MW-5) and two additional off-site wells (MW-6 and MW-7) The most recent quarterly monitoring and sampling event (8/24/93) shows that groundwater was encountered at depths ranging from 42 47 to 45 06 feet bgs Groundwater levels collected to date are summarized in Table 5 A groundwater elevation hydrograph is shown in Figure 8 The most recent groundwater elevation data and inferred potentiometric surface contour lines are shown on Figure 9 Consistent with previous monitoring episodes, Aegis estimates groundwater flow is to the east at an average hydraulic gradient of approximately 0 001 ft/ft No benzene has been detected in the off-site downgradient well MW-7, indicating minimal Iplume migration 3 3 Potential Off-site Sources The groundwater samples collected from MW-3 and MW-6 contained detectable concentrations of TPHg and BTEX (Figure 10 and Table 6) Both MW-3 and MW-6 are up-gradient of the former UST basin, indicating a potential off-site source of petroleum hydrocarbons in groundwater 91-044A RAP 3