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r1 r, <br /> • 1rthoIogy of the Central Valley consists of eroded material from the Sierra Nevada <br /> Mountains (mainly sand, silt, and clay) that extends to depths of approximately 20,000 <br /> feet near the valley's axis Lithologic logs for the monitoring wells indicate that the <br /> shallow sediments beneath the Site consist of a fine-to-medium sand that extends to a <br /> depth of approximately 16 feet below land surface (bis) Beneath the sand is <br /> approximately 8 feet of interbedded silt and clay Beneath the silt and clay layer is a very <br /> fine grained sand that extends to at least 30 feet bls (the vertical extent of the Site <br /> monitoring wells) <br /> The water table beneath the site is present at a depth of approximately 17 feet bls Based <br /> on the groundwater monitoring that is performed quarterly at the site, the direction of <br /> groundwater flow is generally to the northwest at a gradient of approximately 0 0016 feet <br /> per foot <br /> 2.0 BACKGROUND INFORMATION <br /> Several investigations have been conducted at the facility to delineate the extent of <br /> petroleum contamination Eight on-site monitoring wells (MW-1 through MW-8) have <br /> been installed to date, each to a depth of approximately 30 feet bis During installation of <br /> these wells, soil samples were obtained at various depths and analyzed for benzene, <br /> toluene, ethyl benzene, and xylenes (BTEX) using EPA Method 8020, and for total <br /> petroleum hydrocarbons as gasoline (TPHg) using EPA Method 8015 Results of the soil <br /> sampling are compiled in Table 1 The extent of soil contamination at 11-11 5 feet bis, <br /> 16-16 5 'feet bis, and 20-21 5 feet bis is shown to Figures 2, 3, and 4, respectively The <br /> data indicate that no contamination is present above the depth of the former underground <br /> storage tanks (USTs) However, beneath the USTs (which would have been buried to a <br /> depth of approximately 15 feet b1s), soil contamination is present (Figure 3) Once the <br /> contamination reaches the water table, the location of contaminated soil moves <br /> downgradient, as shown in Figure 4 <br /> The quarterly groundwater monitoring conducted at the site is compiled in Table 2 The <br /> results of the most recent groundwater sampling are shown in Figure 1 The data <br /> presented in Figure 1 indicate that the extent of groundwater contairunation has been <br /> defined to the south, west, and north However, one additional well is required to <br /> delineate the extent of groundwater contamination to the east, <br /> 3.0 SCOPE OF WORK <br /> The objective of this workplan is to propose activities that will completely delineate the <br /> extent of petroleum impacted soil and groundwater, and will provide enough irifortnation , <br /> to evaluate whether nsk'assessment or remedial activities are more appropriate for this, /y <br /> Site To accomplish this objecttve, eight soil boring (SB-1 through ISB-8),are'proposed in' <br /> I the locations shown'in Figure 5 One additional shallow on-site monitoring well (MW-9) <br /> !� is also proposed, in the location shown on Figure 1 The actual well location will depend <br /> BRE=F1AAX/96-194/MY41.1 i` <br /> r � � I <br />