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p ��ME . PRO13LEM ASSESSMENT REPORT Moore Petrofeun, Inc. 1399 E. Yosemite Avenue, Manteca, to NGA Project No. 10.90-480 Page 5 well screen were backfilled with approximately two feet of clean soil overlain by plastic sheeting covering the base of the tank basin. The sections of well screen were manifolded together and brought to 2 feet below grade with sections of 4-inch-diameter PVC blank casing. Following manifolding of the PVC ;well screen, the tank basin was backfilled with clean soil. After compaction of sails used to backfill the tank basin, the tank basin was resurfaced with asphalt. A plan view of the below-ground components of the soil venting system is presented as Figure 3. 3.4 site soils and Goo2-ogy The surface of the site consists of a two--inch-layer of asphalt which overlies a four-inch-thick aggregate base. Subsurface geology at the site has been inferred from soil borings drilled by ETS, Delta, and NGA. Examination of soil borings logs indicate that soils underlying the site consist predominantly of strata of silty sand, sand, clayey sand, and silty clay. The uppermost strata encountered by soil borings is a sand which is in places both silty and gravelly. This layer ranges from about 14.5 to 20 feet thick. Thin clay lenses (less than six inches thick) were found at the base of this strata in wells MW--2 and MW-3. Underlying the uppermost sand are layers of silty sand and clayey sandand silty clay. These strata appear to grade laterally into layers of salt and silty send between MW-2 and MW-1A. A silty sand is the deepest unit penetrated at the locations of MW-2 and MW--3. Split spoon samples were not retrieved from the base of NW-1A, however, drill cuttings indicate that the deepest unit penetrated within KW-1A may be a strata of clay and gravel. Subsurface geology at the site can be more easily interpreted by examination � r of the geologic cross--sections presented in Figures 4. Cross- section locations are shown on Figure 2. .4.5 Soil Analytical Results soil samples collected from soil borings drilled by ETS, Delta, and NGA have been submitted to a California-certified laboratory and analyzed for those parameters listed in Section 1.2. Analytical results indicate that TPH have been found in samples from soil borings SB-1, SB-2, SB-3, and MW-2 at either nor,-detectible or very low concentrations. ;,oncentrations of BTEX constituents in soil samples from these borings have either been non-detectible or detected at concentrations not exceeding 0.07 part per million (Ppm) . Soil samples collected from MW-1 and KW-3 have also shown very low concentrations of BTEX. BTEX concentrations in soil -samples from _...---------these borings--do..-spot-.exceed--1.-0...-..ppm•---..-_TPH_.-conc.entr.ations. of --