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c WM MINI NINE- PROBLEM ASSESSMENT REPORT Moore Petroleun, Inc. 14BOO H. Frontage Road, Manteca, CA MGA Project No, 10-90-479 Page 7 3.4 Site Soils and Geology The majority of the site surface is covered by a 2-inch-thick layer of asphalt underlain by a 4-inch-thick layer of aggregate base. Subsurface geology at the site has been determined by information derived from soil borings drilled by ETS and Delta. Examination of soil boring logs shows the site to be underlain primarily by strata of sand, silty sand, silt, and clay. The uppermost strata at the site consists primarily of sand and silty sand with rare interbeds of silt and clay. This unit ranges from 13 feet (MW-7) to 41 feet thick (MW-5) and appears to underlie the entire site. Underlying this unit (except at the location of M11-5) , is a layer of silty and sandy clay and silt. This unit ranges in thickness from about four feat at MW-2, where it consists. of a silt, to a maximum thickness of about 14 feet at MW-7. The lowermost strata that has been penetrated by soil borings at the site consists primarily of sand and clayey sand with interbeds of clay and silt. Subsurface geology at the site can be more easily interpreted by examination of the geologic cross-sections presented as Figures 4 and 5. cross-section locations are shown on Figure 2. 3.5 Soil Analytical Results Soil samples collected during the drilling of monitoring wells NW- 5, MW-6, MW-7, and 14W-8 were submitted to a California-certified laboratory and analyzed for those parameters listed. , Section 1.2. Petroleum constituents were not detected in the soil samples from MW-6 and MW-8 and ware detected at relatively low concentrations in soil samples from MW-5 and MW-1 (1.3 and 10.0 ppm TPH respectively) . The highest concentration of a ETEX constituent in wells MW-5 or MW-1 was xylene, which was detected at a concentration o:: 0.63 ppm from the 20-foot sample at MW-1. The highas;- TPH concentrations detected in soil samples from soil boring:: are 88, 78, and 19C ppm (detected at MW--2, MW-3, and MW-4) respectively. Soil samples were collected by a Delta representative on November 22, 1989, from beneath USTs at the site and at 20-foot intervals along fuel product lines. Th a sa.nples were analyzed by a California-certified laboratory for those parameters listed in Section 1.2. In order to determine the presence and concentration - - --of--diesel;--two-samples- were -also-analyzed for--TPH--by- EPA method - - - -- -- - 8.270•. Results of these analyses indicate. that in the area of, the product lines soils contain high concentrations of, petroleum constituents (1,100-150,000 ppm TPH) . TPH concentrations of samples taken from beneath the USTs indicate significantly lower concentrations. Samples from: the North ends. of the unleaded and leaded gasoline tanks as well as the South end of the diesel tank have TPH concentrations of 1.6, 66.0, and 2.0 ppm respectively. ! y