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Jonas & Associates Inc <br /> ' was detected only in OW-2 on August 8, 1995 The detected benzene concentration <br /> is below the drinking water standard (MCL) of 1 ppb <br /> On April 8, 1996, at the request of the San Joaquin County Public Health <br /> Services/Environmental Health Division (PHS/EHD), all groundwater monitoring <br /> wells were tested for MTBE using EPA Method 5030/8015M/8020 MTBE was <br /> ' detected in well OW-2 at a concentration of 0 029 ppm, and non-detectable in all <br /> other monitoring wells " m - E ca), <br /> vw t <br /> 3 4 Mass Balance Calculation <br /> As requested by PHS/EHD, in 1996 PG&E performed a mass balance calculation of <br /> ' gasoline hydrocarbons that existed in the subsurface,based on data collected during <br /> the site investrgations performed in 1986-88 Table 3-7 presents the mass balance <br /> calculation The data supported an estimate of approximately 129 pounds, or 22 <br /> gallons of gasoline in the subsurface at that time (PG&E, 1996) <br /> ' 4.0 REGIONAL .AND LOCAL HYDROGEOLOGY <br /> 41 Regional and Site Geology and Hydrogeology <br /> The Service Center is situated within the Northwestern part of the San Joaquin <br /> ' Valley between the San Joaquin River and the Coast Range, and bounded on the <br /> north by the Old River (a channel of the San Joaquin River) The San Joaquin Valley <br /> is a topographic and structural basin bounded on the east by the Sierra Nevada <br /> ' Fault block and on the west by the Coast Range <br /> The Service Center is situated on the veneer of recent flood plain deposits that <br /> ' overlies recent deposits This veneer consist mainly of clays and silts The recent <br /> deposits consist of unconsolidated clays, silts, sands, and gavels to the depth of over <br /> 100 feet This sequence overlies the Corcoran Clay Member, a mayor aquitard in the <br /> ' San Joaquin Basin (Hotchkiss and Balding, 1971) <br /> Shallow stratigraphy specific to the Site can also be described from boring logs <br /> ' obtained during the 1986 and 1987/8 field investigations (see Appendix C) The <br /> surficial material typically consists of 10 to 15 feet of silty clay or clayey silt <br /> overlying fine to medium silty sand Sieve analyses performed on selected soil <br /> samples support the soil texture descriptions recorded by the project geologist in <br /> the drilling logs (PG&E, 1988) <br /> 12 <br /> 1 <br />