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Mal r locations of the former structures and tanks. In August „ 1988, following removal of the underground gasoline storage tank, two soil samples and one ground water sample were collected and analyzed for fuel hydrocarbon constituents. Soil and ground water analytical results detected the presence of total petroleum hydrocarbons (TPH), and benzene, toluene, ethylbenzene, and xylenes (BTEX) , (Kaprealian, 1988) . On tdovember 21, 1988, six exploratory borings were drilled to depths ringing from 12 to 13 feet below grade at the site. Analyses .of soil samples collected from depths of 5 and 10 feet showed detectable concentrations of TPH as diesel (TPH-D) up to 2,900 parts per million (ppm) and TPH as gasoline (TPH-G) up to 360 ppm. Ground water was encountered between 11 and 12 feet below grade (Kaprealian, 1988) . in February 1989, additional" soi.l samples were collected at the site from 12 test holes at depths of 5 and 10 feet below grade using a backhoe. Analyses of soil samples detected concentrations of TPH-D of up to 3,600 ppm and TPH-G up to 170 ppm. Results of the investigative work revealed the presence of diesel--range contamination in the subsurface soil extending to a depth of 10 feet below grade and encompassing an area of approximately 3,700 cubic feet. Soil analytical data from test holes suggests that gasoline-range hydrocarbon contamination is limited to the northern portion of the site. " Figure 3 shows the locations of the soil borings and test holes with corresponding TPH concentrations detected in`t%e sail samples (Kaprealian, 1989) . - In February 1990, Mobil Oil Corporation contracted Alton Geoscience, Inc. to characterize in situ for potential offsite disposal. The results of the soil profilii,7 were to be used during soil eAcavation to segregate the soii for disposal at the appropriate offsite facilities. 1.3 RegiQUal Geoloav and ydrD-gao ogy The following description of regional geology with respect to the site was based on the United States Geclogical Survey Professional....Paper....1401-C--(1986).. The site lies in the CenLkal Valley of California, which is about 400 miles long and averages about 50 miles wide, encompassing about 20,000 square miles. Geologically, the valley is a large asymetric trough that is bounded by granitic, metamorphic, and marine sedimentary rocks of. pre-Tertiary age. The trough has been filled with as much as 30,000 feet of sediment in the San Joaquin Valley to the south and as much as 60,000 feet of sediment in the Sacramento Valley to the north. These sediments range in age from Jurassic to Holocene and include both mal-ine and continental rnnir4 and :edi ucartgr.. dcYuai, $ an- L , ,