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y j UM 5 Ms. Vickie McCartney,REHS Page 3 San Joaquin County Environmental Health Department February 13,2004 Site Geology and Hydrogeology The site is located in the northern portion of the San Joaquin Valley. The San Joaquin Valley is part of the Great Valley Geomorphic Province. The San Joaquin Valley is a major California groundwater basin bordered by the Sacramento Valley and Sacramento-San Joaquin River Delta complex to the north-northwest, the Sierra Nevada Mountain Range to the east, the Coast Ranges to the west, and the ' Tehachapi Mountains to the south. The elevation of the site is approximately 15 feet above mean sea level. Based on observations during past drilling activities,the site is underlain primarily by silt with inter- bedded clay to a depth of approximately 25 feet below ground surface(bgs). The silt zone is underlain by a sand zone composed of sand and silty sand in.the depth interval from approximately 25 to 35 feet bgs. A second silty zone underlies the first sand zone in the depth interval from approximately 35 to 45 feet bgs. A second sand zone is present from approximately 45 to 60 feet bgs. The second sand zone is underlain by silt in the depth interval from approximately 60 to 70 feet bgs. A.third sand zone is present from approximately 70 feet bgs to the total depth explored at approximately 75 feet bgs. Depth-to-water measurements at the site have ranged from approximately 34 to 59 feet bgs. Historic data show groundwater flow direction and hydraulic gradient to be variable. The direction of flow and gradient change between sampling intervals. The dominant direction of groundwater flow is to the east- northeast. Groundwater monitoring data are summarized in Table 1. SCOPE OF WORK URS proposes to install one deep groundwater monitoring well, one soil vapor extraction well, and 10 air sparging wells to monitor for the presence of petroleum hydrocarbons in deep groundwater and to remediate petroleum-hydrocarbon-impacted soil and groundwater beneath the site. The well locations were selected based on historic groundwater monitoring results summarized in the URS Fourth Quarter 2003 Quarterly Groundwater Monitoring Report, dated February 5, 2004, and soil analytical results summarized in the URS Corrective Action Plan, dated September 19, 2003. The sections that follow include discussion of permitting, drilling, sample collection, well construction, and report preparation. A site-specific health and safety plan (HASP) will be prepared and will be available on site during field activities. K:1WprocessV5 BP ARCD10218612186 WP 04.doe