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F! S Ms. Lori Duncan,REHS Page 4 of 6 San Joaquin Environmental Health Department November 15,2006 All downhole sampling equipment was decontaminated between samples by washing in a non-phosphate detergent solution. Rinsate water was contained in Department of Transportation (DOT)-approved 55- gallon drums and stored on site,pending disposal. Wells were constructed using i and 2-inch-inside-diameter (ID), schedule 40, flush-threaded,polyvinyl chloride (PVC) well casing. Two feet of 0.020-inch machine-slotted well screen were set at the bottom of SP wells; 10 feet was set in EW wells; and 15 feet were set in MW wells. A Monterey No. 3 silica I sand filter pack was installed from the bottom of the borings to approximately 2 feet above the top of each well screen. A 2- to 6400t bentonite transition seal was placed above the top of each sand pack. Neat cement grout was used to seal the remaining annular space to the surface. The wells were completed with 8- or 12-inch flush-mounted well boxes and locking, water-tight well caps. Well construction details are shown on the boring logs in Attachment C. Field and laboratory procedures are presented in Attachment D. Subsurface Conditions The site is in the northern portion of the San Joaquin Valley, which 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 cast, the Coast Range to the west, and the Tehachapi Mountains to the south. Elevation of the site is approximately 15 feet above mean sea level. �f Based on observations during drilling activities, the site is underlain primarily by silt with interbedded E f , clay to approximately 25 feet bgs. The silt zone is underlain by a sand zone composed of sand and silty Esand in the depth interval from approximately 30 to 35 feet bgs. Soil boring logs are presented in Attachment C. Depth-to-water measurements at the site have ranged from approximately 18 to 45 feet bgs. Historic data show variable groundwater flow direction and hydraulic gradient. The dominant regional direction Fof groundwater flow is to the southeast and northeast. Soil Analytical Data GRO was detected in seven soil samples, at concentrations ranging from 0.39 milligrams per kilogram (mg/kg), in EW-5/SP-5 to 3,700 mg/kg in EW-111SP-11. Benzene was reported in soil samples collected from EW-11/SP-11and SP-22 at concentrations of 4.9 and 0.27 mg/kg,respectively. MtBE was detected in samples from EW-5/SP-5 and EW-8/SP-8 at 0.0064 mg/kg and 0.0045 mg/kg, respectively. Tert-butyl alcohol (TBA) also was detected at concentrations of 0.10 and 0.26 mg/kg in samples collected from ('"f EW-5/SP-5 and EW-13/SP-13, respectively. No other oxygenates/additives were reported above the F1 laboratory reporting limits. Soil analytical data are presented in Table 1. Laboratory analytical reports and chain-of-custody documentation are included in Attachment E. F' 11 K:1WprocessU5$P ARCO%0216812006 System Instal lation\Report.doc