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URS Ms Vickie McCartney, REHS Page 4 . San Joaquin County Environmental Health Department ARCO 2186 All downhole sampling equipment was decontaminated between samples by washing in a non-phosphate detergent solution Rmsate water was contained in Department of Transportation (DOT)-approved 55- gallon drums and stored on site, pending disposal Wells MW-8 and AW-1 were constructed using 2-inch-inside-diameter (ID), schedule 40, flush- threaded, polyvinyl chloride (PVC) well casing Well VW4 was constructed using 4-inch-ID, schedule 40, flush-threaded, PVC well casing Five feet of 0 020-inch machine-slotted well screen were set from approximately 129 to 134 feet bgs in well MW-8, 15 feet were et from 25 to 40 feet bgs in well VW-4, and 5 feet were set from 59 5 to 64 5 feet bgs in well AW-1 IX'Monterey No 3 silica 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 6-foot 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- and 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 east, 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 Based on observations during drilling activities, the site is underlain primarily by silt with interbedded clay to approximately 25 feet 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 50 feet bgs A second sandy zone is present from approximately 50 to 60 feet bgs The second sand zone is underlain by silt in the depth interval from approximately 60 to 75 feet bgs A third sandy zone is present from approximately 75 feet bgs to 110 feet bgs The third sand zone is underlain by a silty clay and clayey silt zone from approximately 110 to 135 feet bgs This clayey zone contains a thin, saturated sand zone from approximately 131 to 134 feet bgs A generalized geologic cross-section is presented in Figure 3 Soil boring logs are presented in Attachment C Depth-to-water measurements at the site have ranged from approximately 34 to 59 feet bgs Historic data show variable groundwater flow direction and hydraulic gradient The dominant direction of groundwater flow is to the east-northeast (Table 1) Groundwater Monitoring Well Development, Sampling, and Surveying On May 3, 2004, Stratus Environmental, Inc (Stratus) of Cameron Park, California, developed the groundwater monitoring and remediation wells by rigorously surging over the lengths of their respective screen intervals and by purging 10 casing volumes of water from the wells Well development field sheets are included as Attachment E K 1WprocessV5 BP ARCO\0218612004 Assessment\Report 04 doc