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, z Atlantic Richfield Comranr Former iLRU()4493,205 N Coiter Strccl_Stockton.CA S� tamper 5.2111)0 • pumped dry of all gasoline, the piping was evacuated and capped, and a temporary closure plan was submitted to the SJCEHD. In July 1997, the SJCEHD sent ARCO a letter stating that the former Shell service station approximately 300 feet east of the site had reported increasing concentrations of MtBE in their upgradient wells. These wells are to the east of the ARCO station, which is the historical groundwater flow direction. In July 1998, SECOR sampled two UST pit observation wells. Analytical results from the sampling event indicated that MtBE was present in one observation well at a concentration of 52.4 ppb. Depth to water during the observation well sampling was approximately 10 feet bgs. On February 10, 11, and 12, 1999, four groundwater monitoring wells (MW-1 through MW-4) were installed at the site. The wells are currently sampled on a quarterly basis. Groundwater flow at the site has ranged from east to southeast at a gradient of 0.003 to 0.008 ft/ft. Maximum concentrations of TPHg and MtBE are present in well MW-4 on the east side of the property. ASSESSMENT ACTIVITIES AND FINDINGS On May 8, 2000, one continuous core soil boring (B-1) was completed at the site by Precision Sampling of San Rafael, California. Soil boring B-l was advanced to 60 feet below ground surface (bgs) using Precision Sampling's direct push Enviro-Core sampling system. Soil boring B-1 was continuously cored and groundwater "grab' samples were collected at depths of 30 feet, 45 feet, and 60 feet bgs. On June 2, 2000, three groundwater monitoring wells (MW-5 through MW-7) were installed downgradient of the site by V&W Drilling of Isleton, California. Wells MW-5 through MW-7 were installed to a total depth of 25 feet bgs and were screened from 5 to 2.5 feet bgs. Boring Logs showing well completion details are included as Attachment B. Technical information on Precision Sampling's direct push Enviro-Core sampling system is included as Attachment C. The findings of this assessment are discussed below: Subsurface Conditions In boring B-1, a silty sand to silty and sandy clay was encountered from approximately 5 to 14 feet bgs. From approximately 14 to 33 feet bgs, a silty sand to sand was encountered. Groundwater was encountered at approximately 16 feet bgs during drilling within the silty sand to sand. A moist to wet clayey to sandy silt was encountered from depths of approximately 33 to 43 feet bgs in boring B-1. Sand and silty sand was encountered from approximately 43 to 56 feet bgs and was underlain by silty clay to the bottom of the boring at 61 feet bgs. Soil samples collected continuously fi-om boring B-1 from depths of 14 to 56 were classified as wet to saturated. At depths of 56 to 61 feet bgs within the silty clay, the moisture content was classified as damp, suggesting that this maybe a potential confining layer. 077.41883.3301well install1doc