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ARCO Products Company 2 ARCO Service Station#4493,205 North Center Street.Stockton,CA Ju! 7,1999 line passed testing. In 1996, fluid was reportedly leaking from UST #3. UST #3 was 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 northeast 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. ASSESSMENT ACTIVITIES AND FINDINGS On February 10, 11, and 12, 1999, four groundwater monitoring wells (MW-1 through MW-4) were installed at the site by Central Valley Drilling of Rancho Cordova, California. Groundwater monitoring well MW-1 was drilled west of the UST complex to a depth of approximately 30 feet below ground surface (bgs). Monitoring well MW-1 was set a 25 '/2 feet bgs and screened from 25 %2 to 5 '/2 feet bgs. Groundwater monitoring well MW-2 was drilled north of the product island to a total depth of 25 feet bgs and screened from 25 feet to 5 feet bgs. Groundwater monitoring well MW-3 was drilled east of the UST complex and southeast of the product islands to a total depth of 23 feet bgs and screened from 23 feet to 5 feet bgs. Groundwater monitoring well MW-4 was drilled east of the UST complex to a total depth of 25 feet bgs and screened from 25 feet to 5 feet bgs. Boring Logs showing well completion details are included as Attachment B. The findings of this assessment are discussed below: • Subsurface Conditions. Drilling conditions encountered at this site indicate this service station was built over fill material. Refusal was encountered several times and in several locations. Refusal was caused by buried concrete slabs. Fill material was present in the borings from ground surface to approximately 4 to 8 feet bgs. Fine grained soils ranging from sandy silt to silt clay were encountered from 8 feet bgs to approximately 17 feet bgs. Sandy silt, silty sand, and sands were present from approximately 17 feet bgs to the total depth of the wells. Groundwater was encountered at approximately 16 '/2 to 18 feet bgs in the borings and stabilized between 13.58 and 14.90 feet bgs in the monitoring wells. • Soil Sampling and Analysis. Selected soil samples collected from each boring were analyzed for total petroleum hydrocarbons as gasoline (TPH-g) and benzene, toluene, ethylbenzene, xylenes (BTEX 7G60o-022-02/weu install