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r' Tosco Markeung Company 2 Tosco Service Station 8671,8606 Thornton Road,Stockton,CA March 16,1999 ASSESSMENT ACTIVITIES AND FINDINGS On January 27 and 28, 1999, four groundwater monitoring wells (MW-1 through MW-4) were installed at the site by Woodward Drilling of Rio Vista, California Groundwater monitoring well MW-1 was drilled near the southern portion of the UST complex to a depth of approximately 45 feet below ground surface (bgs) and screened from 25 feet to 45 feet bgs Groundwater monitoring well MW-2 was drilled west of the UST complex to a total depth of 45 feet bgs and screened from 25 feet to 45 feet bgs Groundwater monitoring wells MW-3 was drilled north of the UST complex to a total depth of 45 feet bgs and screened from 25 feet to 45 feet bgs Groundwater monitoring wells MW-4 were drilled east of the product islands to a total depth of 45 feet bgs and screened from 25 feet to 45 feet bgs Boring Logs showing well completion details are included as Attachment A The findings of this assessment are discussed below • Subsurface Conditions. Clay and silty clay was encountered in the borings from the ground surface to approximately 5 to 10 feet below ground surface (bgs) Beneath the clay and silty clay, vanng forms of clayey silts to fine sands were encountered to the total depth explored of 45 feet bgs Groundwater was encountered at approximately 35 to 39 feet bgs in the borings and stabilized between 28 61 and 29 66 feet bgs in the monitoring wells o Soil Sampling and Analysis. Selected soil samples collected from each boring were analyzed for total petroleum hydrocarbons as gasoline (TPH-g)by EPA Method 8015, benzene, toluene, ethylbenzene, xylenes (BTEX compound), and methyl tert-butyl ether (MtBE) by EPA Method 8020 All analytes were reported as nondetect Soil analytical results are presented in Table 1 Field and laboratory procedures are presented as Attachment B Certified analytical reports and chain-of-custody documentation are presented as Attachment C • Groundwater Monitoring Well Development, Sampling, and Surveying. After installation, the groundwater monitoring wells were developed by rigorously surging over the length of the screen interval and by purging ten casing volumes of water Groundwater samples were collected from the newly installed groundwater monitoring wells on February 15, 1999 Groundwater samples were analyzed for TPH-g and BTEX by EPA Method 8015/8020 and fuel oxygenates by EPA Method 8260A TPH-g was reported in one well (MW-2) at a concentration of 100 ppb Benzene was reported in all four wells at a maximum concentration of 3 56 ppb MtBE was reported in monitoring wells MW-1 and MW-4 at 7G888-010-03/well install doc