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Tosco Corporation 5 Tosco Service Station 8671 8606 Thornton Road,Stockton CA June 21 2002 • TBA was reported in one soil sample at a concentration of 0 86 mg/kg (MW-5@30'), • MtBE (by EPA Method 8260) was reported in the grab groundwater sample at a concentration of 130 µg/L (SBA @64'), • Benzene was reported in one water sample at a concentration of 0 52 µg/L (SB-1 @64'), 9 The groundwater gradient was estimated to be to the south-southeast at 0 013 ft/ft CONCLUSIONS AND RECOMMENDATIONS Based on the data collected during this assessment, the presence of MtBE in soil has been defined laterally by wells MW-5 through MW-7, and vertically in the source area by boring SBA The absence of MtBE in groundwater in wells MW-5 through MW-7 indicates that the lateral extent of the dissolved MtBE plume has been defined downgradient of the site SECOR recommends that the site continue to be sampled on a quarterly monitoring schedule to confirm groundwater gradient and chemical concentrations in groundwater • Additionally, given the detection of MtBE in the deep soil boring and the close proximity of municipal well #3, SECOR recommends a sentry well be installed between the site and the municipal well to a depth of approximately 110 feet bgs If you have any questions or comments regarding this letter, please call us at (916) 861-0400 Sincerely, SECOR International Incorporated Q9,OfESSIp�,q David J Harp ��o�IE BE�r,�o CFy Project Geologist Q50 U, y NO C 058399 rn "T Exp I 1O CW_ �t IN- Rusty E Benk y, PE 'qIF OF CAOF Senior Engineer Attachments Table I - Well Completion Details Table 2 - Soil Analytical Data Table 3 - Groundwater Elevation and Analytical Data Table 4 - Water Supply Well Survey Data Table 5 - Stockpiled Soil Analytical Data 077 43791 2221Assessrnent