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4 Adddionai Site Assessment Report and Corrective Action Work Plan 4 . The Southland Corporation,Store#20W2,4627 daMnci Dnve Stockton CA January 8 4997 The matenals encountered during recent drilling at the site generally consisted of the alluvial silts and clays to the depths explored During drilling,groundwater was encountered at approximately 10 feet BGS in soil boring MW-1 and at approximately 15 feet BGS in borings MW-2, MW-3,and MW-4 4 2 Results of Soil Sample Analyses Table 1 summarizes the laboratory analytical results for soil samples collected on September 21 and 22, 1997 Benzene was detected in samples MW 1-5'and MW-4-5'at concentrations of 0 015 mg/kg and 012 mg/kg, respectively TPH-G was detected in sample MW-4-5'at a concentration of 71 mg/kg MTBE was detected by EPA method 8020 In four of the ten samples analyzed at concentrations ranging from 0 086 mg/kg to 11 mg/kg Presence of MTBE was confirmed by EPA method 8260 at concentrabons ranging from 0 032 mg/kg to 7 3 mg/kg TBA was detected In samples MWA-S',MW-4-5',and MW-4-10'at concentrations of 0 036 mg/kg, 10 mg/kg, and 27 mg/kg, respectively DIPE was detected In sample MW- 4-5'ata concentration of 0 0059 mg/kg ETBE and TAME were not detected in any of the samples analyzed Copies of the analytical results and chain-of-custody documents are Included in Appendix C 4.3 Results of Groundwater Monitoring/Analyses Groundwater monitoring and analytical data from the four wells are presented In Table 2 No measurable separate phase hydrocarbons(SP)were detected in any of the wells monitored During the monitoring and sampling on October 17, 1997,the depth to water was approximately 8 feet BGS Groundwater flow,as mapped from data collected on October 17, 1997,Is to the south-southwest at a hydraulic gradient of 0 01 foot per foot(Figure 3) Figure 4 shows the lateral distribution of dissolved hydrocarbons(benzene,TPH-G and MTBE)in groundwater samples collected on October 17, 1997 Highest dissolved benzene concentrations were detected directly downgradient from the UST pit in well MW-4 at 610 micrograms per liter(pg/L) Highest concentrations of dissolved TPH-G and MTBE were also detected in MW4 at 20,000 and 18,000 pgA., respectively MTBE was confirmed In the same sample using EPA method 8260 at concentration of 23,000 pg/L Copies of the analytical results and chain-of-custody are Included In Appendix C 5.4 WASTE DISPOSAL Four drums of soil and two drums of well purge water and rinse water were removed from the site by Belshire Environmental Services, Inc The four drums of soil were transported to TPS Technologies,Inc in Adelanto,Caldomia on December 23, 1997, for proper disposal On December 11, 1997,water from the one drum of well purge water and rinse water was transported by Nieto&Sons to DeMenno Kerdoon In Compton, California for recycling 40 FLUOR DANIEL GTI