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SUBSURFACE SOIL.AND GROUNDWATER ASSESSMENT REPORT LABORATORY ANALYSIS OF SOIL AND GROUNDWATER Table 4 summarizes the concentrations of petroleum compounds detected by laboratory analysts in soil and groundwater A total of 11 soil samples were analyzed for the presence of TPHG, and BTEX by EPA Methods 1 8015 and 8020, and MTBE and associated oxygenates by EPA Method 8260 The soil sample obtained from soil boring TP-4 at 15 feet bgs had a concentration of 11 parts per billion (ppb)of MTBE 3 Laboratory analysis did not identify concentrations above the detection limits for TPHG,BTEX, DIPE,ETBE, TAME and TBA in the 11 soil samples analyzed A total of 5 groundwater samples were analyzed for the presence of TPHG and BTEX by EPA Methods 8015 and 8020,and MTBE and associated oxygenates by EPA Method 8260 Groundwater samples were below the detection limits for all analyzed constituents, with the exception of 1,300 ppb of MTBE in TP-4, and 14 ppb of MTBE in the groundwater sample obtained from TP-5 ' CONCLUSIONS AND RECOMMENDATIONS CONCLUSIONS FROM INVESTIGATION Soil analysis and field observations during the April 7, 2000 UST removal did not indicate a significant mass of residual petroleum hydrocarbons remaining beneath the former tank This conclusion is consistent with the results of ESA's recent subsurface soil and groundwater assessment Low MTBE concentrations in soils appear localized at the north and south ends of the former UST excavation MTBE concentrations were not detected in soil samples collected 1 east or west of the excavation perimeter MTBE concentrations were not found to have migrated vertically in soil to depths below groundwater(approximately 15 feet) ' Analytical results of the groundwater samples further substantiates the conclusion that the impact from residual MTBE appears localized at the north and south ends of the former UST excavation Non-detectable levels of MTBE in groundwater samples collected east and west of the UST ' excavation confirm the restricted extent of impacted groundwater The highest MTBE concentration detected in soil was collected during tank removal activities at the southern end of the tank,however MTBE was not detected in groundwater samples collected beneath the center ' of the former excavation in the estimated downgradient direction Impact to groundwater therefore appears localized around defined low levels of MTBE remaining in soil Lateral migration of groundwater at the site would be very slow, given the very stiff to hard clayey silts ' identified in this assessment Considering the consistency of these fine-grained soils, hydraulic conductivity for the shallow groundwater could range from 1x10-4 to 1x106 centimeters per ' second or between 10 and 100 feet per year 4 In addition, the presence of a thick clayey zone impedes vertical groundwater migration between the water table investigated, and the lower source aquifer Typically, shallow groundwater is of low quality and not extracted for public 3 Parts per billion or ppb is equivalent to micrograms per kilogram or ug/kg to soil or micrograms per Liter or mg/L in water 4 Hunt,Geotechnical Engineering Investigations Handbook,McGraw Hill,New York,New York, 1984 ' Subsurface Soil and Groundwater Assessment ESA/200252 1081 W Mendocino Avenue Stockton Califomia 10 August 25 2000