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July 20, 2001 2001-0103-01 Page 3 of 7 Samples collected from boring B-17 were selected for analyses based upon stratigraphy and physical observations. Two of the samples collected above groundwater were from fine grained material (silt with clay); one at 40 feet bgs and the second just above first encountered groundwater (49 feet bgs). These samples were submitted for chemical analysis. Because of the fine-grained nature of the soil samples, the samples represent material within the central portion of the area of impact that would have the greatest potential for retaining residual hydrocarbons following vapor extraction and should provide evidence as to the effectiveness of the remedial actions (SVE and overexcavation). Two additional samples were collected within the saturated zone, one from 85 feet bgs (silt) and one from 95 feet bgs (silty-sand). Both samples were submitted for chemical analyses. No sheen or petroleum hydrocarbon odors were noted at total depth, vertical definition is complete, and remedial action effectiveness has been documented as follows: Total petroleum hydrocarbons as gasoline (TPHG) are 2.1 mg/kg at 85 feet bgs and 1.0 mg/kg at 95 feet bgs. Benzene and Methyl-tertiary Butyl Ether (MTBE) were non-detect (ND) at reported detection limits (RDL's) of 5.0 µg/kg. No fuel oxygenates or additives were detected in any samples collected from this confirmation borehole. Samples were analyzed utilizing EPA Method 8260B with standard RDL's. Tabulated results of confirmation sampling and analyses with detection limits for analytes are presented in Table 4 (Summary of Soil Analytical Results, Confirmation Soil Boring/B-17). Contaminant Contours Residual soil contaminants are Iimited and have been defined laterally and vertically. Less than 50 pounds of fuel hydrocarbons remain in the subsurface. Post-remediation monitoring data demonstrates plume stability and subsequent contaminant concentration reduction. Dissolved hydrocarbon concentrations have declined significantly following remediation. Only two monitoring wells contain detectable concentrations of TPHG (MW-13 and MW-14); only monitoring well (MW-13) contains detectable concentrations of benzene. Concentrations of benzene in samples collected from well MW-13 have declined from 920 ug/L in 1993 to 55 ug/L in 1999. Figure 5 presents TPHG iso- concentration contours in soil (25 to 40 feet bgs). Historical groundwater analytical results are presented in Table 3, and concentrations of TPHG and benzene in groundwater (first quarter 1999) are shown on Figure 6. En ineered Remedial Actions SVE SVE performed at the site was effective, as documented during the vertical confirmation borehole work described above. Due to heterogeneity in the subsurface environment, measured radii of influence (ROI) were highly variable (see Emcon PAR and FRP - December 1992). ROI for extraction at MW-13 through MW-1 was approximately 12 feet at 30 and 31 inches of water column (in. we). ROI for extraction at G-2 through MW-3 was 30 feet at 211n. we. ROT for extraction at G-2 through MW-5 was 62 feet at 21in. we. The SVE system test began in October of 1992. The SVE operated from December 1994 to December 1995. An air sparging operation began in June 1995 to \\Front\project file\Teichert\Stockton-103\Reports\Teichert Stockton Closure Report 0701.doc