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' 15 August 1997 . AGE-NC Project No 96-0254 Page 5 of 7 ' water samples analyzed for diesel at concentrations of 44,000 ppb and 310,000 ppb BTE&X were detected in each of the water samples analyzed at concentrations ranging from 0 56 ' ppb benzene in boring B1 to 20,000 ppb of benzene in boring B7 MTBE was also detected at concentrations up to 1,000 ppb by EPA Method 8020 However, the highest concentration of MTBE detected by EPA Method 8260 was 180 ppb ILaboratory results of grab ground water samples analyzed for petroleum hydrocarbons are summarized in Table 2 -Anal vocal Results of Grab Water Samples The laboratory reports (MAI Laboratory ID 77986 to 77993), QA/QC reports and chain-of-custody forms are Included in Appendix B IFigure 4- Dissolved Hydrocarbons, shows an approximate areal distribution of ground water containing at least 100,000 ppb TPH-g and at least 10,000 ppb benzene Noteworthy in respect to the figure is that the ground water sample from B6 contained 310,000 ppb TPH-d and 200,000 ppb TPH-g, but had only 3,400 ppb benzene, and so was not included in the mapped plume despite significant hydrocarbon Impact on the ground water in this area Also, the ground water sample from B3 had concentrations of TPH-g and benzene below the threshold mapping cut-off, but had a hydrocarbon sheen noted on laboratory report I 50 SUMMARY AND CONCLUSIONS The following conclusions were drawn from the results of this investigation - • Ground water at the site was encountered at a depth of approximately 8 feet bsg The prevailing ground water flow direction in this area of Tracy is generally toward the north, based on giound water flow direction observations made on other sites in the vicinity • The most intensely hydrocarbon-impacted soil was encountered in the area of the former USTs Soil contamination was limited to depths of 12 feet bsg and the highest hydrocarbon Iconcentrations were encountered at depths of 6 to 8 feet bsg The area with the highest hydrocarbon concentrations is centered in the former UST area, included borings B2,B3, B4 and B5, and was bounded by borings Bl, B6 and B7 The estimated lateral extent of impacted soil at a depth of 6 to 8 feet bsg is depicted in Figure 3 - Hydrocar bon Impacted Soil IThe highest concentration of dissolved hydrocarbons quantified as TPH-g and overall . BTE&X compounds extends northward of the former UST area Hydrocarbons were encountered in the presumed down gradient direction from the UST to the northern property Adtatreed GeoEn.iron mental Inc