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31 October 2005 AGE-NC Project No 03-1080 Page 6 of 10 6.0. SITE CONCEPTUAL MODEL ' Based on the data collected to date from dispenser and piping soil samples and soil boring soil and ground water samples, AGE has developed a site conceptual model for the release, migration and distribution of the contaminants in the subsurface as described below Release(s)of petroleum hydrocarbons occurred from on-site dispenser D 1 and,to a lesser extent,D4 ' the release(s) probably occurred prior to 1988, when site subsurface investigation began for the leaking USTs According to the analy ,ical laboratory,the petroleum hydrocarbons in soil and ground water collected from the dispenser area are from aged gasoline,compared to the high concentrations ' of BTEX compounds(benzene,toluene,ethylbenzene,and total xylenes)in ground water at the time of the UST investigation, there is only a detection of xylenes, with occasional occurrences of ethylbenzene, in soil and ground water at the dispenser location Ute' ` mtr b- i,� Once released to the subsurface, the contaminants migrated vertically into the silty sand/sandy ' silt/sand between surface grade and approximately 15 feet bsg, further vertical migration was retarded by the stiff clayey sand and silt/clayey silt located between approximately 15 feet and 20 feet bsg(Figures 5 and 7) Concentrations of target analytes in soil decreased below laboratory detection ' limits below the fine-grained clayey sand and silticlayey silt The low concentrations of TPH-g, TPH-d,and xylenes detected in sample B 1-15 are attributed to lateral dispersion along the top of this fine-grained layer, laterally, hydrocarbon-impacted soil is defined to the west by boring B2, to the �E northwest by boring B3, and to the northeast by boring VB-2�—At present, the estimated volume of ' petroleum-impacted soil encompasses approximatelyp 52 lbs, or 7 8 gallons, of petroleum hydrocarbons as aged gasoline (and possibly diesel) The small amount of petroleum hydrocarbons ' in soil is supported by the fact that only very low concentrations of MTand the degradation TO product TBA were detected in the soil stockpiles (Table 1) ld'yAg1 j ' Typically, ground water would represent the migration medium for contaminants over significant horizontal distances, however, sites located in an up-gradient direction from the site are not likely to be negatively impacted In addition, first encountered ground water is not used for drinking or ' irrigation, although all ground water in the State of California is considered to be of potential beneficial use ' Since initiation of ground water monitoring in April 1992, depth to ground water at the site has ranged from approximately 25 feet to 35 feet bsg Ground water flow appears to be northeast, although easterly and southwesterly flow directions have been recorded Based upon laboratory ' analysis of grab ground water samples collected from borings B 1 through B3,the zone of maximum petroleum hydrocarbon-impacted ground water appears limited to the area immediately surrounding ' - boring B1 and dispenser D1 An iso-concentration map of ground water contaminants is reflective of the generally northerly ground water flow pattern (Figure 8), however, the lateral extent of the • dissolved petroleum hydrocarbon plume toward the east or southeast of the area surrounding boring ' Advanced GeoEnvironmentai,Inc