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i • Mass Balance of Gasoline I The residual petroleum hydrocarbons are distributed between the vadose zone and the saturated zone as characterized by soil borings and groundwater monitoring wells installed in the vicinity of this release The available data suggest that the vadose zone contamination is ' confined to a relatively small area in the immediate area of the former underground gasoline tank and pump island Monitoring shows the groundwater contamination has spread more ' extensively Vadose Zone The vadose zone contaminant mass lies in the approximate shape of an inverted cone, with ' the apex situated directly beneath the former pump island and tank pit associate with the "oil house" (see Figures 2 and 4) For the purpose of these calculations, the vadose zone was ' somewhat arbitrarily set at the zone above 30 feet below-grade For purposes of calculations, the vadose zone was split into several stacked vertical "slices", ' 10 to 20 feet and 20 to 30 feet (see maps at the end of this appendix) The approximate lateral limit of each slice was drawn on a site map, and the area of each contour was calculated by using the planimeter function of the drafting program used to create the figures ' (i e TurboCAD, published by IMSI) The volume of the slice was then calculated by multiplying the area by the thickness of each of the slices The approximate average of TPH- . G measurements collected from soil sample in each slice was then applied to the volume to ' arrive at an estimate for the total average contaminant mass in the slice To compute the contaminant mass, the volume of each zone was converted into a total estimated weight of soil taking into consideration the porosity of the soil These calculations assume the average bulk density of the soil is 1 83 times that of an equal volume of water (62 lb/c f} Computation yields the partial result that the soli weighs 113 lbs 1 c f ' Table 1 --Vadose Zone Mass Zone Ave TPH Gross Zone Volume(V) Soil Mass(M) TPH Mass ' Area Thickness =62.Pa.V =M*TPH 10-20 Ft 1,734 ppm 650 s f 10 Ft 6,500 c f 734,500 lb 1,273 lb ' 20-30 Ft 760 ppm 1,440 s f 10 Ft 14,400 c f 1,627,200 lb 1,237 lb TOTAL 2,510 Ib Where, ' V =volume of zone in cubic feet Pd=bulk density of soil= 1 70 (corresponds to�=0 35) C=Soil contaminant concentration expressed as a ratio ' �=effective soil porosity Water weighs 62 pounds per cubic foot Diagrams showing the slices are included at the end of this appendix • 1 2937 Veneman Ave , #B240 Geo-Phase Environmental Inc Phone(209) 569-0293 Modesto, CA 95356 Fax(209)569-0295 1