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1 <br /> tconstant during the course of this assessment. The absence of contamination in MW-5 in <br /> i April and October of 1993 indicates that there has been a recent decrease in the size of the <br /> Plume. Figures 9 and 10 are contour maps of TPH and benzene concentrations in October <br /> 1993. <br />' 5.3 Volume of Contaminated Groundwater <br />' If the dimensions of the plume and the saturated pore space in the aquifer (i.e. porosity) <br /> can be estimated, the volume of contaminated water can be calculated according to the <br /> following equation: <br />' 7r x thickness of plume (in ft) x (radius of plume in ft)' x <br /> porosity x 7.48 gallons/cu ft <br /> Field and laboratory data suggest that groundwater between the depths of 60 and 75 feet <br /> is contaminated. Contouring the data (Figure 9) supports the interpretation that the plume <br />' is relatively circular in shape. Visual observation of samples collected from monitoring wells <br /> MW-4 through MW-8 indicates that the aquifer is a moderately-sorted to well-sorted fine- <br /> to coarse-grained arkosic sand. It is generally brown to gray in color and ,is not strongly <br />' oxidized or weathered. The porosity of the Modesto Formation has not been reported in the <br /> literature or measured in this investigation, but fluvial sand beds like those present in MW-4 <br /> through MW-8 have initial porosity between 30 and 40%; therefore, a porosity of 35% is <br /> used in the following calculation: <br /> 3.14 x 50 ft2 x,,.15 Ift x .35 x 7.48 gal/cu ft = 308,269 gallons <br /> 6.0 REMEDIATION ALTERNATIVES <br />' The Tri-Regional Board Guidelines for Investigation of Underground Storage Tank Sites <br /> require that a number of possible remediation methods be considered and addressed in the <br />' Problem Assessment Report.The following sections discuss remediation alternatives for both <br /> soil and groundwater at this site. <br />' 6.1 Soil Remediation <br /> 6.1.1 Ex-situ Treatment Alternatives <br />' In this section,cthon, flhve methods of ex-situ soil remediation are considered. Aeration, <br />' bioremediation, vapor extraction, and thermal destruction can all be performed on site. All <br /> four_are feasible methods of remediating sandy soil contaminated with gasoline. The first _ <br /> three require the construction of a treatment cell; thermal destruction does not. The dirt <br />' parking area behind. the Art & Artists building (where MW-2 and MW-5 are located) is <br /> owned by Mrs. Lehr and is currently vacant; it could be used for treatment. Excavated soil <br />' �bp.l -7Z. /1.41 . 23 <br /> AdA PAR/1�a8�9a/1.4� <br />