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f <br /> 12 Augu:;t 1991 <br /> GeoAudit. _"V 30-;44E2-1.6 <br /> Page 3 of 5 <br /> 2. The presence of. eontaminated soil in all borings drilled to date <br /> indicates that the, vadose zone contamination plume has a radius in <br /> excess of 40 feet (Figure 3) . The maximum lateral extent has not <br /> been determined. <br /> 3. The water-bearing sand body at 40 feet appears to have sufficiently <br /> good hydraulic conductivity to permit the successful recovery of <br /> free prcduct and/or dissolved contamination. <br /> 4. The water table has apparently dropped approximately 5 feet since; <br /> 1988.; When measured on 4 June 1991, he water table appeared to be; <br /> essentially flat, although there may be a very slight gradient to <br /> the sol.Irth. However, the presence of free product in HW-1 and MW-2 <br /> suggests that gasoline has migrated northeastward along the water <br /> table at least 25 feet from the 10,000-gallon tank. Gasoline may <br /> also have migrated southward, but not in sufficient quantities to <br /> form free product in MW-3. Northward migration of gasoline may be <br /> due to somewhat greater permeability to hydrocarbons in that <br /> dircation. Alternatively, the contamination in the apparently up- <br /> gradient well (MW-2) could be due. bo'..some ' other •source of,; <br /> contamination north of the .tank cluster, such as this dispenser:; <br /> area., <br /> S. The total volume of free product -cannot be accurately determined <br /> until the average thickness and lateral extent of free product have <br /> been defined and the porosity of the soil has been measured. <br /> However, assuming that 1) the plume is cylindrical in shape and has <br /> a diameter of 80 feet and an average thickness of 4 inches (i.e. <br /> roughly 60% of the measured thickness), and 2) the effective <br /> porosity of the soil is approximately 30 %, between 3000 and 4000_ <br /> gallons of product are estimated to be present. . <br /> 6. The lateral extent of the dissolved contamination plume is probably <br /> -• considerably larger than the lateral extent of free product. <br /> RECOMMENDATIONS <br /> Free product floating on the water table continuously introduces <br /> contaminants to the ground water. We therefore recommend that the free <br /> r -__ product_-be-_recovered-as--soon-as-_possible.-.:The-existing-wells -can--be- - <br /> -- --- ------- <br /> utilized as recovery wells. We have already discussed the required <br /> equipment and procedures with Jim Hoblitzell of Falcon Energy, who.will <br /> perform the extraction.. <br /> The depth.to ground water and/or the interface between water and product <br /> should be measured to recheck the ground-water flow direcction. If flow <br /> to the south is confirmed, the possibility of an. additional source of <br /> contamination should be addressed, <br />