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' I <br /> I <br /> 1 <br /> I <br /> These data suggestthat benzene has never been present at high <br /> concentrations in the soil. They also suggest that all benzene has <br /> been removed from the vadose zone. The benzene that was detected in <br /> MW-2 may have been transported from the tank cavity in the <br /> capillary fringe or near the top of the I saturated zone, but has <br /> been immobile since monitoring began, becauselno benzene has been <br /> detected in any of the water samples collected from the monitoring <br /> wells. If one makes the very conservative butIunlikely assumption <br /> that MW-2 is located at the center ofa benzene plume in the <br /> capillary fringe, the maximum radius of this plume is the distance <br /> from MW-2 to the center of the former tank pit, or 32 feet. <br /> Assuming a capillary fringe thickness of 3 feet, the resulting <br /> volume of potentially contaminated soil is approximately 350 cubic <br /> yards. <br /> It is Geological Audit ' s opinion that this estimated volume may be <br /> as much as an order of magnitude too high.'I Were a significant <br /> volume of benzene-contaminated soil present in contact with <br /> groundwater, water samples from - MW--21 would be continually <br /> contaminated because of the high solubility of benzene. The data do <br /> not favor significant benzene contamination. <br /> You have requested that we suggest alternatives for remediation of <br /> benzene in soil. It is our opinion that the small volume and <br /> 11 shallow depth of benzene-contaminated soil make in-situ <br /> biostimulation an attractive alternative. Small quantities of a <br /> commercial fertilizer could be added to both MW--2 and MW-3 to <br /> stimulate naturally-occurring soil microb6s. The soluble nutrients <br /> should increase the effectiveness of thelmicrobes. <br /> A second option, which could be performed in conjunction with <br /> bioremediation, is air sparging . This method can induce oxidation <br /> of the hydrocarbons by increasing the aiflow to the subsurface. <br /> Because soil microbes are aerobic, the inc-�easled oxygen flow should <br /> also stimulate microbial growth. This metLd m1ay be somewhat easier <br /> to permit in San Joaquin County. It alis more expensive than <br /> biorenedia}ion: and may take more- time. <br /> With either method, groundwater samples would be collected at the <br /> it <br /> beginning of the treatment period and aga�n after 90 days to insure <br /> that benzene had not become mobilized Nin Ithe groundwater. If <br /> necessary, a confirmatory soil boring could be drilled to collect <br /> a soil sample at 15 feet. h <br /> A third method that could be considered is vapor extraction. Due to <br /> the presence of the contamination at or bellow the soil-groundwater <br /> interface, this method may be less effective than the others. It <br /> might also be difficult to prevent water vapor from entering the <br /> extraction stream and causing poor engine performance. in addition, <br /> the cost of this method is high, especilally in view of the low <br /> concentrations and small volume of contaminated soil. <br /> 2 I <br />