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These data suggest that benzene has never 66en 'resent at high I <br /> concentrations ' in the soil. They also suggest :that 11 benzene has 3 <br /> been removed from the vadose zone. The beniene; as detected in ; <br /> NW-2 may have: been transported from the tank : cavity - in the . <br /> Capillary fringe or near the top of thej�saturated zone, but has <br /> been immobile since monitoring began, becaUZq` n4 nzene has been <br /> detected in any of the water samples collected fro „ the monitoring <br /> f wells. If one makes the very conservative but: unli' sly ar-Sumpti.oll <br /> that MW-2 is located at the center o€ (1a b1nzen :plume in the <br /> capillary fringe, the maximum radius of this 61Vma ' is the distance <br /> from MW-2 to the center of the former,, tank pit, '` or 3'2 feet. <br /> Assuming a capillary fringe thickness of 3 ifoat,� the resulting <br /> ` volume of potentially contaminated soil is approxi tely 350 cubic <br /> yards. ,� <br /> if J <br /> It is Geological Audit's opinion that this estimat volume may be <br /> as much as an order of magnitude too high. ' Wer , a significant <br /> volume of benzene-contaminated soil present i . contact with <br /> groundwater, water samples from MW-2� world e, continually <br /> contaminated because of the high solubility ofl benzine. 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 s 11 volume anci <br /> shallow depth of benzene-contaminated sail i make in-situ <br /> biostimulation;: an attractive alternative. Si'all uantities of a <br /> commercial fortilizer could be added to both KW: 2 and XW-3 to <br /> stimulate naturally-occurring' soil microbes. The s table nutrient <br /> should increase the effectiveness of thejjmicrobes. ; <br /> A second option, which could be performed In conjunction with 1 <br /> bioremediation, is air sparging. This mathod 'can duce oxidation <br /> of the hydrocarbons by .increasing the air fldw -to .;the. subsurface. <br /> Because soil microbes are aerobic, the increased ax gipn flow should ` <br /> also stimulate A' growth. This method say be ;somewhat easier <br /> to permit in san aoaquin County. it ;his mcxe {iexpensive than <br /> bioremediationi and may take more time. <br /> With either method, groundwater samples mould` be c llected at the 1 <br /> beginning of the treatment period and again after gi: gays to insure <br /> ` that benzene had not become mobilized It in the oundwater. If <br /> necessary, a confirmatory sail boring could 15e drt'. led to collect <br /> a soil. sample it 15 feet. <br /> A third method that could be considered is vapor extraction. DUe to <br /> the presence of the contamination at or I., w the sail-gre�itndwatear <br /> intezfaae, thi6 method may be less effec: <br />