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' Corrective Action Plan <br /> Montgomery Ward Auto Service Center, Stockton, CA <br /> by air-sparging The unit used to treat vapors extracted by soil vapor extraction <br />' also can be used to control and treat the vapors generated by air sparging ground <br /> water Therefore, vapor extraction is a very feasible and practical soil remediation <br /> method that is also applicable to ground water remediation A grade of 6 was <br />' assigned to this criteria for this remedial alternative <br /> 6.4 Ground Water Remedial Alternative <br /> There are three main types of ground water remediation techniques that can be applied to <br /> the Montgomery Ward Site, namely <br /> t ■ Passive Bio-remediation <br /> ■ Pump and Treat <br /> Air Sparging <br />' Passive bio-remediation does not appear to be an acceptable option due to the relatively <br /> high concentrations of contaminants present in ground water beneath the Former Unocal <br /> and Montgomery Ward sites, the presence of these contaminants in an off-site Delta <br />' College irrigation well (see Section 2 3), and the County's request to implement active <br /> ground water remediation Therefore, only pump and treat and air spargmg methods were <br /> evaluated <br /> Pum and Treat <br /> Pump and treat is a conventional method of ground water treatment in which ground water <br /> is extracted through one or more extraction wells and the water is treated aboveground and <br /> discharged into the sanitary sewerage system, storm drain system or reinjected on-site <br /> ■ Probabiht of Achieving Cleanup Goals <br /> The current understanding on the effectiveness of a pump and treat method is that it <br /> will initially lower contaminant concentrations sharply but then reach a point where <br />' additional pumping only extracts clean, or relatively clean, water However, if the <br /> system is deactivated for a period of time, the concentrations tend to increase <br /> sharply Basically, this happens due to the inability of the pump and treat system to <br /> cleanup the contamination held in soil pores <br /> A site specific limitation associated with this method is the presence of an <br /> immediately adjacent upgradient off-site source of contamination, i e , the Former <br /> Unocal Site (see Figure 1) Since pump and treat physically draws contamination <br /> into a cone-of-depression around the point(s) of extraction, pump and treat on the <br /> Montgomery Ward Site would increase the rate of contaminant transport from the <br /> Former Unocal Site to the Montgomery Ward Site Further, since as of the date of <br /> this report the County has not directed Unocal to cleanup its site (15 ppm of <br /> benzene is present in the most downgradient well [MW-11] associated with the <br />' Former Unocal Site [see Table S]) there would be a continuous source of <br /> contamination Unless Unocal is directed to cleanup its site, the probability of <br /> I- to^"" aiv '.io,inrs tl,ie tnethnri is lnw and consequently a grade of 2 <br /> acillG v Sllg CILaIILLJJ 6vµ•0 rJ <br />' was assigned to this criteria for this remedial alternative <br /> Project No 1232 - 16 - <br /> SAB <br /> 16 -SAB WORT)1232CAP <br />