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Corrective Action Plan Page 12 <br /> required to reduce their contaminant load to a low level falls somewhere between the two <br /> extremes <br /> The principal advantages of remediation by groundwater recovery and treatment are an <br /> array of wells can be installed so that an extensive cone of depression will form in the <br /> water table and contaminated water will be drawn to the wells from throughout the <br /> affected area and thus contain it hydraulically so that any down-gradient and co-gradient <br /> migration of contaminants is prevented, wells can be optimally located within the limits of <br /> the property such that groundwater containing the highest concentrations of contaminants <br /> can be rapidly extracted from the subsurface in areas under the sidewalks and streets that <br /> are inaccessible to other methods of remediation, the required groundwater pumping rate <br /> is within the discharge Iimits set by CRWQCB Order No 92-150, at the pumping rate <br /> required to achieve hydraulic containment of the plume of contamination, the uncontrolled <br /> discharge of VOCs to the air will be within the limits set by the SJVUAPCD (see later <br /> sections of this document for discussions of pumping rates, well drawdowns and radius of <br /> influence, discharge flows and emissions to the air) and, recovery wells and groundwater <br /> treatment equipment can be cost-effectively constructed and operated on the site <br /> Vacuum Extraction ,and Other Technologies At sites where the geology and <br /> hydrogeology are favorable, volatile components of gasoline can be removed from soil in <br /> the vadose zone by vacuum extraction techniques As noted previously, early removal of <br /> contaminants from soil can accelerate remediation by eliminating them as a persistent <br /> source of groundwater contamination However, at the subject site, the water table is <br /> shallow (at 7 to 8 ft beneath the surface) and almost all of the affected soil is submerged <br /> Under these conditions, vacuum extraction methods can not be applied unless the water V <br /> table is drawn down below the affected soil strata <br /> In the type of soil found beneath the 152 East I lth Street property, a grid of vacuum <br /> extraction wells on 10 to 20 ft centers would be required to remove VOCs from the soil <br /> and to lower the water table to sufficient depth for them to be effective would require <br /> groundwater to be pumped from each of them, or from an extensive array of separate <br /> recovery wells, at a combined rate significantly in excess of the 100,000 gallons per day <br /> limitation imposed by CRWQCB Order No 92-150 Also, the rate at which contaminated <br /> air would have to be extracted from the vacuum wells would generate VOCs at rates <br /> greatly in excess of the permissible limits for uncontrolled emissions set by the <br /> SJVUAPCD This would require installation of major systems to treat the discharged <br /> vapors and would greatly increase the capital and operating cost of the remediation <br /> system Furthermore, the vacuum extraction sites would be constrained within the <br /> property boundaries and, would be ineffective in treating contaminated soil that lies <br /> beneath East l lth and "F" Streets, except in a narrow zone extending a few feet beyond <br /> those frontages <br /> For the reasons stated above, it was found that use of vacuum extraction technology at <br /> this site was would be both impractical and ineffective <br />