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the aquifer surrounding a monitoring point can be 90 percent less than the <br /> thickness measured at the monitoring point. This means that the volume <br /> of the petroleum product floating on the A-zone aquifer is very limited. <br /> This concept is well supported by data gathered during the dual phase <br /> pilot test conducted at the site at wells TW-2 and TW-3. A floating <br /> product removal rate of only 1 lb. per hour was observed at TW-2, while <br /> no floating product was recovered from TW-3. Therefore, under <br /> continued dual phase extraction, both the thickness of floating product at <br /> the site and the corresponding quantity of floating product influent to the <br /> Treatment System are expected to decline very rapidly. <br /> Technical Design Basis for A-Zone Ground Water Remediation <br /> The focus of this remedial action is to remediate vadose zone soil, remove <br /> floating product at the ground water table, and remove ground water <br /> containing high concentrations of VOCs, thus mitigating all sources of <br /> ground water impacts. The system has been designed to provide control <br /> of the A-zone ground water VOC plume through a combination of: <br /> • Ground water extraction within the plume via proposed dual phase <br /> extraction wells; and, <br /> • Ground water extraction at the plume margin via ground water <br /> extraction wells installed at the downgradient margin of the plume. <br /> Ground water extracted from both these sources will be treated to <br /> required levels at the Treatment System, and reinjected at an upgradient <br /> reinjection system. The basis of design for control of the A-zone ground <br /> water plume and treatment of extracted ground water is described in the <br /> following subsections. <br /> Ground Water Plume Margin Control <br /> To provide ground water containment at the plume margin, five 4-inch- <br /> diameter ground water extraction wells will be installed at the <br /> downgradient (northern) edge of the plume margin, as shown on <br /> Drawing C-1. These wells will be installed to the base of the A-zone <br /> (approximately 25 feet) and screened from 10 feet bgs to 24.5 feet bgs. The <br /> anticipated ground water extraction rate at each well is 2 gpm. Extracted <br /> ground water will be routed to the Treatment System for treatment prior <br /> to discharge at the reinjection system. <br /> 4-5 <br />