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(YVATe 4u ASSOCIATES I N C . activated carbon vessels to remove gasoline hydrocarbons. The discharge from the first carbon vessel would be periodically analyzed to establish carbon breakthrough. Treated water would be discharged into the sanitary sewer system pending approval by the City of Tracy. If the City of Tracy will not issue a discharge 6W permit,the extracted water would be disposed of off-site at a liquid recycling facility. Air Sparging This remedial approach for accelerated cleanup involves injecting air into the groundwater table. An air compressor capable of delivering 10 cubic feet per minute(cfm) at 20 pounds per square inch(psi)would be utilized to inject air into the groundwater table. The injected air serves two purposes: 1) removing gasoline '-• hydrocarbons from the saturated soil and groundwater by air stripping, and 2) providing increased oxygen to the ground water to promote bacterial action to degrade the hydrocarbons. ATC would need to install a soil vapor extraction(SVE) system to remove the stripped hydrocarbons from above the water table. In addition to the air sparging wells, two-inch-diameter vapor extraction wells would be utilized in order to ensure the capture of the hydrocarbons stripped from the saturated zone. All of the wells would be plumbed 6, to a treatment system area. The two-inch-diameter vapor extraction wells would be plumbed into a vapor extraction manifold. The air sparging wells would be manifolded into an industrial duty air compressor. Solenoid valves and timers would be used to cycle the compressed air between the air sparging wells to prevent preferential air channeling and to maximize the efficiency of the system. A series of filters would be used to prevent any contaminants in the compressed air stream from impacting the groundwater. Water samples would be collected and analyzed from the existing ground water monitoring wells to evaluate the effectiveness of the treatment system. Dissolved oxygen and pressure readings would be measured in all of the groundwater monitoring wells to provide an indication of the radial influence of the air sparging wells. In addition, vapor concentrations of the influent to the extraction system would be monitored monthly. The SVE system would be operated to remove the hydrocarbons being stripped from the saturated zone by the air sparging system. An estimated 1,000 to 2,000 pounds of carbon would be required for six months of system operation. Vapor samples would be obtained from the SVE system during operation to chart the remediation progress and to ensure that breakthrough has not occurred through the carbon canisters. The ... effectiveness of the vapor extraction and mobility of stripped gasoline vapor vary depending on permeability of the soil. Natural Attenuation Natural attenuation is a viable remediation method for the groundwater provided that the source of groundwater contamination has been removed and that the contaminant plume in the groundwater is stable or decreasing. Generally, eight quarters of stable to decreasing gasoline concentrations in the groundwater would indicate that no further action at the site is appropriate. Enhanced Natural Attenuation 'wo Natural bioremediation can be enhanced by the addition of oxygen to the subsurface. Several approaches have been developed to enhance the amount of oxygen available in the subsurface. As previously mentioned, v Former Cheaper#37 January 29 2004 ATC Project Number 5425847.0037 Page 8 v