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PROPOSAL FOR REMEDIATION SERVICES <br /> MW MOBIL SERVICE STATION #10-NP4 <br /> �r TRACY, CALIFORNIA <br /> Contaminated groundwater will be pumped from the trench using <br /> a single explosion-proof above-ground suction pump. The <br /> trench will be equipped with a low level groundwater sensor <br /> and solenoid-operated shut-off valve. Groundwater will be <br /> pumped to an above-ground air stripper. Air stripping is a <br /> mass transfer process which volatile contaminants in the <br /> groundwater are transferred to gas. The groundwater stream <br /> flows down through the packing while the air flows upward. <br /> Volatile, soluble gases have an affinity for the gas phase and <br /> tend to leave the aqueous phase during this process. The gas <br /> that is exhausted through the air stripper will be routed <br /> through two vapor phase carbon units in series. <br /> The treated groundwater from the air stripper will be <br /> reinjected hydraulically upgradient through a matrix of <br /> infiltration trenches as shown on Figure 1. The purpose of <br /> reinjecting the groundwater is to elevate the water table and <br /> thus flush petroleum hydrocarbon contamination from the vadose <br /> zone. <br /> The exact location of the pump, air stripper, vapor phase <br /> carbon units, and infiltration trenches will be evaluated <br /> based on the results of Task 1 and facility constraints. <br /> REMEDIATION ALTERNATIVES <br /> Alternative options are available if it is desired to speed <br /> the site remediation process, or if a higher degree of site <br /> cleanup is desired by Mobil or mandated by LHD or the RWQCB. <br /> These are briefly discussed below. <br /> Option I - Soil Venting <br /> A soil venting system could be installed in areas of the site <br /> that contain high concentrations of volatile hydrocarbons. <br /> This process would expedite the overall soil cleaning by <br /> stripping the volatile compounds from the soil particles and <br /> removing the interstitial soil vapors. The process involves <br /> inserting well points around the site, and connecting the well <br /> points to a suction-blower. <br /> Such a system would cost about $15, 000 to $20, 000 to install, <br /> including a soil gas survey and laboratory analyses of soil <br /> gas permeability. <br /> Option 2 - Additional Groundwater Recovery Wells <br /> An additional row of groundwater recovery wells could be added <br /> to speed the rate at which the contaminated groundwater is <br /> -7- <br />