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URS Mr. Robert McClellon July 7, 2006 Page 2 trench.The results of the soil vapor extraction pilot test conducted in December 2005 were discussed with Mr. Cruz. During pilot test, soil vapor samples were collected from the two extraction wells(EW-1 and EW-2)and analyzed for volatile organic compounds (VOCs).The analytical results(Attachment 1) of the soil vapor samples showed that the total VOC concentration in either of the vapor extraction wells was less than 5 ppmV. In addition,a human health risk screening analysis for five potential operating scenarios were presented to Mr. Cruz(Attachment 2). The human health risk screening analysis was completed using the Screen3 model. The highest concentration of each VOC constituent was entered into the Screen3 model for the following potential scenarios: 10 cubic feet per minute(cfm), 25 cfm, 50 cfm, 100,cfm, and 250 cfm.The scenarios were selected as a range of potential flow rates that could be expected by operating soil vapor extraction trench passively or actively(using a blower).The Screen3 results showed that the total VOC air emissions ranged from 0.018 pound(lb)per day for 10 cfm to 0.46 ib per day for 250 cfm. The calculated cancer risk ranged from 2AE107'to 4.9E10-7 for all of the scenarios.Based on the information presented in this meeting,Mr. Cruz thought that an SJVAPCD permit would not be required in order to install or operate a passive soil vapor extraction trench,but Mr. Cruz requested that a work plan be submitted to the SJVAPCD for a formal decision. This work plan describes the planned activities to install a passive soil vapor extraction trench,and provides the necessary information for SJVAPCD to reach a formal decision. Field Activities URS proposes to install the passive soil vapor extraction trench by one of two methods. The first method would be to install the passive soil vapor extraction trench using a trackhoe excavator or equivalent. The alternate method would be to install the passive soil vapor extraction system using one-pass trenching equipment. If the passive soil vapor extraction system is installed using the trackhoe excavator,trench width will likely be 3 feet. If the passive soil vapor extraction trench is installed using the one-pass trenching equipment,the trench width will likely be 1.5 feet. For either method,we propose to install the trench to a depth of 5 feet below the static groundwater surface.We estimate the total trench depth to be 25 feet below the ground surface(bgs).The trench will be placed along the northern boundary of the site starting from at a location near vapor monitoring well VW-5 and ending at a location near vapor monitoring well VW-4. The length of this trench is estimated to be approximately 200 feet.The trench will be backfilled with washed,poorly graded drainage gravel to a depth of 7 feet bgs. Slotted piping constructed of 4-inch schedule 40 polyvinyl chloride(PVC)or equivalent will be placed on top of the gravel. At each end of the pipe and at 50-foot intervals, vent pipes will be installed. Vent pipes will be positioned to be above the breathing zone(extending approximately 6 to 8 feet above the ground surface). The slotted pipe will be covered with 2 feet of gravel,then site soil will be placed within the trench to the ground surface, and compacted. Proposed Schedule We would likely begin to install the passive soil vapor extraction trench within two to three weeks following your approval of this work plan and the concurrence of this work plan from the SJVAPCD.