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, 5 �[aRO m Ms. Mary Meays December 20, 1995 (9347-3011) The current VES (installed June 23, 1993) and GWO system (installed on July 11, 1994) are composed of venting wells VEW-1 through VEW-4, and MW-3 (since July 1994) manlifolded to a 5- horsepower regenerative vapor extraction blower unlit and five oxygenation points (wells MW-1, MW- 2,MW-3, RW-1, and RW-2) connected to an oil-less, low-flow, air compressor. The VES and GWO system layout are shown.on Figure 3. The VES unit uses granular activated carbon for emission control(two 2000-pound carbon units in series). A conservative estimate of the radius-of influence for the current VES is shown on Figure 3. An estimated total of 3,300 pounds of petroleum hydrocarbons have been removed by vapor extraction, GWO, and ground water recovery at the site to date. The initial start-up petroleum hydrocarbon extraction rate by the VES (based on air sampling conducted on April 29, 1992) was 192 pounds/day. However, based on the July 3, 1995, sampling event,the petroleum hydrocarbon extraction rate of the VES has dropped to 0.7 pounds per day with a corresponding decrease in,the efficiency of the VES. Laboratory analysis of the influent air sample collected on July 3, 1995, indicated a concentration of total petroleum hydrocarbons of 16 ppm volume (ppmv), with no benzene detected above the reported laboratory detection limits. The July 1995 extraction rate corresponding to the soil vapor hydrocarbon concentration of 16 ppmv is less than one- half of 1%of the April 29, 1992, rate of extraction. To increase VES efficiency, Fugro has been pulsing the VES from February 1995 to present. Pulsing includes the cycling of the operation of the VES on 2-week intervals. The VES is alternately shut down for .2 weeks.and then operated for 2 weeks. Pulsing increases the VES efficiency by optimizing the vapor-phase petroleum hydrocarbon recovery. In addition to pulsing, selected extraction wells have been from the VES to alter the airflow pathways and to minimize stagnation zones. The analytical data indicate the soil venting system has removed all of the hydrocarbons that can be practically and economically remediated by vapor extraction. The graphed concentrations shown on Figures 4 and 5 indicate that petroleum hydrocarbon extraction rates and cumulative extraction have achieved asymptotic limits. The ground water recovery system (pumping 3 to 5 gallons per minute each from wells RW-1 and RW-2) commenced on March 2, 1993. Ground water recovery from RW-1 was discontinued on September 20, 1994, since no petroleum hydrocarbons were detected in the water samples collected from RW-1 during the previous three quarterly monitoring and sampling events(August 16,November 14, 1994 and February 22, 1995). Pumping from.RW-2 was discontinued on October 3, 1995, based on conversations between Shell and the SJCPHS/EHD. An estimated 5,772,087 gallons of ground water have been removed from the water-bearing zone beneath the site.. However, the extracted ground water only contained approximately 5.8 gallons of petroleum hydrocarbons. Therefore, petroleum hydrocarbons have comprised 0.0001 percent of the total volume of the fluids removed from the water-bearing zone beneath the site. 93473011.BRP -4- I I