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4.0 CONCLUSIONS AND RECOMMENDATIONS <br /> Groundwater levels have dropped an average of 2.64 feet since groundwater levels were last <br /> measured on May 20, 1999. The groundwater gradient was calculated at 0.0004 foot/foot in a <br /> northeasterly direction. This result is consistent with historical groundwater flow and gradient data. <br /> F. L <br /> In general, concentrations of dissolved gasoline hydrocarbons in the groundwater samples <br /> collected this quarter from MW-6, MW-7, VW-1 and VW-2 have declined since startup of the soil <br /> vapor extraction system. In VW-2, the concentration of TPH-as-gasoline decreased from 1,200 <br /> ppb on May 20, 1999 to 440 ppb this quarter. The concentrations of benzene and MTBE also <br /> decreased since the last sampling event. Benzene decreased from 16 ppb in groundwater <br /> collected from VW-2 on May 20, 1999 to non-dectect this quarter. TPH-as-gasoline, BTEX and <br /> MTBE were not detected in MW-6 and MW-7. <br /> f Between June 2 and August 18, 1999, the SVE system has recovered 3,947.32 pounds of gasoline <br /> hydrocarbons from the subsurface. During this period, the system has operated consistently and <br /> without significant problems and downtime. Significant reduction in extraction rates indicate a <br /> reduction in the concentrations of constituents of concern. <br /> Ramage Environmental recommends the following course of environmental corrective action: <br /> 1. Continue to monitor and sample groundwater from the four water table wells on a quarterly <br /> basis. To better evaluate the concentration of MTBE in groundwater, future samples should be <br /> analyzed by EPA Method 8260. The next quarterly groundwater monitoring is scheduled for <br /> November 1999. <br /> 2. Conduct a water well survey to evaluate any possible impact to sensitive receptors in the'site <br /> f vicinity. The water well survey should encompass a 2,000-foot radius from the site, and should <br /> include a review of well completion reports at the California Department of Water Resources, a <br /> call to public and private water purveyors in the area, and a walk of the site vicinity to confirm <br /> the locations and operating status of any identified wells. <br /> 3. ENSR to continue operating and monitoring the soil vapor extraction system. ENSR should <br /> evaluate the use of air sparging in conjunction with the vapor extraction operations. <br /> 7 <br /> r <br />