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50 CONCLUSIONS AND RECCOMENDATIONS ' Groundwater elevations recorded during the Second Quarter 2005 sampling event were an average 0 52 feet higher than those recorded during the First Quarter 2005 sampling The rise in groundwater ' elevation is typical to historical data for the same time period ' Second Quarter 2005 is the first quarter that marks MTBE concentrations in groundwater below the MCL of 13 ug/l in all wells associated with the Site In the previous three quarters, the only well with MTBE exceeding the MCL was MW-5 located offsite and downgradient of the former tank pit In ' Second Quarter 2005, benzene was the only contaminant of concern that exceeded the MCL It was detected in well MW-3 at a concentration of 43 ug/l ' Prior to the implementation of remedial actions at the Site, the estimated soil contaminant mass was calculated to be 25,264 pounds of gasoline within the vadose zone and capillary fringe The mass removal calculations for the SVE system indicate that through April 25, 2005, the SVE system has removed approximately 17,981 pounds of TPHg from soil The SVE system removed contaminant ' mass from the soil at an average rate for the reporting period of 5 pounds per day In December 2004, the ozone sparge system became non-operational due to system component failures In January 2005, parts from a similar system were scavenged to return the ozone sparge system at the Site back to service By April 2005, several mechanical failures have again rendered the ozone sparge system non-operable The manufacturer of the sparge system is now defunct and replacement parts are not available Geocon recommends that the existing SVE system remain operational Geocon does not recommend replacing the ozone sparge system, as concentrations of the contaminants of concern in groundwater are below the MCLS with the exception of benzene in MW-3 Geocon recommends that an interim remedial effort be implemented to reduce contaminant mass and ' concentrations in groundwater in the vicinity of well MW-3 The interim effort would include the mobilization of a high vacuum dual-phase system that will remove groundwater and soil vapor A ' 20,000-gallon temporary tank will be staged on-site for use as a temporary holding container for the purged water The dual-phase system will be operated for a period not to exceed 5 days or 20,000 ' gallons of water purged Upon completion of the dual-phase extraction, a groundwater sample will be collected from MW-3 to determine the effectiveness of the interim remedial effort I i Project No 38643-06-01 -10- May 5,2005