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Mr Jeff Wong Page 9 of 10 Soil samples collected from groundwater monitoring wells contained no detectable gasoline constituents, except for the sample collected from well MW12 at 80 feet bgs, which contained MTBE at 0 37 ppm Soil samples collected from vapor extraction test well VW1 near the former USTs contained low to moderate levels of gasoline constituents, including MTBE No soil samples were collected from wells MW13C or MW14B for analysis due to the nature of mud-rotary drilling Groundwater samples collected from wells MW10, MW11, and MW14B were free of gasoline constituents, except for a questionable detection of toluene in the October 2003 sample collected from well MW14B MTBE has been detected in each of the three samples collected from wells MW12 and MW13C MTBE in well MW12 has decreased from 2,500 ppb in October 2003 to 1,500 ppb in March 2004 MTBE in well MW13C has increased from 36 ppb in October 2003 to 110 ppb m March 2004 No MTBE has been detected in the facility's water supply well, which is located east of well MW13C, or in the nearby domestic wells north of well MW12, during that period I Analytical results suggest that MTBE has migrated at least 220 feet northeast of the former USTs in the shallow water-beanng zone, although well MW6, located approximately 130 feet northeast of the USTs, does not contain detectable MTBE Analytical results also suggest that the former facility well, which was cased to approximately 190 feet bgs and open to at least 220 feet bgs, has acted as a vertical conduit for MTBE, as evidenced by the presence of MTBE in well MW13C Soil vapor extraction appears to be a feasible method to reduce dissolved (and free-phase, if present) gasoline contamination at the site The effective rade of influence of the vapor extraction wells is estimated to be approximately 35-50 feet and should encompass a large portion of the source and immediate down-gradient areas of the plume Based on the laboratory analyses of the vapor samples collected during the VET, the average TPHg concentration at VES startup is estimated to be approximately 150,000 ug/1, with an estimated initial mass extraction rate for TPHg of approximately 450-600 lbs/day However, groundwater extraction and remediation at the site since the VET was completed have reduced contaminant concentrations in groundwater near the source (well MW1) by 43-87% Accordingly, a more realistic initial extraction rate of approximately 60-350 lbs/day would be expected The initial extraction rate may decline as much as 90% during the first six months of operation and then continue to decline slowly or stabilize RECOMMENDATIONS Ground Zero recommends installing additional groundwater monitoring wells downgradient of well MW 12 Dependent upon the analytical results of groundwater samples collected from the proposed wells, an additional 4-inch diameter groundwater extraction well would be installed in the area of this "downgradxent plume" The wells would be installed pursuant to the previously approved well installation workplan Proposed locations for the additional A Zone monitoring and extraction wells are shown on Figure 12 Drilling permits will be submitted under separate cover Ground Zero also recommends collecting additional soil vapor samples from wells MW 1 and VW 1 to evaluate whether vapor concentrations have been reduced as a result of ongoing groundwater remediation activities at the site G IGROUNDZEITULEBURGIReportsWSA_Mud wells Report 2 doe