SITE INFORMATION AND CORRESPONDENCE

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San Joaquin Uey Air Pollution Control District Page 5 In Ground Zero's experience at other sites we have found that the common effective radius of influence of a vapor well is approximately 30 feet. The minimum radius of influence typically measured in fine-grained soils is 15 to 20 feet. Based upon the lithology exhibited in site cross sections and the data from the VETs, we expect that subsurface vapor flow near the water table will be primarily within the thin sand unit between the depths of approximately 55 and 60 feet bgs, with a radius of influence of approximately 35-50 feet. However, analytical results of soil samples collected at the site suggest that the bulk of residual soil contamination in the vadose zone lies in fine-grained sediments. The primary mode of vapor removal within this zone is via diffusion due to the low permeability of the soil zone (silticlay). Thus the mass loss rate from contaminated soil will be limited by the rate of diffusion of contamination from the less permeable zones to the more permeable sand zones. Laboratory Analyses and Results Vapor samples were collected from the vapor stream prior to the addition of dilution air. The samples were collected in Tedlar bags and transported under chain of custody to State-certified Air Toxics, Ltd. (ELAP # 1149) in Folsom, CA. The vapor samples were analyzed for total petroleum hydrocarbons (C5+) as gasoline (TPHg), C2-C4 hydrocarbons referenced to gasoline and for benzene,toluene, ethylbenzene, and total xylenes (BTEX)using EPA Method TO-3. Reported TPHg (C5+) concentrations ranged from 220,000 to 240,000 micrograms per liter (ug/L) in well MW1 and from 65,000 to 66,000 ug/L in well VW1. Benzene was detected at 6,300 ug/L in MW1 and 2,400 ug/L in VW1. MTBE was detected at 3,800 ug/L in MWI and 1,700 ug/L in VW1. Vapor analytical results are summarized on Table 6. Projected Gasoline Removal Rates The VET was conducted to determine if soil vapor extraction would be an effective method to remediate vadose soil contamination and reduce or eliminate free-phase gasoline hydrocarbons associated with a former gasoline UST. Analytical data from soil samples in and around the former UST pit suggest that a small volume of affected soil remains in the vadose zone, most of the impacted soil lies within the capillary fringe and saturated zones. The water table beneath the site currently exists at a depth of approximately 66-70 feet bgs. However, vapor extraction should be effective in reducing free-phase gasoline that may remain on the water table, and may accelerate the reduction of dissolved gasoline constituents. Based on the test results, the effective radii of influence for the wells is estimated to be approximately 35-50 feet, which would provide adequate coverage of the source area as well as some coverage of the area immediately downgradient (east and northeast) of the source area. The projected flow rate at startup of a vapor extraction system (VES) would be approximately 25 to 30 cfm from well VW1 and approximately 15 to 20 cfm from well MW L Assuming an average gasoline vapor concentration (C5+) of approximately 65,000 ug/L from VW1 and 200,000 ug/L from MW 1, this corresponds to an initial gasoline extraction rate of approximately 450-600 pounds per day. The initial recovery rate would not be sustainable, however, due to substantial decreases in vapor concentrations that typically occur within the first few months of VES operation. i GAGROUND?_LITULEBURGWaporExtraction permits\VE-Scvrltr.DOC I