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average concentration of 0 007 mg/L, average flow rate of 67 cubic feet per minute (cfm), see Table 3 and Appendix D — Field notes/Chromatograms Previous vapor extraction removals were controlled by Thermal Oxidation and removed 74,849 pounds of gasoline range hydrocarbons Weekly bio-venting of MW-8 and MW-12 are being conducted by Lawrence Tank Testing Currently the groundwater is too high, no slotted casing is exposed above groundwater at MW12 to conduct bio-venting As of July 30, 1997 the 2 hour once a week bio venting of MW-8 has removed a calculated 305 pounds of gasoline range hydrocarbons and a total of 3985 pounds of hydrocarbons as gasoline and CO2 The average flow rate is 50 8 cfm, with a vapor stream consisting of TPHg of<0 001 mg/L and CO2 at 15%, see Table 4 and Appendix D Update of groundwater contaminant plume migration controls Groundwater plume containment is being provided by vapor extraction of MW1, MW2, MW3 and MW10 Also the weekly purging and venting of MW8 and MW12 by Lawrence Tank Testing MW8 is purged of approximately 40 gallons of groundwater per week and MW12 is purged of approximately 70 gallons of groundwater per week This purged groundwater is stored on site in the 1000 gallon ConVault and is air stripped prior to discharge to the City of Stocktons sewer system under permit The most recent groundwater quarterly sampling occurred on June 11, 1997 Monitor wells MW1, MW2, MW3, MW8, MW10 and MW12 were sampled, see Appendix E STable 1 represents groundwater chemical results MW1, MW3 and MW10 showed slight increased in gasoline range hydrocarbons MW2, MW8 and MW12 showed decreases in gasoline range hydrocarbons and indicate that the bio venting is helping reduce and control the groundwater contaminant plume, see Figure 5 Figure 4 represents the groundwater gradient present during the last sampling, June 11, 1997 Groundwater lows are associated with vapor recovery wells MW1, MW3 and MW10, and a groundwater high at MW2 Overall flow direction is to the east at a gradient of 0 004 feet Vapor extraction has reduced the contaminant levels in the soil and groundwater Fluctuating groundwater elevations most likely influence fluctuations in groundwater concentration levels Summary SOIL Soil sample borings S134, SB5, SB6 and SB7 were drilled and sampled near previous sample boring SBI, SB2 and SB3 see Figure 3 Charts were developed to show the reduction comparisons of the soil sample results, see Table 2 for results No MTBE was noted in any of the soil samples Comparing the historic highest contaminated depth, fifty feet below ground surface (bgs), the following reductions in contaminants are noted and are representative of the overall site conditions 7