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4 EXXON,4444 PERSHING -3- 12 November 1996 STOCKTON,SAN JOAQUIN COUNTY Most of the treated groundwater leaving the upper screened section of the well circulates through the UVB sphere of influence, and re-enters the Iower screen section a number of times The treated groundwater leaving the UVB well is almost saturated with oxygen,thereby enhancing aerobic biodegradation in the circulation cell Bacterial growth will be monitored by plate count and epnfluorescence If microbial growth is limited by rutrogen, phosphorus, soil moisture, or pH, amendments will be introduced to the subsurface Two groundwater monitoring wells will be installed I foot from the UVB well to monitor the groundwater entering the lower screen and leaving the upper screen The UVB well will be installed near existing well RW3, which will be overdrilled and converted into one of the UVB momtoring wells The circulation cell width was calculated to be approximately 28 meters The maximum time to complete one pore volume flush of the circulation zone is 40 to 68 days The UVB well will draw in approximately 200 mYhr of air, and the pump will provide a ground water pumping rate of 2 m3/hr into the air stripper These flow rates produce an air to water ratio of 100 1, and a stripping efficiency of 90- 99% It is estimated that 90% of the influent water will be recirculated more than once The VOC concentrations leaving the circulation cell are "expected"to be lower than the influent concentrations COMMENTS ON UVB 1 The ability of the UVB well to drive vertical circulation through a thick interval of low hydraulic conductivity is limited From the cross sections, it is not clear that the effluent will be discharged into a sandy zone, although that appears to be the intent 2 The water discharged from the UVB well is unlikely to meet the discharge requirements in the General WDRs for petroleum cleanups 3 If bacterial growth is limited by nutrients, they propose to add them to the subsurface 4 This proposal is not intended to provide hydraulic control The downgradient flow rate appears to be increased for a limited distance However, it is not clear how the discharge will effect the downgradient plume outside the circulation zone 5 The subsurface zone influenced by the UVB well may be thought of as the treatment zone, rather than the discharge point Outside the treatment zone,there appears to be no increase in ground water flow rates, although this is not clear 6 No proposed discharge concentrations were provided 7 The reason for installing the two UVB wells only 1 foot from the UVB was not explained I understand that it is difficult to install wells so close together because augers do not always remain perfectly vertical after entering the subsurface 8 This proposal is not intended to treat the entire contaminant plume and a significant portion will not be affected by the UVB well 9 According to the case studies, ground water contamination concentrations may increase after startup of the UVB well due to "mobilization of contaminants from the pore spaces", presumably due to the flushing effect of the water circulation 10 The case studies show a decrease in contaminant concentration in some monitoring wells, however, not all monitoring wells within the treatment zone were impacted and the impact of the UVB appears limited