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EXXON,4444 PERSHING -2- 12 November 1996 STOCKTON,SAN JOAQUIN COUNTY THE REMEDIATION PROPOSAL A soil vapor extraction system (SVES) was installed and began operation in 1994 The system consisted of four vadose zone wells screened from 10 to 30 feet bgs A total of 16,000 lbs of hydrocarbons have been removed However, significant soil contamination remains trapped in soil below the water table, between 27 and 42 feet bgs In March 1996, depth to water was approximately 20 to 21 feet In addition to two air sparging wells and six vapor extraction wells, Exxon's consultant, EA Engineering, Science, and Technology, has proposed to use UVB (Unterdruck Verdampfer Brunnen- vacuum vaporizer well) technology The UVB system extracts groundwater through a lower well screen section, strips VOCs from the water in a negative pressure stripping reactor (air stripper), and discharges the treated groundwater through an upper well screen section, back to the aquifer It also uses an aboveground vacuum extraction blower Gases from the UVB, along with the gases from the air sparging and vapor extraction system, will be treated with a catalytic oxidizer The system does not lower the water table The proposed UVB treatment well is 16 inch galvanized steel, with two screened zones below the water surface, one at the bottom of the treatment interval, and one at the top The borehole annulus between the two screened zones is sealed with bentonite A packer is installed in the treatment well between the two screened zones to ensure one directional flow of the groundwater A pipe is placed through the packer, extending from the pump, which provides groundwater from the lower zone, to the air stripper in the upper zone The upper part of the well, is maintained below atmospheric pressure by a centrifugal blower The air for the in-situ stripping is drawn in through a fresh air pipe, the upper end is open to the atmosphere, and the lower end terminates in a pinhole plate (diffuser) in the air stripper The negative pressure within the upper part of the well causes a water level rise within the treatment well The air stepper is balanced below the groundwater level, within the treatment well, and is free floating to allow for changing water levels Between the pinhole plate and the water surface is the stripping zone, in which air bubble flow strips VOCs from the groundwater The rising air bubbles also produce an air lift pumping effect, which moves the water up, and causes a suction effect at the bottom of the well The upward streaming, stripped groundwater leaves the well casing through the upper screen The mass of flowing groundwater leaving the upper screen is counterbalanced by the flow of groundwater toward the lower screened section,thereby setting up a three-dimensional groundwater circulation cell around the UVB well Contaminated groundwater flows toward the well through the lower screen and is moved up in the treatment well to the stripping zone by the action of the pump and the air lift effect The water is stripped of VOCs, and is returned to the groundwater through the upper screen The artificial groundwater circulation cell established by the UVB system is superimposed over the natural groundwater flow surface, and results in a three-dimensional flow regime around the UVB well