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The groundwater level in the well casing is also raised by the rising air bubbles (airlift effect) The reduced pressure in the well casing above the stepping reactor accelerates the nse of the bubbles When the bubbles reach the air/water interface inside the well casing, they burst and allow the VOCs to be transported upward through the well shaft to the offgas treatment unit As ' the bubbles burst, the groundwater falls along the interior wall of the well, producing a significant hydraulic pressure which drives the water honzontally into the aquifer through the upper screen section at the top of the aquifer Groundwater from the aquifer flows into the lower part of the well to replace the water exiting from the upper screen A vertical circulation pattern develops, with water entering the screen in the lower part of the well, and leaving through the upper screen segment It is expected that VOC concentrations leaving the circulation cell into the aquifer will be significantly Iower than the influent hydrocarbon concentrations As the system functions over time, the VOC concentrations should approach asymptotic levels ' Most of the treated groundwater leaving the upper screened section of the well circulates through the sphere of influence, and re-enters the lower screen section a number of times Since the treated groundwater leaving the UVB well is almost saturated with oxygen, the effluent ' groundwater enriches the phreatic zone with dissolved oxygen, enhancing aerobic biodegradation throughout the circulation cell 2.3 UVB MONITORING WELLS Two groundwater monitoring wells will be installed adjacent to the UVB well to monitor the groundwater entering the lower screen and leaving the upper screen The monitoring wells will be 2-inch PVC wells located 1 foot from the 24-inch UVB well annulus The lower-screened UVB monitoring well (UVBMW 1) will be placed downgradient of the UVB well This well will placed at a depth of approximately 50 feet bgs and be used to monitor groundwater entenng the UVB well UVBMW 1 will consist of a 4-foot screen with 0 020-inch slotted Schedule 40 PVC The upper-screened UVB monitoring well (UVBMW 2) will be placed upgradient of the 1a UVB well The 2-inch well will be placed at a depth of approximately 40 feet bgs This well will consist of a 6-foot screen and also have 0 020-inch slots schedule 40 PVC UVBMW 2 will be used to monitor groundwater leaving (discharged from) the UVB well 1 WP61 73942 WP1096 TX 5 3