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The VOC concentrations in groundwater leaving the upper screen of the well are significantly ' lower than the influent hydrocarbon concentrations Most of the treated groundwater leaving the upper screened section of the well is retained in the UVB circulation cell 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 saturated zone with dissolved oxygen, enhancing aerobic biodegradation throughout the circulation cell ' The UVB percent stripping efficiency is defined as (1- Co,,, / C,,,) x 100, where Coin is the concentration of volatile and semivolatile hydrocarbon constituents in groundwater leaving the upper screen of the UVB well, and C,,, is the concentration of volatile and semivolatile ' hydrocarbon constituents in groundwater entering the lower screen of the UVB well Representative samples of groundwater entering and leaving the lower and upper screens of a UVB well, respectively, are collected from monitoring wells installed as close to the UVB well as possible while avoiding any damage to the UVB well packing 3.2 DESCRIPTION OF UVB SYSTEM AT FORMER EXXON RS 7-3942 ' The UVB treatment system at former Exxon RS 7-3942 consists of an in situ, 400-millimeter- diameter reduced pressure air stripper and a 5-HP blower and includes a weatherproof cover and ' moisture knockout drum The moisture knockout drum has a built-in stripping unit to treat any accumulated water prior to removal Vapors stripped from groundwater in the UVB air stripper are extracted by the UVB vacuum blower A Grundfos pump (1/3 HP, 110 volt, 6 3 amp, single phase) is used as the support pump to maintain the desired flow rate of water through the UVB system The pressure outlet of the UVB blower is connected to a Thermtech VAC 25 ' thermal/catalytic oxidizer, to destroy VOC vapors in a heated chamber prior to release into the ambient air The Thermtech VAC 25 was already onsite to treat soil vapor extracted by the SVE system The existing piping manifold at the site was modified to connect the UVB vacuum ' blower to the oxidizer (Figure 3) The UVB well consists of an upper and a lower screen and 16-inch nominal diameter well ' casing The well casing is made of mild galvanized steel with a 0 25-inch thickness The lower (Irrigator®) well screen has 4 feet of 0 025-inch slots and a 2 5-foot-long tight-wind sump with plate bottom The upper well screen is double-cased and has approximately 6 feet of 0 040-inch slots The UVB well completion diagram, along with the diagrams for adjacent monitoring wells UVB-MW 1 and UVB-MW2, is provided as Figure 4, and on its own in Appendix A ' The wellhead is sealed with a steel flange and HDPE cover The internal UVB well components are a fixed inflatable packer, a single-speed support pump, and an air stripper The support pump allows a regulated amount of water to enter the air stripper, independent of the airlift pump effect ' created by the rising air bubbles in the stripper The UVB well components are depicted in Figure 5 The design of the UVB system installed at former Exxon RS 7-3942 was based on the assumptions and technical data outlined in the Technical Plan (EA 1996) Based on those assumptions and technical data, the upstream and downstream stagnation point distance S and capture zone widths B, and Bh were calculated (EA 1996) and are shown in Figure 6 ' WP61 73942 R01397 3 6