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Jun 6 1995 4 21PM 1EG TECHNOLOGIES No 2615 P 23/34 • Prior to initiation of active remedial measures, dissolved petroleum hydrocarbon constituents historically were present in all five original monitoring wells lMW-1 through MW-5, Figure 2). Additional monitoring wells (MW-6 and DW-1) were installed after UVB system activation to better define both the lateral and vertical extent of petroleum hydrocarbon contamination The lateral extent of dissolved BTEX in groundwater has been interpreted from analytical data gathered prior to activation of the UVB and after system start-up. Vertical hydrocarbon extent is being monitored through the Type III monitoring well DW-1 completed in bedrock and located cross gradient to the UVB system. The UVB system was activated on September 17, 1992. The capture zone, circulation cell, and release zone were calculated to encompass the areas shown in Figure 4. The capture zone is defined by the width St at the top of the aquifer and the width Bb at the bottom of the aquifer. The initial hydrocarbon removal rate in the off-air stream was estimated to be 34.3 lbs/day (Table 1). Since a portion of the upper screen is open to the vadose zone, soil vapor extraction is occurring However, about 95% of the initial removal rate was attributed to hydrocarbon removal from the saturated zone and capillary fringe. One day after start-up, the removal rate was 23 lbs/day Since operating day 58 to the present (operating day 796), the removal rate has remained below 1 .3 lbs/day. Prior to the most recent sampling event the stripping reactor efficiency has ranged from 58% to 100% for BTEX and from 95% to 100% for IPE, based on the analytical results for the shallow and deep UVB annulus wells (Table 2). Results from the most recent sampling event (operating day 796) revealed that the BTEX concentrations were higher in the shallow annulus well than in the deep annulus well, which is a reversal of the normal trend This was likely due to the fact that sampling occurred . only a few hours after restarting the system following a shut-down. Hydrocarbon removals and reductions are summarized in Tables 1 and 2, and Figures 5 and 6 A significant decrease in the concentration of dissolved BTEX near the center of the plume at MW-3 (10 ft from the UVB well) has been Observed since the first round of sampling after start-up through the most recent sampling event on operating day 796 (Table 2) At startup, dissolved BTEX was 1719 ug/) at MW-3 All post start-up dissolved BTEX concentrations at MW-3 have been less than 254 ugA. Also, comparing dissolved constituent concentrations in the other monitoring wells, the lateral extents of the dissolved BTEX plume (Figures 2 and 6. Table 2) and dissolved IPI: plume (Table 2) have diminished. In addition to the physical removal of VOCs which takes place within the stripping reactor,the UVB also introduces dissolved oxygen (DO) into the circulating groundwater. As expected, there is a characteristic increase in DO between the deep and shallow annulus wells of the UVB (Figure 7) Moving out into the aquifer, there :s a rapid decrease with distance from the UVB as evidenced by the DO measurements in MW-3 (Table 4). This decrease is likely due to dilution, mixing, and biodegradation of hydrocarbons in the aerobic zone surrounding the UVB well 3