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ki• 202/799.5045 Mr. Michael Rab'slh Page 9 February 6, 1991 The capture zone of the recovery well is defined by the region in which the veior"/fieko is directed towards the recovery well,and Is determined by the vector sum of the natural groundwater flow direction and the radial flow direction toward the well Induced by pumping, The capture zone differs from the radius of Influence because the latter does not Include the natural hydraulic gradient. Graphically,the capture zone Is determined by contouring the measured water table elevations at the conclusion of the pump test pure 4). The graphical method Is the most reliable, because it incorporates ail the subsurface heterogeneities. MW-8, located downgradient of the recovery well, retarded a 10 hour drawdown of 0.13 feet, and the gradient is directed towards the recovery well. Hence,the downgradient capture zone pumping at 19.5 Spm is approximately 360 feet. As expected from the low natural hydraulic gradient,the capture zone and radius of influence are nearly equal. Using the most conservative T and 5 values of e,10O gpd/ft and 0.03 In the Theis equation predicts that a 5 gpm pumping rate is sufficient to provide hydraulic control over the dissolved hydrocarbon plume, as presently defined (Appendix C). 4.0 AIR STR1PPFR DESIGN At the conclusion of the pump test, water samples were collected to assess the adequacy of the water treatment system described in the FFIP. Samples were analyzed for BTEX, TPH, EDB, total iron, and hardness. These data allow prediction of a treatment efficiency for hydrocarbons, and the likelihood o encrustation by iron or calcium precipitates. Chemical analysis of water pumped from the recovery well after 10 hours indicate concentrations of 4,100 Ag benzene and 9,700 pg/I TPH (Appendix D). Because of its toxicity, the benzene concentration will govern air stripper design. The design as previously proposed in the FRP was based on 23,000 µg/l benzene. The same tower design was re-evaluated for a 5 gpm flow rate and 4,10O pg/I benzene Influent, removal efficiency is greater than 99.9 percent(see Appendix D). Although conservative,the tower design originally specified is still recommended for the system because it is expected that any discharge alternative will require a low to non-detectable benzene concentration. Total iron is 1,000,ug/l,and should not cause encrustation. High oxygen fugacity or microbial activity could induce iron precipitation,but only system operation will determine this. The carbonate hardness Is 540 pg/l. _._ -- This value is likely to lead to scaling in the tower If left untreated. Periodic of.the.lower.an packmg material will minimize scaling problems. ��. GROUNDWATER TECHINOt.001',INC.