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Please explain whether the cause of the downward gradient is understood <br /> (water supply well extractions?). <br /> 11. Section 3.6 describes aquifer pump tests. Two pumping tests are planned; one in the <br /> Middle Horizon and one in the Lower Horizon. <br /> Please indicate whether the hydraulic characteristics of the Upper Horizon in <br /> the vicinity of the eastern arm of the plume is adequately defined for well field <br /> design. It appears that a pumping test might be needed to help design the <br /> capture zone well spacings and rates in that area. <br /> 12. Section 3.7 describes the injection well malfunction study. Besides consideration of <br /> pretreatment needs, the work plan should consider "post-treatment" needs to prepare <br /> the treated water for injection. <br /> Please modify the work plan to clearly identify as an objective the <br /> determination of injection water quality criteria (inorganic chemistry and <br /> solids). The results of the work plan studies should identify water quality <br /> design parameters as well as determine their specific values, to increase the <br /> probability for a successful injection system. The studies of the existing <br /> injection wells, and other injection well fields, should provide this criteria. <br /> Please include in the work plan an identification potential site-specific injection <br /> water-aquifer water incompatibility problems, and provide water quality criteria <br /> to minimize their probability of occurrence. (The effluent water quality <br /> characteristics, based on the bench scale treatment studies, should be compared <br /> with the water quality characteristics of each injection zone.) <br /> 13. Section 3.7.1.1 describes how the existing injection wells are reported to have <br /> extensive biological and chemical encrustation. <br /> Please clarify whether rehabilitation of these wells is on-going, or whether <br /> rehabilitation methods will be field-tested during the planned pre-design phase <br /> of data collection. (Are the bench tests with Well Mean II intended to address <br /> this issue?) The results of rehabilitation tests on the existing interim wells <br /> could provide useful information for the one-year operation and maintenance <br /> plan for the final system. <br /> EPA/11 JAN94 4/4 <br />