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w Draft Final FS Report and PP Memo -3- 12 November 1992 DDR`IW, Tracy of all of the treated ground water may not be technically feasible without complications. It is highly desirable to maintain flexibility in disposal by using alternative disposal options such as ponding, irrigation and surface water discharge to an irrigation canal . Other more economical means of disposal may ultimately be needed which were eliminated in this evaluation (Table 2.4-2) . If reinjection of all of the treated ground water is not technically feasible and with the lack of investigation of alternate disposal methods, delays in the implementation of the full scale remedial action for OU-1 may result. 5. I have several concerns with the preliminary designs for the extraction and injection wells. These concerns are discussed below. a. The preliminary extraction well field design includes the installation of extraction wells at 500 to 550 foot spacings along the down gradient north- northeastern edge of the plume (page 3-20 and Figures 3.3-2A to 3.3-2C) . These extraction wells appear to have been placed to created a hydraulic barrier near the leading edge of the plume. Although the response to our comments indicated that extraction wells will be installed in "hot spot" areas, the Figures have not been changed. Too many extraction wells appear to be proposed near the leading edges of the plume. By not installing a sufficient number of extraction wells within the plume, higher concentrations of contamination may be allowed migrate toward the wells near the leading edge of the plume. The installation of too many extraction wells near the leading edge of the plume may cause needlessly excessive water to be pumped and treated, thereby reducing the effectiveness and increasing the duration of the remedial action. It is unclear why so many extraction wells are needed in areas of the plume that are already near the aquifer cleanup level . We recommend that the number of extraction wells be increased within the plume in the areas of highest contaminant concentrations and that the number of extraction wells near the leading edges of the plume be reduced where the contaminant concentrations are at or below the aquifer cleanup level . b. The Draft Final FS Report indicates that installation of monitoring wells may be difficult due to access issues. In lieu of these wells the hydraulic barrier of extraction wells was proposed. Monitoring wells must be installed to define the leading edge of the plume and access related issues may be eliminated if the Raspo property is purchased or if the wells are installed in the rights-of-way along county roads. Furthermore, monitoring wells will ultimately be needed within the plume to evaluate the effectiveness of the ground water remedial action. c. The injection wells are proposed to be installed in the upper and middle horizons along the southern and western boundaries of the site (page 3-21 and Figures 3.3-2A to 3.3-2C) . It is unclear whether the potential hydraulic affects from the on-site supply wells on the injection wells were considered.