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r • • installation of these extraction wells should be sealed by placing blank casing and grout in the annular space adjacent to the aquitard to prevent an undue amount of silt and clay from entering the well bore. This design will have the same effect as installing two individual wells at each location. 9.2 Well Discharge and Injection Rates Proposed discharge and injection rates are based on the Optimal Design Scenario and are -1, 9-2 and 9-3 for the Central, South Balloon, and North Balloon Areas, provided in Tables 9 respectively. It is probable that new extraction wells will be capable of obtaining somewhat higher extraction rates than those proposed due to increased well efficiency and full penetration. Increasing extraction rates beyond those proposed is not recommended, however, as model results indicate that increased extraction rates will not measurably increase TCE removal efficiency or containment. Several precautions have been taken in the conceptual design of the injection system. These precautions are related to the presence of elevated concentrations of nitrate and/or arsenic within the A-zone at proposed extraction well locations 455-B, 460-AB, and 475-A (MacPhee, 1992d). In order to prevent the inorganic contamination of the B- and C-zones, the volume of water 1 extracted from wells located at 455-B, 460-AB, and 475-A will be injected solely into the A- J zone under the Optimal Design Scenario. 1 Extraction rates for North and South Balloon extraction wells have been optimized based on the J results of the solute transport modeling. In addition, five South Balloon wells are no longer loperational under the proposed design (A-1, A-3, A-4, B-3, and C-2). 9.3 Extraction Well Design The leaky nature of the Sharpe Site aquifer system allows existing partially penetrating extraction wells to operate with minimal loss of efficiency. In order to maximize well efficiency and increase operational lifetime, it is recommended that new extraction wells be fully penetrating. Screened intervals should extend across transmissive portions of the zone. Significant fine- grained silt or clay lenses occurring within the zone may be sealed by placing blank casing and l grout in the annular space adjacent to these intervals. This should prevent an undue amount of J silt and clay from entering the well bore and reducing well efficiency. C-zone wells may be partially penetrating for economy given the large saturated thickness of the interval, but should 1 penetrate the upper 70 percent of the zone at a minimum. The precise depth of well screens for new extraction and injection wells are best determined in the field using a variety of techniques including 1) geological observations of sediment grain-size and degree of saturation, 2) geophysical well-logging, and 3) field sieve analysis. These same techniques are also commonly used to select the proper size of well screen openings and filter- pack material. 9 - 2