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<br />Mr. Bruce Humphries <br />January 3, 2000 <br />Page 3 <br />r~ <br />~J <br />Therefore, [he groundwater extracted to achieve hydraulic control in the alluvial aquifer would <br />be handled within the TR-026 water management plan. <br />The quality of the extracted groundwater would probably be of a higher quality than the water <br />currently contained in the West Pit backfill. Therefore, BMRI proposes to use the groundwater <br />pumped from the downgradient hydraulic barrier wells to accelerate the flushing of the secondary <br />sulfate salts associated with [he West Pit backfill materials. The basis for this proposal is the <br />relatively rapid rate of flushing for backfill materials intercepted by the backfill monitoring well <br />BF-l, which is located in the portion of [he West Pit and is most likely affected by the inflow of <br />upgradient Rito Seco alluvial groundwater. Water quality at BF-1 is better than at any other <br />location within the West Pit backfill, and the concentrations of constituents have been essentially <br />constant throughout the period of record. Thus, the potential exists that the bulk oaf the backfill in <br />the West Pit would respond similarly to the influx of clean water. In addition, the relatively <br />clean groundwater from the downgradient pumping wells should be more effective at flushing <br />[he secondary sulfate salts from the West Pit backfill because clea~ter water is generally more <br />effective in dissolving salt minerals as evidenced by laboratory data for leaching experiments <br />(e.g., see Appendix B, TR-026). <br />Groundwater extracted from [he downgradient alluvium would be pumped [o the West I'it and <br />delivered directly to the West Pit backfill groundwater system through either [he existing <br />intiltration trench that runs parallel to the highwaii along the north side of the West Pit or the <br />monitoring/injection wells currently in place within the backfilled pit. The inffiItration trench <br />was previously used as part of the seepage control system, and effectively managed <br />approximately 150 gpm. Thus, the existing intiltration trench should be capable of managing <br />[he flows associated with the downgradient water management plan. If neces3ary, additional <br />points of injection associated with the current groundwater monitoringMnjection well <br />configuration in the backtilled pit may be proposed in the future to ensure an even distribution of <br />the downgradient groundwater within the West Pit backfill. <br />