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6.2.1 Passive Water Treatment <br />The bench-scale testing conducted to date simulates a passive water treatment system that <br />consists of two phases of water treatment: (1) an initial phase to cause reduction of soluble <br />sulfate to less soluble sulfide and (2) a final treatment to oxidize and precipitate manganese. <br />Field-scale testing is required to optimize the engineering design of the passive treatment system, <br />but the laboratory testing demonstrates the effectiveness of the basic process. <br />Two possible methods exist for field-implementation of the passive treatment system: (1) <br />construction of two passive treatment cells external to the West Pit or (2) combining an in-situ <br />passive treatment in the backfill followed by a separate passive treatment cell to oxidize <br />manganese. Field testing of the passive treatment options will be conducted during the initial <br />phase of the water management plan implementation. <br />6.3 Estimated Time for Rinsing of the Rito Seco Alluvial Aquifer <br />Groundwater in the Rito Seco alluvial aquifer downgradient from the zone of outflow from the <br />West Pit is a mixture of ambient groundwater and backfill groundwater. The reversal of <br />hydraulic gradient will eliminate the flow of groundwater from the West Pit, but the chemical <br />mass loading from the West Pit into the Rito Seco alluvium that has occurred since mid-1998 <br />will remain in the alluvial aquifer. Natural groundwater transport will eventually rinse the <br />portion of the Rito Seco alluvial aquifer that has been affected by groundwater outflow from the <br />West Pit. <br />To a first approximation, the time required to flush that portion of the Rito Seco alluvial aquifer <br />that has been affected by West Pit outflow can be estimated assuming that sulfate, manganese, <br />and TDS aze chemically conservative. Under this assumption, the constituents will migrate at <br />approximately the same rate as the groundwater. Dispersion along the hydrologic flow path <br />would cause the concentration of each constituent to spread such that the increase in <br />concentration at a given location, e.g., M-I1R, would occur before the calculated travel time for <br />groundwater flow. The travel time from the zone of outflow from the West Pit to monitor well <br />Battle Mouuain Ruources, fnc. <br />paf002671reportslmarchrprttrl6wtrmng(march.doc 68 Afarch 22, 1999 <br />