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rock and sediment, then the leachate will migrate from the backfill to adjacent rock and sediment. (a clay liner will prevent leachate migration from A and B pits where the backfill material is ash.) The migration of spoil leachate from A pit will occur through the interconnected, intergranular pores (effective porosity) in the backfill, rock, and sediment. The migration will be gravity-driven, and will be controlled by the inherent capacity of the rock to transmit water (the hydraulic conductivity). The velocity of this subsurface migration will be limited by the hydraulic conductivity and the hydraulic gradient in receiving aquifers, downgradient. In the confined Laramie aquifer (sandstone and coal beds), the Darcy equation can be used to predict that the velocity of leachate and ground water flow will be on the order of 1 ft. per yeaz, as calculated in Table 1. The Darcy equation cannot be used to predict the ground water flow velocity in Ennis Draw alluvium or eolian sand because these units are unconfined. Ground water flow velocity in the Ennis Draw alluvium can be expected to be on the order of 170 ft. per yeaz as documented for similar hydrogeologic conditions in the High Plains aquifer of eastern Colorado. The direction of all subsurface flow (leachate and ground water) would be northeast from A pit towazd Ennis Draw alluvial monitoring well DH-96, approximately 6,000 fr. downgradient from the pits. Potential for /eachate discharge from mine backfill to land surface. The relatively flat topography of the reclaimed pit areas and subsurface migration would prevent significant formation of surface spoil leachate springs or seeps at the Keenesburg Mine. The relatively flat topography of the reclaimed pit areas would prevent significant formation of surface ash leachate springs or seeps at the Keenesburg Mine. Potential for degradation of ground water in rock and sediment next to mine site. As spoil leachate migrates through the subsurface from A pit through adjacent sediment and rock, it will mix with ground water along the flow path and load the ground water with dissolved solids. The amount of loading will diminish with distance from the pit, as the leachate becomes diluted by ground water the leachate mixes with along the migration pathway. This dilution process along the migration pathway is called dispersion, and can be thought of as a repeated series of cycles of mixing of ground water with leachate. In each mixing cycle the leachate will become successively more dilute, and eventually the leachate will approach the composition of the native ground water. Eolian sand and Ennis Draw alluvium -Table 2 shows calculations of resulting leachate concentrations resulting from successive mixing cycles in the sand and alluvium. TDS and SAR of leachate will be diluted to within 25% of uncontaminated ground water afer one dilution cycle. If a dilution cycle in the alluvium and sand occurs within 100 ft. of the pit, then a significant leachate plume in the alluvium and sands would not extend more than 100 ft. downgradient from the pit. SAR values would also be less than the potential impairment level of SAR 10 after one dilution cycle. A plume would take less than one year to form (after complete saturation of backfill), based on the 170 f11yr flow velocity mentioned above. The plume would spread out (attenuate) vertically and laterally along its flow path in the alluvium and sand. This attenuation would further reduce the concentration of the plume.