2008-09-17_REVISION - M1980244 (28)

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North Cresson proposed mining area, and its potential to possibly generate acidic • discharges, would it not make sense to completely back fill this area to original contour? In looking at Drawing C-6, Mine area Cross section labeled as North Cresson Mine Area A-A', it could potentially trap drainage from the Globe Hill area to the Schist Island partial backfill. Please Explain how CC&V will mitigate if such occurrences take place, given the permeability of the Schist Island area and the potential for standing water to exist through time in the partial back fill area. The legend for this map labels green sites as areas to be reclaimed, does that mean the green line in x-section A-A' between elevations 10,400 and 10,500 labeled in green, is part of a small backfill area? Please explain. Response: Based upon the reclamation plan submitted in the MLE Project Application, the current Schist Island portion of the North Cresson configuration is free draining with provisions for positive surface drainage. As such, there is not an issue. However, based upon public comment, CC&V has agreed to submit an operational change (in the form of a technical revision for DRMS purposes) after issuance of approval of the MLE Project Application to backfill and regrade the Schist Island mine highwall view shed area to a maximum grade of 2H:1 V, provide growth medium, and revegetate in accordance with the existing approved reclamation plan. This commitment has been memorialized in the approval of the Teller County Mine Development Plan for the Cresson MLE Project. The positive surface drainage as addressed in the MLE Project Application will be maintained in the regrading proposal to be submitted. • DRMS has raised the issue of the potential that water could flow from the Globe Hill mine, which is currently scheduled to not be backfilled, to the eastern highwall of the partially backfilled Schist Island mine. This possibility has been evaluated (Attachment 7) using a reasonable range of assumptions about the flozn characteristics of the diatremal rockmass. The result of this evaluation is that all precipitation falling within the perimeter of the Globe Hill mine will infiltrate vertically into the diatreme, and will eventually migrate to the regional ground water table that is intersected by the Carlton Tunnel. There is no credible hydraulic conductivity condition that would allow flo717 from the Globe Hill mine to pass laterally to the Schist Island mine highwall. In the unanticipated event that water was to flow laterally from the Globe Hill mine to the Schist Island mine, the water would infiltrate into the Schist island surface mine backfill material, and flow vertically into the dewatered diatremal bedrock beneath. In order for Water not to flow out of the Schist Island mine on the surface, the CC&V proposed final topography will mitigate this (remote) possibility by backfilling the mine to no higher than 5 feet below the lowest elevation of the western highwall. This will ensure infiltration of all water that enters the perimeter of the Schist Island mine will enter the diatremal rockmass, come into chemical equilibrium with the high carbonate rockmass beneath, and eventually migrate to the regional ground water table that is intersected by the Carlton Tunnel. The green line between 10,400' and 10,500' on cross section A-A' is a reclaimed portion of the • SGOSA which exists to the east of the proposed Globe Hill mining area. This area will be reclaimed in accordance with Figure F-1. I i. 9