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III IIIIIIIIIIIII III June 20, 1994 Mr. Joe Dudash Colorado Division of Minerals and Geology 1313 Sherman, Room 215 Denver, CO 80203 ~E~'Ef VET, JAN 2~ ~ e~.;~; .,.pn w;Nine~a~s ~ CEaeny Subject: Basin Resources, New Elk Mine C-81-012, Technical Revision 34 Dear Mr. Dudash, Basin Resources is responding to the fourth adequacy letter on TR 34, dated June 9, 1994. Questions will be answered in the order in which they were asked. Three questions, numbered 6, 7, and 8 will arrive under separate cover from the mine office. 1. The current inflows into the mine indicate that the void space is being filled from an aquifer outside the former workings. This aquifer could be either an unconfined aquifer above the mine or from an confined aquifer from below the mine. Either source will contribute to [he inflow until a hydrostatic balance condition is met. This means that the pressure of the water currently entering the mine is currently greater than that of the void space. In order for water [o migrate from the void space in the former mine workings, these workings will have to be completely filled. This condition was estimated in the correspondence of May 11, 1994 to require 7.76 years. At that time a condition will be established where the hydrostatic head of [he inflow will then encounter pressures that may inhibit flow. This will then allow the aquifers (either unconfined above, or confined below) to then reach an equilibrium condition. Since the flow is currently into the mine, the current pressure gradient indicates that the flow of water out from the former mine working is unlikely. Furthermore, once the mine is filled, the likelihood of this pressure gradient continuing to exist is high; therefore, effectively not allowing the migration of [he water from the current mine spaces. 2. Table 26C has been modified to compare projected water quality of the Middle Fork of the Purgatoire with Receiving Stream Standards and NPDES effluent standards, should the mine discharge water and should the Colorado Department of Health establish a discharge point at the site. The NPDES standards on the chart reflect the standards currently being addressed at GE-002 minus the sediment based standards. It is assumed that all of the solids will stay in [he mine, and that the discharge will basically be clear and full of dissolved constituents. It is difficult to anticipate 5990 Greenwood Plaza Blvd. Suite 250 Englewood, CO 80111 (303)850-0930 Fax(303)721-9298