2013-07-25_REVISION - C1981010 (2)

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STATE OF COLORADO DIVISION OF RECLAMATION, MINING AND SAFETY Department of Natural Resources 1313 Sherman St., Room 215 Denver, Colorado 80203 C O L O R ADO D I V I S I O N O F Phone: (303) 866 -3567 M FAX: (303) 832 -8106 II NTGON — St— INTEROFFICE MEMORANDUM SAFETY John W. Hickenlooper Governor Mike King TO: Jared Ebert Executive Director Loretta E. Pineda FROM: Mike Boulay M Director DATE: July 25, 2013 SUBJECT: Trapper Mine, Permit No. C- 1981 -010, Permit Revision No. 7 (PR -7) I have reviewed Trapper Mining, Inc.'s permit revision application (PR -7) with regard to groundwater hydrology information as requested. I am in agreement with the conclusions and most of the assumptions drawn by Trapper with regard to, mine inflow, drawdown in the coal aquifers, pit dewatering, and potential impacts to groundwater and adjacent wells. Pertinent calculations are given in the permit application package (PAP) in Section 4.8.2.2 Groundwater Inflow to the Mine for drawdown calculations for Ashmore Pit (p. 4- 225 -226) and for travel times for groundwater that flows through the backfill aquifer in the Browning and Colt pits (p. 4- 226a). I noted some errors with these calculations and a lack of clearly stated assumptions in some instances. In several of the sections I also noted that the term permeability seems to be consistently misused as being the same as or equal to hydraulic conductivity. For the drawdown calculation very little specifics were given about the characteristics of the aquifer discharging or how the Transmissivity value was derived. However, in earlier text there is a general statement that pump tests were carried out to determine transmissivities of these units and to estimate the amount of water that would flow into the initial mine boxcuts with no other more detailed information provided. That being said; Trapper used an appropriate equation and given the variables used in the calculation, I agree with the numbers derived for drawdown at 1,000 feet from the pit after a year (39 ft) and at one mile from the pit after four years (3.5 ft). I could not replicate the estimated travel times calculated for groundwater that flows through the backfill aquifer in the Browning and Colt pits. In the third paragraph of page 4 -226a, Trapper states an average permeability of 0.5 ft/day used in their calculation. I assume they meant for this to be an average hydraulic conductivity value. The difference being, permeability is the effective cross - sectional area for flow and it is a measure of (length)2. Hydraulic conductivity is a flow rate and it is a measure of (length) /(time). Assuming that the value given in the permit text was hydraulic conductivity as opposed to permeability I calculated both a seepage velocity and discharge velocity then used the Distance - Rate -Time formula and achieved different results than those in the PAP. Again I don't think this is a significant issue and will have little to no bearing on the PHC predictions. As indicated in the permit text the wells that could be impacted by mining that occurred in Browning and Colt pits are completed above the H coal seam. Therefore, it is likely that these wells will not have direct communication with groundwater that flows through the Office of Office of Mined Land Reclamation Denver • Grand lunction • Duran¢o Active and Inactive Mines