2011-06-13_REVISION - C1981019 - Laserfiche WebLink

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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 Phone: (303) 866-3567 FAX: (303) 832-8106 MEMO DATE: June 13, 2011 Ifiz FROM: Tom Kaldenbach To: Janet Binns Re: Colowyo Mine (C-1981-019), PR-3, Ground Water Review Follow-up comments to the operator's responses of May 20, 2011 are provided below. 2.05.6(3)(a) Protection of the Hydrologic Balance 1. Response is adequate. COLORADO D IV IS I ON Of RECLAMATION MINING - SAFETY John W. Hickenlooper Governor Mike King Executive Director Loretta E. Pineda Director 2. No further further response is needed for this comment, as pitwall seepage in Collom Lite pit is addressed elsewhere in this letter. 5. As previously requested, please use in the calculations of spoil saturation time a reasonable surface infiltration rate into Collom Lite spoil of 3.1 inches per year, calculated as 18% X 17 inches per year average precipitation. In order to reflect the 3.1 inches per year infiltration, please change the 0.016 cfs value on page 133 to 0.31 cfs, and change the 0.75 inches per year recharge rate at the bottom of page 136 to 3.1 inches per year. Our letter of April 22, 2011 explained that the 18% factor is based on Williams and Clark's (1994) spoil lysimeter study which found that approximately 18% of annual precipitation recharge spoil at a Routt County mine. The lithologic differences between the Danforth Hills area and the lower Williams Fork Formation that are cited on page 23 of your 5/20/11 response letter would not cause a significant difference in spoil permeability ( and, consequently, spoil saturation rate) because the effective porosity in spoil is between particles of spoil, rather than inside the particles of spoil. Also, a spoil infiltration study found the Colowyo spoil infiltration rate was not less than infiltration rates at mines developed in the lower Williams Fork coal sequence (Table 10, item 6 of Exhibit 7 of the Colowyo permit application). 6. Our original comment was a request to use in the Collom Lite pit backfill saturation calculation that currently is on page 137, a value for effective porosity of 15% (based on spoil swell of that amount), rather than the total porosity of 35% that is shown on that page. Most of the flow and resulting saturation in spoil will occur between particles of spoil, rather than through particles of spoil; therefore; the porosity between spoil particles is the effective porosity, and effective porosity is the appropriate porosity value for performing a spoil saturation calculation. It seems reasonable in a spoil saturation calculation to add 5 percentage points for ineffective porosity inside spoil particles that may become partially saturated, resulting in 20% porosity for the percentage of spoil backfill that will become saturated (15% effective porosity between particles, plus 5% partially saturated particles). A value of 20% is consistent with the value of 20% that is Office of Office of Mined Land Reclamation Denver • Grand Junction Durango Active and Inactive Mines j