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III IIIIIIIIIIIIIIII <br />999 <br />o~"w, ~Jnited States Forest Grand I11esa, Uncompahgre and P.O. Box ]030 <br />~ ~ Department of Service Gunnison National Forests N Rio Grande Ave. <br />Agriculture Paonia Ranger District Paonia, CO A1428 <br />PHONE NO. 970-527-4131 FAX 970-527-475] <br />File Code: 2$2Q <br />Date: August 23, 1999RE(i~~VEp <br />Mr. Joe Dudash <br />Colorado Division of Minerals and Geology <br />1313 Sherman Street, Room 215 <br />Denver. CO $(1203 <br />Dear Joe: <br />AUG 2 51999 <br />DNision of Minerals & Geology <br />Thank you for sending the notification of tte Sanborn Creek Mine Technical Revision 35, and <br />the updated Probable Hydrologic Consequences (PHC) for our review. We have the following <br />comments on the PHC: <br />1) In the section Protection of Ground Water Ouanttty and Quality (bottom of page 12), the <br />discussion refers to "...potential to effect ground water quantity through int7ows to the mine <br />workings from the water table aquifer...and will result in temporary localized depression of <br />the piezometric surface..." The PHC on pages 4 to 5 describes ground water contained in the <br />D coal seam as a component of the "water ta61e aquifer". Given that, [his statement is mis- <br />leading in that it references effects to the "water table aquifer", and then uses the term "piezo- <br />metricsurface", which implies that a confined ground-water system is present. The <br />unconfined/cont3ned nature of the ground water in the D Seam is unclear and needs to be <br />clarified and supported by any existing data. In context of the discussion, it would be clearer <br />to discuss that mining operations have the potential to effect the quantity of ground water in <br />[he D Seam through int7ows to mine workings. <br />2) In the section Underground Coa] Removal (page 13) the end of the first paragraph dis- <br />cusses "localized reduction in the potentiometric surface" and goes on to say that the potetio- <br />metric surface will reestablish post-mining and reach equilibrium with the surrounding water <br />table. As in Item 2, this statement is misleading in that it is unclear whether the ground water <br />in the coal seam has been fowrd to be unconfined or confined, or whether it is unconfined <br />near the outcrops, becoming confined further "inland". This point needs to be clarified in the <br />PHC. If data show that it is unconfined, then the water table associated with the ground water <br />in the coal seam will depress and may return to pre-mining levels. If it is confined, then min- <br />ing would depressurize the system and alter the related potentiometric surface. The potentio- <br />metric surface may or may not return to pre-mining levels after strata alteration. <br />3) On page 27, the last paragraph references that some tension cracks may extend to the <br />ground surface. This statement needs to address whether these cracks aIE contiguous to the <br />mine level, thereby providing a pathway for water to he diverted, or whether they are surface <br />tension cracks. <br />ups ~~ <br />Caring for the Land and Serving People Pnnletl on Reryebtl Paper <br />