2005-07-26_REVISION - M2000016 - Laserfiche WebLink

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M-2000-016, Lafarge West, Inc.: Riverbend Operation, AM-01 July 25, 2005 Adequacy Memo No. 4 Page 4 If possible, please lighten the aerial photo background to more like the latest Monitoring Wells location map to improve readability and imaging quality. Rule 3.1.6(1) of the Construction Material Rules and Regulations requires that "disturbances to the prevailing hydrologic balance of the affected land and to the surrounding area and to the quantity and quality of water in surface and groundwater systems both during and after the mining operation and during reclamation shall be minimized...." The hydrologic model predicts significant impacts to the hydrologic balance, including offsite drawdowns of up to -23 feet during operations. Efforts have not been proposed to minimize impacts to the hydrologic balance, only some plans for mitigation. The applicant must identify and propose measures to comply with Rule 3.1.6 and minimize disturbances to the prevailing hydrologic balance. The applicant may want to consider using slurry walls, recharge trenches, and/or other methods. 16. Response 15 provided in the June 24~' Adequacy Response and the additional response in the July 12, 2005 Adequacy Response regarding Well Permit No. 2216, does not satisfactorily address the conclusion in the AMEC report that, "The estimated drop in groundwater level at this well should have a minimal impact to the historic use of the well." The applicant must provide data that supports that the well will still be able to pump its full yield if the static water level should drop 3 feet, as expected by the hydrologic model. (AMEC, pp.11-12) 17. Since Well Permit No. 13697 is expected to be unusable during the life of Phase 6 Cell 1, a written agreement with the owner of Well Permit No. 13697 must be obtained prior to approval of this amendment, even though the applicant reports the owner does not object to this permit. (AMEC, p. 12) 18. With permission from the well owner, it would be acceptable to use Well Permit No. 7280, as a monitoring well. However, based upon the Maximum Disturbance maps, the original conclusion that wells opposite the South Platte River from the mining activities, including Well Permit No. 7280, are not expected to be significantly affected has not been substantiated. The applicant must install or identify monitoring wells in the vicinity of all affected wells suitable for establishing a baseline and monitoring for potential impacts. See Item 15. (AMEC p. 12) 19. As requested in the April 22, 2005 adequacy review, what were the criteria and how was it determined that "only one of these wells [of the 4non-Lafarge wells within 600 feet] should not have the ability to pump at historic rates?" Please also see Item 16. The applicant must commit to speck criteria for establishing that "well yields cannot be met and mining dewatering is determined to be the cause of reduced well yield." (AMEC p. 12) 20. Response 21 of the June 24, 2005 adequacy response contains the statement, "To our knowledge the surrounding ditches are not hydraulically connected to the ground water in the area... " This does not adequately address the Division's concern over potential effects of pit dewatering on the amounts of water conveyed by the Lupton Bottom Ditch, the Brighton Ditch, and Big Dry Creek. The Division requires a demonstration that the amount of water carried by these ditches and the creek will not be adversely affected by the mining operation. Alternatively, the operator could obtain signed agreements with the ditch owners prior to approval of this amendment.