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Ms. Paige Beville -2- January 13, 1982 I. ARCO Coal Company has incorrectly stated that O.Z acre-feet of sediment per acre of disturbed area was used to calculate sedirrent storage for the Zoad- out sediment pond. By the Division's calculations, 1,285 cubic feet of sediment storage is required for 0.295 acres of disturbance rather than 8 cubic feet. ARCO must supply further justification that the figure of 24 cubic feet is adequate or revise the plan to allow for adequate sediment storage. II, It appears, after review of the pond dimensions, that the pond size is inadequate. The pond dimensions from drawing C-006R are roughly 38 feet Zong, 9 feet wide and 2.5 feet deep. A pond with vertical sides and these dimensions would have a capacity of 855 cubic feet, so a pond with sloping sides would have even less capacity. The pond at the Zoadout must have a capacity of 2,577 cubic feet using ARCO's figures, and 2,838 cubic feet if 0.2 acre-feet per acre is used to calculate sediment storage. ARCO Coal must supply supporting information to verify that the sediment pond has adequate capacity for sediment storage and washdown storage below the primary dewatering device, and additional capacity for the 10-year, 24-hour inflow without discharging from the emergency spillway. . ~~ 3. The Division has previously reviewed your October 27,~I98I response to the requirements of Stipulation No. 14, dealing with the design of the future upper waste rock pile. Your response included an Exhibit 3.2.9.E entitled "Prelim- inary Stability Evaluation, Upper Waste Rock Pile, Mt. Gunnison Coat Mine, for ARCO Coat Company", and a statement in the form of a revised page to the permit application stating that the waste pile would not be required during the first permit term. This being the case, the Division replied on December 10, Z98Z that the requirements of Stipulation No. Z4 were not applicable to the initial ' 5-year permit term and that ARCO Coal would not be required to comply with the terms of stipulation No. 14 during the initial 5-year permit. However, in order for the Division to be able to approve the waste pile during the future permit term in which the pile is to be constructed, ARCO Coal must follow the recommendations crontained within the preliminary stability evaluation. This wilt entail conducting some additional field work in the near future, primarily involving the installation of an appropriate surface monitoring system in accordance with recommendation No. 8 of the preliminary stability evaluation. This recoimnendati~ indicates that prior to the driving of any mine entries beneath the proposed area of the upper waste pile, an appropriate surface monitoring system should be installed to detect any deformation of the upper bench or the slope between the crest of the bench and the portal entries. Because ARCO is currently installing the mine entries and will commence the development of sub-mains shortly thereafter, it is essential that the anni- toring system, as recommended by Geo-Xydro Consulting, Inc., be installed prior to the driving of the entries beneath the pile. Therefore, the Division's acceptance of Exhibit 3.2.9.B will be contingent upon ARCO's submission of, our approval of, and ARCO's installation of the surface monitoring system. ~~.~~ ~~~„ f~~v~~S~c~~ ~ p. c~~~~~`II~ ~~~~~ -~'//l~ire. I Z-~~P~ ~ouvt~ r~? S~~.i~. ~,~5p0V1 ~ S .~