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• • III IIIIIIIIIIIII III <br />999 <br />STATE OF COLORADO <br />DIVISION OF MINERALS AND GEOLOGY <br />Departmem of Natural Resources <br />1313 Sherman $t., Room 215 <br />Denver, Colorado 80203 <br />Phone: (3031 866-3567 <br />FAX: 13031 832-81 06 <br />DATE: March 22, 2000 <br />DIVISION OF <br />MINERALS <br />GEOLOGY <br />RECLAMATION <br />MINING•SAFETY <br />TO: Erica Crosby Bill Owens <br /> Governor <br />FROM: Allen Sorenson <br />~ Greg E. Walther <br /> / , Executive Director <br /> Michael B. Long <br />RE: Slurry Wall and Interior Dike, L.G. Everist, Inc., Division Dnector <br /> Ft. Lupton Sand and Gravel Mine, File No. M-99-120 <br />Introduction <br />The application proposes the installation of a slurry wall around the pit area prior to commencement of <br />mining. Design details are provided but no specifications or quality control/quality assurance plan is <br />included in the application. The application proposes to bond for the cost to install the slurry wall. <br />Such a bond could serve as a contingency if leakage were to occur in excess of criteria established by <br />the Office of the State Engineer or if the slurry wall were to be damaged by pit slope failure. The unit <br />cost in Exhibit L used to establish the amount of required bond is $3.00 per square foot of slurry wall <br />assuming a 36 foot depth for the wall. <br />Lined reservoirs that employ slurry walls to prevent hydrologic communication with ground water are a <br />viable water storage alternative and are adequate to meet the developed water resources post mining <br />land use. However, leakage of ground water through or around the slurry wall into the pit a[ a rate in <br />excess of 3x0.03 ft3/day/fr' would constitute a failure to achieve the designated post mining land use <br />under leakage criteria established by the State Engineer. If an operator were to mine out a pit within the <br />perimeter of a previously installed slurry wall, and it were subsequently determined that ground water <br />was leaking into the pit a[ a rate in excess of 3x0.03 ft3/day/ft', the operator would be faced with the <br />following three options: <br />Improve the seal in the pit to meet the Stale Engineer's leakage criteria. <br />Change the post mining land use and provide water to augment for ground water leaking into the pit <br />that is lost to evaporation. <br />• Backfill the pit so that water is no longer exposed to the atmosphere. <br />In the event the Division were to assume responsibility for reclamation of the pit through bond <br />forfeiture, we may be confronted with the same situation and the same three options. Of these options, <br />water augmentation in the South Platte River Basin and backfilling of the pit with inert fill would be the <br />most costly. Also, neither of these options would result in reclamation to the develo~ed water resources <br />use. This leaves the option of assuring that leakage into the pit is less than 3x0.03 ft /day/ft' through <br />reinstallation or repair of the slurry wall. <br />