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The Union Milling Company <br />P.O. Box 620490 <br />Littleton, CO 80162 -0490 <br />+1.303.947.3499 <br />July 16, 2013 <br />Mr. Michael Cunningham <br />Environmental Protection Specialist RECEIVED <br />Division of Reclamation, Mining and Safety <br />Department of Natural Resources <br />1313 Sherman Street, Room 215 JUL 16 2013 <br />Denver, CO 80203 <br />michael.cunningham @state.co.us DIVISION OF RECLAMATION <br />MINING AND SAFETY <br />RE: Leadville Mill; M1990 -057, TR -4; Adequacy Review No.1 — Extension Request No. 2 <br />Dear Mr. Cunningham, <br />Union Milling Company (UMC) requests a 14 -day extension to allow for review of our responses <br />by the Division. We have observed the perched water in the TSF embankment since the spring <br />and have based our Adequacy Review No. 1 responses based on these observations. <br />Following are our work -in- progress responses. <br />The Division has reviewed the Applicant's proposed methods for addressing the <br />perched aquifer which was discovered under the tailings storage facility (TSF). The <br />Divisions' primary concern is with the long -term stability of the TSF and the structural <br />integrity of the composite liner. Two of the proposed methods of remediation temporarily <br />eliminate the inflow of groundwater and allow for installing the geosynthetic clay liner <br />(GCL) without premature hydration. However, since there is not a defined gradient to <br />the groundwater flow, it is not clear where the water will be routed. Saturation of the toe <br />of the TSF could potentially lead to failure of the impoundment. <br />The Division has concluded that the only suitable option for addressing the perched <br />aquifer is to install a sub - surface drain along the seep. Please respond. <br />Response: UMC concurs with the Division that installation of a sub - surface drain along <br />the seep is the only suitable option to address groundwater inflow, given the amount of <br />inflow experienced last fall during the construction of the embankment. However, inflow <br />has decreased significantly since last winter and currently consists of small seeps. <br />There was no increase in the flow from these seeps during spring thaw. We have <br />continued our monitoring and to -date there is no apparent difference in the amount of <br />inflow. Approximately 8 "primary" seeps are identified. Flow from each of these seeps is <br />difficult to measure, but is estimated at less than 1 gpm per seep. See attached photos <br />showing inflow in September 2012 and in July 2013. <br />Seepage from the side wall of the TSF ponds at the base of the embankment, then <br />infiltrates into the native colluvial soils. The flow out of the side wall has equalized to the <br />infiltration into the colluvium with the exception of minor evaporation. <br />UMC received an opinion from a Tailings engineer as to how to address this situation. <br />Based upon this opinion, UMC proposes that a geonet liner be placed over the seep <br />area starting 1 0ft above the seep on the embankment to 1 0ft past where water infiltrates <br />