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Columbus Mine Portals <br /> Page 2 <br /> Site Loading Estimate (cont.) <br /> Based on these assumptions and procedures, SGC estimates that the average daily loading for <br /> this site may be as much as 22.6 pounds of dissolved zinc per day. SGC is under no obligation to <br /> defend these estimates and they should only be used as an estimate. SGC does not represent that its <br /> mitigation project at this site will remove any specific percentage of metals loading from this site: the <br /> loading estimate contained herein does not form the basis of an enforceable permit obligation. <br /> Description of Project <br /> The waste dumps will be isolated from portal flows as well as stormwater events. The dumps <br /> will be regraded and hydrologic diversions installed to prevent run on conditions. The dumps would <br /> then be stabilized with basic pH material and revegetated to minimize percolation from stormwater <br /> events. <br /> The two portals will be opened and studied for the placement of hydraulic seals. In order for <br /> hydraulic seals to be placed, SGC feels that sites meeting the following conditions need to be found. <br /> 1) Location far enough underground to avoid the near surface fractures and joints caused <br /> by weathering. <br /> 2) Adequate rock compressive strength for structural stability. <br /> 3) A length of the tunnel with minimal faulting or other geologic features that may serve as <br /> a leakage pathway. <br /> 4) Adequate ground cover over the potential site to resist the hydrostatic forces from the <br /> potential maximum head. <br /> If an acceptable location can be found, SGC will design and install two hydraulic seals. After <br /> sealing is complete, the seal will be contact grouted and the diversion pipes, if necessary for <br /> construction, grouted. The near surface fracture flows typically found will be diverted to avoid <br /> contact with waste material and the portal closed. Design of hydraulic seals will be according to <br /> current enginnering practices. The design approach will be similar to the attached paper on Tunnel <br /> Bulkheads. <br /> This plan is also subject to notification requirements for disturbance of Historical Mining Sites to the <br /> board of the San Juan County Commissioners. <br /> Work Plan <br /> 1) Build catchments for potential adit releases. <br /> 2) Open and evaluate adits for hydraulic seals. <br /> 3) Design and install hydraulic seals. <br /> 4) Grout seals and bypass pipes. <br /> 5) Portal closure. <br /> 6) Surface diversions and dump regrading. <br /> 7) Addition of pH neutralizing material, seeding and mulching. <br />