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reached. The spillway will route excess water to the treatment plant or Ute Creek. Pumping <br />only will be required for additional draw-downs below the spillway level. <br />• Geochemical Stability: Although in limited use in the United States, closure of <br />potentially acid generating (PAG) tailings impoundments using water covers has been utilized <br />successfully throughout Canada at a number of metallic mineral milling sites. The principles are <br />neither new nor untested. Flooding of PAG tailings minimizes atmospheric contact and, <br />accordingly, oxidation. Previous research conducted at the Upper Urad tailings impoundment <br />showed that up to 80 percent of dissolved metals in those PAG tailings were generated within <br />the vadose or unsaturated zone. By maintaining a water pool on the Henderson tailings <br />impoundment, the phreatic surface within the impoundment will be higher than with a fully dried <br />system. Therefore, oxidation and dissolution of metals will be significantly less resulting in <br />better quality water entering the subdrains and flowing toward the water treatment plant. <br />• Constructability: The reclamation plan in the original Permit predicted that <br />access for men and equipment on the tailings impoundment would be difficult, resulting in <br />significant winter work under frozen conditions and a lengthy period for completion of <br />reclamation. Even with the incorporation of geogrid material, access over the entire surface of <br />the impoundment will be difficult. <br />The new scenario is expected to significantly reduce access problems and, possibly, the time Line <br />for completion. Heavy equipment access to highly unstable western side (low point) of the <br />impoundment will be eliminated. Material handling requirements from that end wil} be provided <br />by floating equipment such as barges and dredges as necessary. Soon after cessation of <br />deposition, tailings within the 30 percent of the area designated for the dry cover will be dry <br />enough to easily handle the personnel and equipment required to apply and vegetate the dry <br />cover. Any constructed wetland areas under this scenario would be emplaced at the wetted <br />mazgins of the water pool on an area where stability would be arguably greater than in the low <br />point, but somewhat less than on the dry cover azeas. The degree of stability and the type of <br />equipment that could be used in this area is yet to be determined in this conceptual plan. <br />2.4.3 Henderson Proposal <br />Henderson is seeking conditional a~roval from DMG of the alternate conceptual scenario to <br />reclaim the tailings impoundment with a partial water and wetland cover. Accordingly, <br />Henderson is proposing to change the post-mining land use for the wetted portion of the <br />impoundment to "Developed Water Resource" with the remaining dry portion to remain as <br />"Rangeland" per the general change requested in Section 2.2. Henderson intends to submit a TR <br />at a later date to provide DMG with engineering design and information necessary to evaluate <br />9 <br />