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continue to do so until the neutralizing capacity of the diatreme is exhausted (mainly by the <br />• products of oxidation of sulfide in the non-mined rock, which represents more than 99% of the <br />sulfide), which is predicted to be thousands of years from the present, if it occurs at all (MLE <br />Project Application, Volume 11, Appendix 1, Section 3.5.4). <br />Because of the geochemical buffering from the abundance of carbonate that exists within the <br />diatreme, no temporal variation in the quality of zvater in the regional saturated ground water <br />system that is intersected by the Carlton Tunnel is expected or predicted, except as a result of <br />physical covering of previously existing preferential flow paths in the rockmass, which will <br />(and has) served to reduce rapid flow of infiltrating water to the regional ground water table. <br />This ultimately reduces the variation of flow from the Carlton Tunnel, and thereby decreases <br />mobilization of colloidal zinc, and increases alkalinity in the flow. Most other constituents <br />have remained constant during the mining period to date, and are expected to remain <br />constant in the future. <br />Further, the transit time for water through the overburden at the surface (-10 years), through <br />the vertical flow regime from the rock surface to the saturated portion of the diatreme (-1 <br />year), and through the saturated portion of the diatreme to the regional ground water table <br />and ultimately flow from the Carlton Tunnel (-50-100 years) is sufficiently long and difficult <br />to define that the exact timing of the placement of the overburden material is not a significant <br />additional variable in that transit time computation. <br />• Finally, the location of placement of the overburden material from each proposed mine as <br />backfill can not be defined with certainty. Changes in the details of mine plans are the norm in <br />mining, driven by economics, exploration, operation philosophy, equipment availability, and <br />other factors. It is not possible to credibly anticipate the actual mining and overburden <br />placement sequences, or their interrelation ahead of time. With the exception that the material <br />from excavation in the Precambrian and the high-sulfide South Cresson mine that will be <br />placed low in the backfill of the Main Cresson mine or in the engineered ECOSA, all other <br />material could and would be placed on the most convenient operating storage facility. The <br />geochemical makeup of the rock would have no impact on the flow of any water that would <br />potential leave the overburden, nor any potential impact on the quality of the ground water <br />monitored at the portal of the Carlton Tunnel. <br />B. The description of the "additional conservative design features "for the ECOSA, is a bit <br />confusing. On page 11-10, last paragraph, 6th line, a statement is made that the footprint <br />area of the ECOSA will be left in a roughened condition to encourage the infiltration into <br />the Grassy Valley feeder. Later, measures are described to "minimize infiltration through <br />the pile". What hydrologic component is being encouraged to infiltrate to the Grassy Valley <br />feeder and why ?Is the roughened condition of the footprint designed to inhibit saturation <br />of the ECOSA, while at the same time facilitate infiltration that may originate as percolation <br />through the ECOSA, and runoff from the ECOSA and surrounding area that migrates <br />beneath the ECOSA toward the feeder ? It is not explicitly stated in the section, but is the <br />• advantage of channeling infiltration to the feeder to facilitate capture by the Carlton <br />Tunnel? <br />Response. <br />22