2016-07-28_REVISION - M1980244 (2)

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"19, 63, 212, 319". If the values listed on Plate 19 are correct,then the result of the Acid/Base Accounting Flowsheet on Plate 25 (assuming oxidation of the vadose zone) would be 88 million tons CaCO3 equivalent exiting the Carlton Tunnel (rather than 30 million tons). Please correct or explain this discrepancy. RESPONSE. The set of values cited by DRMS differ because the Plate 19 values refer to the total rockmass, while the Plate 25 values refer to the unmined vadose zone rockmass. The difference is the mined vadose zone rockmass, which is accounted for separately in Plate 25 under the column "Mined Rock Acid Generation (million tons CaCO3)". The ABA results have been further checked as a result of this question, and are correct. Additionally, in the Acid Neutralization Computation table, for Cell 2: Vadose Zone Rock, under the column Vadose Zone Acid Neutralization(million tons CaCO3), the first four values listed are"7, 15, 56, 181". However, on Plate 20—Acid Neutralizing Potential,the values associated with these elevations are listed as"9, 21, 61, 191". If the values listed on Plate 20 are correct(and the values above for Vadose Zone Acid Generation are corrected to the values listed on Plate 19),then the result of the Acid/Base Accounting Flowsheet on Plate 25 (assuming oxidation of the vadose zone)would be 64 million tons CaCO3 equivalent exiting the Carlton Tunnel(rather than 30 million tons). Please correct or explain this discrepancy. RESPONSE. The set of values in cited by DRMS differ because the Plate 19 values refer to the total rockmass, while the cited Plate 25 values refer to the unmined vadose zone rockmass. The difference is the mined vadose zone rockmass, which is accounted for separately in Plate 25 under the column "Mined Rock Acid Neutralization (million tons CaCO3)". The ABA results have been further checked as a result of this question, and are correct. 4.2.3 Static Evaluation of Performance of the Groundwater Neutralizing System • Page 24: The text summarizes that"the acid generated by the mined material to the completion of AM-I I will be readily neutralized by the in-situ inventory of calcium carbonate." And that the "very slow—natural weathering rate—oxidation of the material in the vadose zone can also be accommodated by the system neutralization, providing permanent protection against the flow from the Carlton Tunnel turning acidic."The rocks of the mining district are, in general,net acid producing with twice as much acid generating potential than acid neutralizing potential,and the system neutralization is based solely on the currently saturated bedrock remaining saturated. Therefore, as mentioned above, if the diatreme water table were to drop,there may be a significant loss in the system's neutralization potential, potentially creating acid rock drainage at the Carlton Tunnel portal. In this case,there would be no "permanent protection from the flow from the Carlton Tunnel turning acidic". Have you developed a plan to mitigate impact to Fourmile Creek if this situation were to occur during the operation,or post-mining? RESPONSE.DRMS is not entirely correct in stating that "the system neutralization is based solely on the currently saturated bedrock remaining saturated. " The system neutralization in the long-term case is based on all the material in the system yielding its entire neutralizing capacity, which is considered to be reasonable in the long term. It is the system acidification that is dependent on the location of the water table: if the water table were to drop below the current average elevation (which is estimated to be 7,500 ft amsl in these computations and is likely higher, but can never be lower than 6,985 ft amsl, the elevation of the Carlton Tunnel), then more sulfide would be exposed to potential oxidation—at natural geological rates—and there would be a somewhat larger shortfall Page 12 of 28