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Cresson Project – AM-11 Adequacy Review
<br />Page 5
<br />May 03, 2016 (Revised May 31, 2016)
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<br />page 8)? Will the portal sediment pond system capacity be sufficient to
<br />accommodate this increased flow rate? Will maintenance of the pond system
<br />continue post-mining? Will monitoring of the water exiting the portal continue
<br />post-mining?
<br />4.2.3 Static Evaluation of Performance of the Groundwater Neutralizing System
<br /> Page 24: Under Item #1 - Plate 24 – Neutralizing Capacity – All Mined Material, the
<br />text states “when the remaining acid enters the saturated zone in Cell 3, no acid is
<br />added because the cell is saturated, and sulfide oxidation cannot occur. However, the
<br />remaining acid is neutralized by the carbonate in the cell.” This statement might be
<br />considered accurate as long as the diatreme water table remains at the level of the
<br />Carlton Tunnel (~7,000 feet elevation). However, throughout the mining operation,
<br />the Carlton Tunnel flow rate appears to show an overall decreasing trend (except for
<br />during the high precipitation events of 2014-2015; see Plate 8 – Carlton Tunnel
<br />Flow), which may be attributed to less infiltration to the diatreme due to construction
<br />of VLFs, large overburden stockpiles, etc. If the diatreme water table lowers, there
<br />could be a substantial loss of Acid Neutralizing Potential in the mining district. For
<br />example, on Plate 24, the Acid-Base Accounting Flowsheet assumes that vadose zone
<br />rock is not oxidized (not mined) and saturated rock remains saturated (offering
<br />neutralization), giving a net 0 million tons CaCO3 acid equivalent (exiting the
<br />Carlton Tunnel). However, if the diatreme water table were to drop, thereby losing
<br />the neutralization potential of previously saturated bedrock, this would change the
<br />flowsheet results. The estimated 35 million tons CaCO3 acid equivalent from the
<br />mined rock would pass through both the vadose zone and the previously saturated
<br />zone with no net change, leaving 35 million tons CaCO3 acid equivalent. Because the
<br />Carlton Tunnel only drains diatreme groundwater at approximately 7,000 feet
<br />elevation, a reduced water table may not drain to this tunnel. In this case, there could
<br />potentially be acidic groundwater sitting below the tunnel elevation, which only exits
<br />the portal during high infiltration events (as experienced in 2015). Do you have a
<br />plan in place to mitigate impact to Fourmile Creek in case of loss of Acid
<br />Neutralization Potential in the diatreme due to lowering of the water table?
<br />
<br /> On Plate 25 – Neutralizing Capacity – All Vadose Material, some of the data listed
<br />appears to be inconsistent with what was listed elsewhere. In the Acid Generation
<br />Computation table, for Cell 2: Vadose Zone Rock, under the column Vadose Zone
<br />Acid Generation (million tons CaCO3), the first four values listed are “13, 45, 194,
<br />303”. However, on Plate 19 – Acid Generating Potential, the values associated with
<br />these elevations are listed as “19, 63, 212, 319”. If the values listed on Plate 19 are
<br />correct, then the result of the Acid/Base Accounting Flowsheet on Plate 25 (assuming
<br />oxidation of the vadose zone) would be 88 million tons CaCO3 equivalent exiting the
<br />Carlton Tunnel (rather than 30 million tons). Please correct or explain this
<br />discrepancy.
<br />
<br />Additionally, in the Acid Neutralization Computation table, for Cell 2: Vadose Zone
<br />Rock, under the column Vadose Zone Acid Neutralization (million tons CaCO3), the
<br />first four values listed are “7, 15, 56, 181”. However, on Plate 20 – Acid Neutralizing
<br />Potential, the values associated with these elevations are listed as “9, 21, 61, 191”. If
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