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- s `. .- - _... ~ irk-.,. ZO present in ttte system as further discussed in response to subsequent questions and in the technical memorandum included as Attachment 1 l to this submittal, (b) 7fte Text indicates, last paragraph, page 25, that less than one third ojthe 578 samples Jested siww signs of acidifrrntion from sulfide, and refers to figure 4-4, yet clear support jot this st4tement based on}lgure 4-4 is nor apparent. (This discussion is re-visited on p 36-3y) The figure depicts approxirnnrely J 13 samples, nor 578 as stated The figure shows that 70°~ of the samples have a net negative NNP. •Please address the apparent discrepancies between jtgure 4.4 and the text. Please re-visit the presumptions about poste pN, lack of oxidation at depth, evidence far lack of "acid production in the shallow sections, and the use of paste pH as a predictor of NNP. Please provide adegaate figures to support the statements in the text or remove the statements altogether. Rpsponae: We agree that only a fraction of the 578 static test samples aze shown in Figure 4.4. These are the samples fqr which paste pH testing was performed. We apologize for net specifying this in the text. Please see the discussion in the following responses as it relates to NNP and paste pH values. 5. Ppge 36-37, Val. 11 i'7te text states: "Taken together, the NNP and paste pN rrendr with depth [Figure S-4) indicate there was a ~<higher su~ide to carbonate ratio originally present in the diatreme at elevations above about 9000 fi Otherwise, the paste pH values in the oxidized Zone would be expected to be entirely `alkaline. as they are at lower elevations, where the NNP values increase to large positive values." a i;:. ~1While the utility of this presumption is nor evident, the statement nonetheless is unrupporred Please explain this conclusion. ReSponset° On average, the shallow NNP values are lower than the deeper NNP values. We believe that the cause of the lower NNP in the shallow diatrcme system is that proportianaUy less carbonate was deposited originally at shallow depths than at deeper depths. Another explanation could be that some acid generation has occurred to shallow depths reducing the overall csrbonates in this area. We know from geologic reports that the diarretne is strangiy oxidised to at [east 1,000 feer in depth. Additionally, the lower paste pH values measured in the shallow zones indicate that acidification has occurred at shalloty depths, as a result of natural oxidation, and that materials with a NNP of less than zero are potentially acid generaring. The fact that surficial materials are oxidized, will not reduce ttte NNP of the materials. The preponderance of NNP values greater than zero at depths beyond the 9,000 foot level (and the general tack of low paste pH values) suggest that NNP values were always htghcr in this portibtt of the dietreme and that acidificatioa will not occur in overburden mined below this depth, even if it"is oxidized at some tune in the future. The rationale for the statement "higher sulfide to carbonate rado:arigfnally present in the diaa~eme at elevations above about 9000ft" is provided from this reasdtiing.