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C /y'sl/ o/S <br />pR -z <br />c~.tA- Qua/ys%S <br />MEMO <br />Date: Apri130, 2007 <br />From: Tom Kaldenbach <br />To: Jim Stark <br />Re: Colowyo Mine (C-1981-019) <br />PR-2, South Taylor Creek area <br />I have assessed the cumulative hydrologic impact of adding approximately 6,000 acres of <br />mining to the Colowyo permit azea. I am currently in the process of completing a major <br />revision of the Yampa CHIA; therefore, rather than updating the current version, I <br />conducted a sensitivity analysis to determine if the proposed mine expansion could cause <br />material damage in the Yampa River watershed. <br />This sensitivity analysis consisted of plugging into amass-balance spreadsheet what I <br />believe is an extreme value for flow from new spoil springs that could result from <br />Colowyo's proposed expansion. If this extreme value does not indicate material damage, <br />then it can be safely assumed that a more reasonable value would not cause material <br />damage. <br />The attached spreadsheet shows that during the month of August in a historically dry <br />year, a 2-cfs flow of 4,000 mg/1 spoil leachate would raise the TDS in the mainstem of <br />the Yampa River from 308 mg/I to 390 mg/1. The source of the 308 mg/1 value is the <br />August spreadsheet in the existing Yampa CHIA, which includes underground mine <br />pumping in a historically dry yeaz. The predicted 390 mg/1 is well below the approximate <br />700 mg/1 material damage suspect level; therefore, the proposed mining would not cause <br />material damage. This sensistivity analysis is conducted only for the month of August <br />because this is the month when mine impacts to streams are likely to be greatest due to <br />low stream flows and large imgation use. <br />cc: Sandy Brown <br />Dan Hernandez <br />