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RESPONSE: <br />Although the effect of rising groundwater levels flushing soluble or weakly sorbed uranium from <br />the alluvium and fill has been observed and documented, the Operator has not performed <br />geochemical analyses to determine the form of these sorbed or precipitated compounds. <br />However, the geochemical literature indicates the likely forms. In the highly oxidized form <br />(U + ), uranium forms the uranyl ion (UO2 + ) (Hem, 1972; Garrels and Christ, 1982). The uranyl <br />ion is known to form compounds such as uranyl carbonate, uranyl chloride and uranyl sulfate. <br />Given the available alkalinity, chloride, and sulfate in the alluvial groundwater at the site, any of <br />these uranium salts could exist (alone or in combination). <br />Additional studies on the precipitation and sorption of uranium (U + ) under various aqueous <br />conditions (Kang, Han, and Haun, 2002) indicated that, in addition to uranium precipitation as <br />UO2(OH)2 at neutral pH, uranium may adsorb onto kaolinite (clay) in a complexation reaction. <br />At a neutral pH, the adsorbed uranium fraction exceeded the precipitated fraction. The <br />sorption/desorption process could explain the relatively rapid release of uranium due to flushing <br />with rising water levels in the alluvium at the Schwartzwalder Mine. <br />Hem, John D., 1985. Study and Interpretation of the Chemical Characteristics of Natural Water, <br />Third Edition, U.S. Geological Survey Water Supply Paper 2254. 263 pp. <br />Garrells, Robert M., and Christ, Charles L, 1982. Solutions, Minerals and Equilibria, Freeman <br />Cooper Co, 464 pp. <br />Kang, J.K., Han, B.E., and Haun, P.S., 2002. Precipitation and Sorption of Uranium (VI) Under <br />Various Aqueous Conditions. Environmental Engineering Research, vol 7, no. 3, p. 149- <br />157. <br />Page 6, Item 11(E) — Section 15(b)(iv) Permeable Reactive Barrier (PRB) <br />RESPONSE: <br />The permeable reactive barrier that was presented as a potential mitigation alternative in the July <br />31, 2010 EPP is no longer a proposed alternative. Based on a more detailed analysis of <br />alternatives, discussions with DRMS and other parties, and preliminary engineering feasibility <br />studies, the mitigation actions for the site do not include the PRB. The discussion of the PRB <br />has been removed from the revised EPP Section 15 presented in this transmittal. <br />Page 7, Item 11(F) — Strategically Designed and Constructed Wetlands <br />RESPONSE: <br />The engineered wetland that was presented as a potential mitigation alternative in the July 31, <br />2010 EPP is no longer a proposed alternative. Based on a more detailed analysis of alternatives, <br />discussions with DRMS and other parties, and preliminary engineering feasibility studies, the <br />mitigation actions for the site do not include the wetland. The discussion of the wetland has been <br />removed from the revised EPP Section 15 presented in this transmittal. <br />2 <br />