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Page 4, Item 6(B) — Radium in the Mine Pool Section 9(b) (iv) 2. Evaluation of Chemical Trends in Mine Water as a Function of Time (page 9- 43). Uranium is but one of several constituents that are of concern in the mine pool. For example, radium -226 concentrations in the mine pool are significantly elevated above standard and have shown a disturbing long -term increasing trend going back to December 1998. Please provide your assessment as to why the radium -226 concentration has shown such a significant increase in the mine pool). RESPONSE: Cotter Corporation (N.S.L.)'s Response to December 14, 2010 Adequacy Review, Comments 6(B) & 11 (B) May 6, 2011 It appears that the basis for the "significant increase" conclusion in the above statement is the single December 1998 sample of mine pool water analyzed for radium -226. The radium -226 sample collected in 1998 did not represent mine filling, because the pumps were not shut off on the 19 Level until May 2000. Radium -226 concentrations increased as water levels rose and water in the mine pool contacted wall rock. (Figure 1, Figure 2). The radium -226 concentration of the first sample collected during initial mine filling was 150 pCi/L. Radium -226 concentrations slowly increased to about 207 pCi /L as the mine pool water level rose to 5,749 ft amsl on January 23, 2002 (Figure 1). The increase in radium -226 concentrations during mine filling was likely due to the mine pool coming in contact with wall rock. Since 2007, radium concentrations have ranged from 207 pCi/L to 254 pCi/L (Figure 3) with the lowest values measured in March, 2009 (135 pCi /L). Profile sampling of Schwartzwalder mine pool water conducted in 2010 indicated that the mine pool was highly reduced from the surface to a depth of 800 ft below the surface. Under these highly reducing conditions, uranium concentrations were consistent throughout the mine pool while radium concentrations varied at depth (Figure 4) showing no discernible pattern. Although radium -226 concentrations are elevated in the mine pool, radium concentrations are typically well below water quality standard (5 pCi /L) in samples from alluvial monitoring wells and in Ralston Creek, because radium is insoluble in oxidizing environments. Radium's immobility in oxidizing environments and mobility in reducing environments is well - documented (Ames, et al. 1983; Banks, et al. 1995; Langmuir & Riese, 1985; Szabo and Zapecza, 1991). Radium immobility in oxidizing environments results from both solubility and sorption reactions. Radium forms a positively charged divalent cation in solution, similar to barium, calcium, and strontium, in natural waters. Radium solubility is typically controlled by its co- precipitation within sulfate minerals (e.g. barite and gypsum). Radium adsorption on iron - hydroxides is well- documented in natural systems (Schott and Wiegand, 2003). Under oxidizing conditions iron- and manganese- hydroxides can form, adsorbing radium efficiently (Zwalen, et al. 2007). 1