2010-04-20_REVISION - M1977300 (6)

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Environmental Protection Plan, Schwartzwalder Mine 9-4 Uranium concentrations in alluvial groundwater exceed the Colorado drinking water standard of 0.03 mg/L in all alluvial monitoring wells including upgradient well MW0015. As shown in Figure 9-2, uranium concentrations in alluvial monitoring wells fluctuate in response to seasonality, shutting off the sumps and water treatment plant, and to surface disturbance associated with reclamation activities (e.g., the higher concentrations observed from 2002 to 2003 are attributed to surface reclamation operations). Higher uranium concentrations in 2009 may be related, in part, to turning off the Sump 1 recirculation and to increased precipitation which solubilized uranium from reactive surfaces on the waste rock and fill. The primary source of molybdenum, uranium, and zinc in groundwater appears to be the fill material, rather than metals loading from the flooded mine, based on several lines of evidence: • Uranium and molybdenum concentrations in the alluvial groundwater were elevated above current Colorado groundwater standards16 before the mine began refilling. At that time, mine water could not contribute metals to the alluvial groundwater because the direction of potential flow was toward the mine from the creek. Uranium and molybdenum concentration trends in alluvial groundwater do not coincide with mine refilling trends. Uranium and molybdenum concentrations did not increase in alluvial groundwater when the water level in the mine reached the level of the creek 17 in February 2007. Monitoring data indicate that the highest uranium concentrations in alluvial groundwater occurred in 2002 and 2003 when water levels in the mine were 400 - 600 feet below the creek level. Results from 2009 may show impact from mine pool water, shutting off the sumps, and from increased precipitation which solubilized uranium from reactive surfaces on the waste rock and fill • Seasonal variations in uranium and molybdenum concentrations in the shallow groundwater are consistent with chemical loading from alluvium and fill. While surface water in Ralston Creek exhibits low concentrations during spring (high flow) months and higher concentrations during winter (low flow) months, alluvial groundwater often exhibits higher concentrations in the spring or during particularly wet months. Rather than diluting alluviual groundwater (as happens with surface water) rising water levels in the alluvium/creek system spring bring groundwater in contact with oxidized salts from above the normal water level in the alluvium and fill, resulting in higher concentrations. Groundwater in the mine and in the alluvium have different chemical signatures. Mine water is a strongly-buffered calcium-magnesium-sodium-sulfate type water with elevated TDS (about 3,300 mg/L), sulfate (2,060 mg/L), bicarbonate (480 mg/L as CaC03), calcium (428 mg/L), sodium (190 mg/L) and magnesium (250 mg/L). In contrast, alluvial groundwater is a calcium-sulfate-type water with low dissolved solids (about 380 mg/L in the vicinity of the mine). The ratio of uranium to molybdenum is about 30:1 in the mine water and 12:1 in alluvial groundwater from representative wells (MW-6 and MW-9). Manganese and iron concentrations average 5.55 and 0.13 mg/L, respectively, in mine water and are below detection in alluvial groundwater. Although the flooded mine workings are not a primary source of chemical loading to the alluvium adjacent to Ralston Creek, they may be a minor source. Potential conduits from the flooded mine workings to the alluvium included historical exploration boreholes that were drilled into the deposit from the valley floor. 15 Although uranium concentrations in excess of the groundwater standard reported by the lab for upgradient monitoring well MW00 are questionable, the data are reported exactly as contained in the electronic data deliverable (EDD) from the lab. 16 Colorado adopted new groundwater standards for uranium (0.03 mg/L) and molybdenum (0.035 mg(L) in 2008. • 17 When the flooded mine pool reached the 6540 ft level in about February 2007, the groundwater gradient was essentially zero (no driving head between the creek and the mine pool). Since that time, a low hydraulic gradient has existed from the mine to the creek. The 6540 level represents the elevation of Ralston Creek near the SW-BOS station, near MW-6. 4109C.100419 Whetstone Associates