2010-08-02_REVISION - M1977300 (27)

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Environmental Protection Plan, Schwartzwalder Mine 9-4 • 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 16 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. 0 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. Increases in sulfate and uranium concentrations in alluvial groundwater that were observed in Fall 2008 and Spring 2009, suggest that the flooded mine could contribute 0.8 to 8% of the alluvial flow. However, increases in chloride concentrations cannot be explained by hydraulic connection with the mine because concentrations in mine water are about half the concentrations in alluvial groundwater. Drill holes, near-surface fractures, or rock foliation could provide potential conduits for low rates of flow from the mine to the valley floor alluvium. Fifteen boreholes were plugged in May 2000 with a mixture of Portland cement and bentonite (7%), including 11 long holes at the Montana (Atlantic) adit, and four surface core holes (DHH#3, L-1, L-4, L-5). A targeted search was conducted for an additional 33 exploration holes drilled from pre-1956 to 1977 (White, 2000). The boreholes could not be located, and some or all of them may have been left unplugged. Flow from boreholes and fractures is believed to be low, based on the mixing calculations performed in Table 9-2 and the observation that flow in Ralston Creek did not decrease noticeably by capture from the boreholes when the mine was fully dewatered17 to 16 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. • 17 During the last four years of mining, at its total maximum depth of 2,220 feet, groundwater inflow to the Schwartzwalder mine was only 189 gpm. 4109C.100731 Whetstone Associates