2010-04-20_REVISION - M1977300 (6)

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Environmental Protection Plan, Schwartzwalder Mine 9-3 • The drinking water standards listed in Table 9-1 are not considered to be directly applicable to the alluvial monitoring wells or to groundwater in the underground void, because these are not direct sources of drinking water supply. Although water in the alluvium feeds into the Ralston Reservoir drinking water source, physical and geochemical processes reduce concentrations along the 1.6-mile flowpath between the alluvial groundwater at the Schwartzwalder site and the outlet for impounded water at Ralston Reservoir. Notably, mixing occurs when surface water in Ralston Creek interacts with alluvial groundwater. Significant additional mixing also occurs as water from the Moffat collection system enters Ralston Reservoir and mixes with water from Ralston Creek before being pumped to the Moffat water treatment plant. Also, note that the uranium MCL is based on uranium toxicity, rather than radioactivity. If the standard were based on radioactivity, it would be higher (approximately 1.6 mg/L11) (ii) Water Quality in Alluvium (ii).1 Alluvial Monitoring Wells Water quality data are available for 10 monitoring wells completed in alluvium adjacent to Ralston Creek (Figure 2-2) which have been monitored for up to 20 years. Six of the wells (MWO, MWO1, MW02, MW04, MW06, and MW07) were sampled and analyzed for uranium, radium, and field parameters (temperature, pH, and conductivity) for approximately 9 years, prior to the start of the 1998 - 1999 baseline hydrologic study. In the twelve years during and after the baseline hydrology study, these six wells have also been analyzed for an extended analytical suite at least once per year. Three additional wells (MW9, MWSR, and MW00) were constructed in the alluvium in 1999 as part of the mine closure baseline hydrology study. One monitoring well (MW 12) was drilled in November 2008 to serve as a replacement for downgradient alluvial monitoring we1112. These wells have been routinely monitored to the present, and • several are slated for abandonment (in accordance with Colorado water well regulations) as part of mine closure13 Water quality for the alluvial monitoring wells is provided in Table 9-3 and Appendix B. The water is a calcium-sulfate-type water (Figure 9-1), with low dissolved solids and near neutral pH. TDS averages 170 mg/L in upgradient well MW00, and increases to about 380 mg/L in the vicinity of the mine. Trace metals are generally below detection in alluvial groundwater, with the exception of molybdenum, zinc, and uranium. Zinc concentrations are below the Colorado groundwater standard of 5 mg/L. Molybdenum and uranium concentrations exceed Colorado groundwater standards by factors of 1.5 and 40, respectively, and have been identified as Constituents of Concern (COCs) in alluvial groundwater. Uranium concentrations show a general trend of increasing concentrations from upgradient well MW00 to downgradient well MW 12. Uranium concentrations in alluvial wells generally increase from upstream/upgradient to downstream/downgradient14. Proximity to Ralston Creek also affects water quality. Wells located closer to the creek (e.g. MW2) have lower uranium concentrations than distal wells despite their proximity to other facilities (Figure 9-2, Figure 8-3). 11 The effective dose from ingestion of 1 g pure natural uranium is 1.23 mSv. The 1 mSv annual standard for the public is equivalent to 0.813 g. Consumption of 500 liters/yr corresponds to a uranium concentration in drinking water of 1.6 mg/l (based on ICRP72 dose factors for adults). 12 Alluvial monitoring well MW12 is located approximately 450 feet upgradient (northwest) of the property boundary, and is completed partially in fill material, rather than in the native alluvium. Drill rig access to the saturated native alluvium along the property boundary was limited by thick vegetation and the steep embankment along the access road, so the well was located as close to the property boundary as possible, without additional earthwork. 13 Alluvial monitoring well MW4 was removed during reclamation activities in 2008. • 14 The order of increasing uranium concentrations in wells is as follows: MW-00 (lowest uranium concentrations), MW2, MW-1, MW-0, MW-9, MW-7, MW-6, MW-12 (highest uranium concentrations). 4109C.100419 Whetstone Associates 6