2010-08-02_REVISION - M1977300 (27)

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Environmental Protection Plan, Schwartzwalder Mine 9-3 • (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 (MW12) was drilled in November 2008 to serve as a replacement for downgradient alluvial monitoring well12. 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). Uranium concentrations in alluvial groundwater exceed the Colorado drinking water standard of 0.03 mg/L in all alluvial monitoring wells including upgradient well MW00. 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 standards 15 before the mine began refilling. At that time, mine water could 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). 15 Colorado adopted new groundwater standards for uranium (0.03 mg/L) and molybdenum (0.035 mg/L) in 2008. 4109C.100731 Whetstone Associates