2010-04-20_REVISION - M1977300 (9)

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Environmental Protection Plan, Schwartzwalder Mine • (a) Description of Source 15-2 As described in Section 9(b)(ii), uranium and molybdenum are constituents of concern (COCs) in alluvial groundwater. Molybdenum and uranium concentrations exceed Colorado groundwater standards by factors of 1.5 and 40, respectively. Concentrations of other metals are generally below analytical detection limits, with the exception of zinc which is below the Colorado groundwater standard of 5 mg/L. TDS increases with distance downgradient in the alluvium, but remains low to moderate, averaging 170 mg/L in upgradient well MW00, and increasing to about 380 mg/L in the vicinity of the mine. Uranium concentrations show a general trend of increasing concentrations from upgradient well MW00 to downgradient well MW12. Uranium concentrations in alluvial wells generally increase from upstream/upgradient to downstream/downgradient30. Proximity to Ralston Creek also affects water quality, with wells located closer to the creek (e.g. MW2) having lower uranium concentrations than distal wells. (i) Source of Chemical Loading The primary source of molybdenum and uranium in alluvial groundwater is the fill material, based on several lines of evidence described in Section 9(b)(ii). The flooded mine workings may be contributing a small amount (less than 0.8% to 8%) of flow and therefore chemical loading to the alluvium and fill, as described in Section 9(b)(ii).1. The waste rock dumps do not affect water quality in Ralston Creek, according to a November 2008 electrical conductivity (E.C.) survey of the creek and over a decade of surface water monitoring results from Stations above and below the waste rock dumps. However, it is unclear whether water quality in alluvial well MW-0 is affected by the East Waste Rock Dump or by other sources in the alluvium. Uranium concentrations in MW-0 peaked at 0.98 mg/L in 2003 during surface reclamation operations, and • have declined to their present levels of about 0.3 mg/L. No consistent trend of increasing uranium concentrations has been observed in the well, and radium concentrations are currently at the lowest concentrations observed in 10 years of monitoring. (ii) Thickness and Extent of Alluvium Alluvium along Ralston Creek ranges in thickness from 5 feet to over 30 feet in the vicinity of the mine. In general, the alluvium is confined to the valley floor along Ralston Creek and pinches out near the property boundary, where a natural constriction in the valley occurs. This "choke point" occurs where the valley narrows and its walls rise steeply from the creek bed. Bedrock is exposed along the width of the valley floor and groundwater previously flowing in the alluvium is forced to the surface where it enters Ralston Creek. Alluvium thickens again downstream from the choke point. Monitoring Well MW8 is installed in the alluvium on the downstream side of the choke point and has always been dry. Alluvial monitoring well MW4 was also frequently dry as a result of its distal location on the southwestern fringe of the alluvial system31 (iii)Potential Pathways for Alluvial Groundwater The alluvial aquifer near the mine is of limited areal extent and is not directly connected with alluvium below the choke point, except via Ralston Creek. At present, contaminants in the alluvial aquifer are released to potential receptors through the surface water pathway. Since Ralston Creek is the only conduit between the upgradient alluvial groundwater and the downgradient alluvial groundwater mitigation strategies, would logically focus on treating groundwater before it reaches Ralston Creek, or isolating the 30 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). 31 Monitoring well MW-4 was damaged and removed during site reclamation activities in 2005. 4109C.100419 Whetstone Associates