2010-08-02_REVISION - M1977300 (36)

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Environmental Protection Plan, Schwartzwalder Mine 15-3 • (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 system3o (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, the only potential contaminant pathway to receptors from the alluvial aquifer is limited to releases into surface waters of Ralston Creek. For this reason, mitigation strategies logically focus on removing source term material in the alluvial fill, or on treating alluvial groundwater before it reaches Ralston Creek and/or isolating the creek from the alluvial fill, thus breaking this exposure pathway. No drinking water wells are installed within impacted aquifers at the site, so at present, there is no direct pathway for human health risk from drinking alluvial groundwater. If targeted source term removal is not successful at meeting groundwater standards in the affected portions of alluvial fill near the mine, institutional controls may be required to ensure that water wells are not installed in those areas. Alternatively, additional remedial measures may be considered to meet surface water quality standards in • Ralston Creek. A detailed human health risk assessment can be conducted to determine whether any other potential pathways for human exposure to impacted alluvial groundwater exist. If no potential for exposure exists, alternative concentration limits (ACLs) that are not based on human health drinking water standards may apply to the alluvial aquifer. The Operator's goal and commitment is to ensure that all potential exposure pathways from impacted alluvial groundwater are mitigated or managed to the extent that water quality standards in Ralston Creek are met and that public health and the environment are fully and permanently protected under any potential future uses of the site. (iv) Effect of Alluvial Water Quality on Ralston Creek Ralston Creek above the mine site generally has uranium concentrations below the 0.03 mg/L stream water standard, but concentrations increase as it flows past the mine and interacts with groundwater in the alluvium. The waste rock dumps have had no discernable effect on water quality in Ralston Creek. Uranium concentrations at surface water stations above (SW-AWD) and below (SW-A001) the waste rock dumps are consistently low (Figure 14-1). However, uranium concentrations in Ralston Creek increase as the creek flows past the mine site downstream from SW-A001, then decrease past the property boundary (Figure 11-6) as sorption and precipitation processes remove uranium from solution. Uranium concentrations in Ralston Creek spiked after the alluvial sumps were turned off 2002, and spiked a second time after Sump 1 recirculation was discontinued in 2008. In 2002-2003, reactive surfaces on alluvium and fill had been exposed to subaerial weathering for over a decade while the sumps were in operation. The "first flush" caused uranium concentrations to spike and then decrease in later sampling events. A similar spike occurred in 2008, several months after the cessation of Sump 1 recirculation. Since the spike in 2008, uranium concentrations have fluctuated seasonally, meeting water quality standards • so Monitoring well MW-4 was damaged and removed during site reclamation activities in 2005. 4109C.100731 Whetstone Associates