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ARCADIS <br />the mine site. In addition, the system will facilitate dewatering of the alluvial fill to allow for excavation as a <br />remedial measure. The effectiveness of the system will be ascertained as additional water quality and <br />quantity measurements are obtained. <br />The Reclamation Plan views the diversion pipeline as "temporary" and it will remain until the other <br />mitigation measures are in place. However, the diversion pipeline should remain in place until the other <br />mitigation measures are demonstrated to be working as designed. It may be necessary to consider the <br />diversion pipeline a "permanent" water management feature if the mine pool is allowed to rebound after in- <br />situ treatment to ensure that the creek is protected (discussed later). <br />Recommendations: Provide the criteria to be used to decide whether the diversion pipeline can be <br />removed or if it should be retained as a "permanent" water management feature. <br />3.3 Angled Coreholes and Hydraulic Connection <br />Overview: The Reclamation Plan states that a comprehensive grouting program will be implemented to <br />seal historic exploration coreholes or natural faults/fractures through which mine pool water could migrate <br />to the creek. Figure 25 in the Reclamation Plan shows this program being scheduled over an <br />approximately 2 -year period (late 2012 to late 2014) when the mine pool level is high enough to provide <br />adequate hydraulic head to locate the coreholes, and the alluvial fill is sufficiently dewatered to allow <br />excavation and seep investigation activities. <br />Three to four angled coreholes have been identified from maps provided by Cotter. These coreholes are <br />collared near the elevation of the creek and extend back into the mine. The coreholes provide a conduit <br />from the mine pool to the alluvium and eventually to the creek as demonstrated in July 2010, when a <br />corehole was unearthed and found to be discharging mine pool water at a rate of 20 gallons per minute <br />(gpm) into the alluvium. New evidence of the continued, strong hydraulic connection of mine pool with the <br />alluvium is documented in the AMEC Technical Assessment as to the Probable Source of Uranium in <br />Sump 10 (AMEC 2011). Sump 10 was installed in June 2011 and started pumping in October 2011. It is <br />located near the Pierce Adit (approximately 250 feet northwest of the Steve Adit) very close to the location <br />of the open corehole that was unearthed in July 2010. Uranium concentrations have been as high as 20 <br />milligrams per liter (mg/L) in the sump water, which is similar to concentrations in the mine pool (26 mg /L). <br />This high level of uranium in the sump means that the open corehole was not properly plugged, there are <br />other open coreholes in the area, or the mine pool is discharging directly into the alluvium via fractures or <br />possibly along the Pierce Adit trend. <br />Data Adequacy. The inclusion of the grouting program in the Reclamation Plan is likely to help minimize <br />migration of mine pool water to the creek via improperly plugged and abandoned coreholes and <br />faults /fractures. However, a more detailed overview of the grouting program should be provided. In <br />Page: <br />4/16 <br />