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Arcadis Review of Cotter Technical Revision 23 <br /> For on-site disposal to be a successful option, TR-23 should include updated, detailed information on the <br /> eventual cover material and vegetation. Information on storm water run on and runoff controls should also <br /> be provided because the additional waste rock will change the configuration and slopes of the waste piles. <br /> This information could not be located in TR-23. Further, there is no information on the material that will be <br /> used to backfill the excavated areas along Ralston Creek. Several feet of material is planned to be <br /> excavated and this should be backfilled with suitable material. Information on the origin of this fill material, <br /> and its grain size and chemical makeup should be provided in TR-23. Or, if no backfill is proposed, <br /> justification should be provided because excavation will change the landscape of the site and could <br /> present potential safety hazards, or excessive erosion and sedimentation of Ralston Creek. <br /> At first glance, excavation of the residual waste rock may appear to be a reasonable measure to remove a <br /> contaminant source; however, available data continue to show that the major source of contamination <br /> (uranium) is from the mine pool and this will continue to be a source of uranium to the alluvium along <br /> Ralston Creek. Consequently, the area of proposed waste rock excavation will likely re-contaminate from <br /> mine pool water. The following explains the continued connection between the mine pool, alluvium, and <br /> Ralston Creek. <br /> Figure 1 is a graph of the mine pool water quality since 2012, showing concentrations of uranium and <br /> other constituents through time. In 2013, there was a noticeable drop in the uranium concentration from <br /> approximately 22 milligrams per liter(mg/L)to below 10 mg/L, which was due to the mine pool treatments. <br /> The uranium concentration has generally remained in the 4 to 7 mg/L range for the last few years and this <br /> range of uranium concentration is essentially a "fingerprint"of the recent mine pool water quality. <br /> Figure 2 is a graph that shows uranium concentrations in alluvial monitoring well MW-6, which is near the <br /> downstream boundary of the site, and concentrations in Ralston Creek below the downstream property <br /> line (SW-BPL). Figure 2 also shows the elevation of the rising mine pool since the pumps were shut down <br /> in May 2000. Since 2009 when the mine pool reach the elevation of the water treatment plant discharge <br /> at Ralston Creek (SW-BDIS), uranium concentrations in MW-6 began to increase. Prior to 2009, the <br /> uranium concentrations in MW-6 were in the 1 to 2 mg/L range and likely due to leaching of the residual <br /> waste rock in the alluvium along the creek that is proposed to be excavated in TR-23. However, since <br /> 2009 the mine pool has risen to the Steve Adit elevation that is well above the creek, and water from the <br /> mine pool is able to flow into the alluvium. The result of this is that the mine pool "fingerprint"of uranium <br /> concentrations is now being observed in MW-6. The variations in uranium in MW-6 can be explained by <br /> seasonal changes in recharge from precipitation and snowmelt, which are typical of shallow alluvial <br /> groundwater systems. A couple of the uranium concentrations in MW-6 are greater than concentrations in <br /> the mine pool, which is likely due to the additional uranium originating from the waste rock in the alluvium. <br /> This coincidence of rising mine pool and increased uranium concentrations in MW-6 is strong and can only <br /> be explained by water migrating from the mine pool into the alluvium. Using uranium concentrations <br /> shown on Figure 2 with an understanding the timing of the mine pool's impact on MW-6, it is reasonable to <br /> estimate that approximately 80% of the uranium at MW-6 is from the mine pool and the remaining 20% is <br /> from the residual waste rock in the alluvium. Excavation of the waste rock, as proposed in TR-23, could <br /> potentially remove this 20%, but there will still be appreciable uranium sourced from the mine pool. Unless <br /> the mine pool is lowered to below Ralston Creek, there will be a continued hydrologic condition for the <br /> mine pool to feed the alluvial groundwater and eventually enter Ralston Creek. <br /> arcadis.com <br /> Page: <br /> 2/2 <br />