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ARCADIS <br />Alexco Resource Group (Alexco) began pretreating water in the underground mine pool in September <br />2000 over approximately a 2 -day period annually (Alexco 2009). The treatment approach, which is similar <br />to the proposed approach for the Schwartzwalder Mine, resulted in a reduction in the volume of treatment <br />residue generated by the WTP. <br />Regardless of the in -situ treatment being conducted, a Notice of Violation /Cease and Desist Order <br />(Number 10- 060616 -1) was issued to the Platoro Joint Venture in June 2005 due to permit limit <br />exceedances for total suspended solids (TSS), arsenic, lead, silver, and zinc at the point of compliance in <br />the receiving streams (CDPHE 2005). <br />In June 2009, Alexco was contracted to transfer the lime treatment sludge generated since 1985 into the <br />mine pool and conduct in -situ treatment, as approved by the U.S. Environmental Protection Agency (EPA) <br />under the Class V underground injection control regulations (Alexco 2009). Detailed information pertaining <br />to the in -situ treatment approach (initial or sludge treatment), resulting water quality, and long -term <br />stability were not readily available for review. <br />Data Adequacy: The geochemical conditions at the Platoro and Schwartzwalder sites are markedly <br />different: <br />• The Platoro Mine pool contained acidic low pH water, whereas the Schwartzwalder Mine pool <br />contains alkaline pH water. Both water types will be characterized by distinct chemical <br />speciation including a predominance of free divalent ions at low pH (Platoro) and carbonate <br />complexes at higher pH (Schwartzwalder). <br />• Lime treatment residuals are composed of relatively insoluble (under neutral pH conditions) <br />metal precipitates including metals hydroxides (e.g., Cu(OH) Ag(OH) and metal carbonates <br />(e.g. PbCO ZnCO <br />• RO treatment residuals are concentrated high ionic strength solutions made up primarily of <br />dissolved ions that, if dewatered, would consist of highly soluble salts. <br />ARCADIS is not aware of an in -situ treatment project that has been conducted that includes placement of <br />RO concentrate into a mine pool. The Platoro Mine example (and other examples previously provided by <br />Alexco) is insufficient to demonstrate that the proposed Schwartzwalder Mine pool treatment approach is <br />likely to be successful or protective of human health and the environment. It follows that the Platoro Mine <br />example should only be considered evidence of the potential viability of the proposed treatment approach. <br />ARCADIS highly recommends that bench -scale studies be conducted under site relevant conditions prior <br />to implementation of the pilot testing phase. <br />Recommendations: Conduct bench -scale testing to better understand the system conditions prior to <br />implementation of the small -scale and full -scale treatment. <br />Page <br />10/16 <br />