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Daniel Arnold, Esq. January 25, 2011 <br />Denver Water Page 18 of 21 <br />Permeable Reactive Barrier <br />A PRB is evaluated for in situ treatment of alluvial groundwater with zero - valent iron to <br />remove uranium. The location of the PRB is proposed near the downstream end of the <br />mine where the alluvial aquifer is constricted. It is acknowledged that the PRB may have <br />to be replaced every 10 to 20 years as the reactive surfaces of the iron fill and the PRB <br />becomes less effective. <br />A PRB in the downstream end of the mine may reduce some of the uranium from <br />entering the creek, but the creek is affected by groundwater discharges further upstream <br />between the SW -OS and SW -BOS surface water locations. A series of PRBs would <br />therefore be needed to treat groundwater within this area. This remedial alternative, <br />alone, does not address the sources of contaminated groundwater from the waste rock <br />piles and from the mine pool, and these other source need to be addressed for PRBs to be <br />effective in the long term. Periodic replacement and disposal of the iron would have to <br />be performed and this presents a long -term maintenance issue. <br />In addition to uranium treatment, the PRB would need to address molybdenum and <br />radium. Molybdenum removal from groundwater has been demonstrated for zero - valent <br />iron, through sorption to iron corrosion products as well as precipitation through the <br />formation of lower oxidation state forms of molybdenum and co- precipitation with iron <br />sulfide that will form in the barrier. Performance data for radium removal by PRBs is <br />lacking in the technical literature; this requires verification prior to further consideration <br />of this technology. Radium sulfate precipitation may be a removal mechanism and <br />sorption of radium to reactive iron; however radium solubility can be enhanced under the <br />sulfidic, reducing conditions that may form in the PRB. <br />Constructed Wetlands <br />Constructed wetlands are an alternative considered for the mine in the EPP. They are <br />self - sustaining wetlands that naturally remove uranium from solution with plant rhizomes <br />and root uptake. Approximately 5 acres of wetlands are proposed covering the valley <br />floor of the central mine area. <br />There are several factors at the mine that would limit the effectives of constructed <br />wetlands. The foremost factors are climate and growing season. The mine is at an <br />elevation of approximately 6,600 feet receiving snow in the winter and the snow cover <br />can persist through the winter. Wetlands rely on some level of water flow through the <br />wetlands, which would likely be frozen with ice for several months of the year. The <br />growing season at the elevation of the mine is limited to June through September; <br />therefore, metals removal during most of the year is reduced, and no removal may occur <br />during some months. The growing season is further impacted by the aspect of the mine, <br />which is northeast- facing in a canyon with steep walls that limit the solar radiation onto <br />the valley floor where the wetlands would be located. Like the PRB alternative, this <br />remedial alternative, alone, does not address the sources of contaminated groundwater <br />