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ARCADIS <br />response to the carbon addition, and their growth and activity is supported by "alternate electron <br />acceptors" including solid -phase iron, manganese, and dissolved nitrate and sulfate. There are a number <br />of key aspects of this treatment process that require careful consideration and must be evaluated prior to <br />(i.e., bench and pilot scale) and during the implementation of the approach, including: <br />• Secondary effects resulting from the creation of strongly reducing conditions in the mine pool <br />• The effect of the presence of RO treatment residuals on the treatment process <br />• The effect of organic carbon on the RO treatment processes <br />Secondary effects include the dissolution of arsenic, manganese, and radium. These are discussed in the <br />Reclamation Plan as being of negligible consequence if produced; however, due to the need to prevent <br />additional impacts to the creek, testing should be conducted to evaluate the potential concentrations of <br />these elements that may be produced. Bench -scale tests cannot simulate all of the possible reactions that <br />can occur at the field scale, but they can provide useful information upon which to base decisions about <br />the full -scale treatment process and to evaluate the risks associated with mine pool treatment. <br />The presence of RO treatment residuals in the mine pool will increase the ionic strength of the mine pool, <br />as well as the concentrations of uranium and other dissolved ions. Microbial processes are sensitive to <br />ionic strength, and although the current population of microorganisms in the mine pool has likely adapted <br />to elevated ion strength, their activity may be suppressed with rising ionic strengths over time. Tests <br />should be performed to ensure that the mine pool treatment process will not be severely compromised by <br />the Reclamation Plan for disposal of the RO brine. <br />Organic carbon is anticipated to affect the RO treatment process as described in Section 5.2 -Mine <br />Dewatering with Ex -situ Water Treatment. <br />As discussed above, the geochemical interactions and viability of the microbial community during in -situ <br />treatment of mine pool water containing RO concentrate are not well documented. ARCADIS' experience <br />with water treatment and uranium reduction suggests that properly designed bench -scale studies can <br />provide extremely useful information to assist with fine - tuning the pilot- and full -scale studies, including, <br />but not limited to: <br />• Timeframe required to stimulate anaerobic conditions sufficient to initiate uranium reduction and <br />sulfide precipitation <br />• Dosing requirements to maintain a sufficient carbon source while limiting the excess organic <br />matter that could reduce RO treatment efficiency <br />• Residence time needed to meet the treatment objectives <br />• Approximate stable concentrations of COCs <br />• Byproduct release due to changes in redox conditions <br />• Effect of increasing dissolved metal /uranium concentrations and salinity on the viability of the <br />microbial community <br />Page. <br />12/16 <br />