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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B <br />TOXICITY REDUCTION EVALUATION — LINES OF INVESTIGATION <br />Design Basis <br />East Taylor Modifications <br />The sizing, construction, and operational costs of evaluated BSR systems are directly related to the flow <br />rate and in some cases, including the proposed technology, the sulfate concentrations. Limiting the <br />volume of water to be treated is by far the most significant costing factor; therefore, Stantec recommends <br />isolation of the seep water from stormwater by lining the channel and sedimentation basin and installing a <br />gravel-backfilled seep water collection trench and conveyance line that would discharge directly into the <br />proposed bioreactor. This will eliminate surface water and potential inputs of unwanted dissolved oxygen <br />into the BSR. <br />Given the direct correlation of the seep flow rate and the sulfate concentration to the proposed treatment <br />system construction and operational cost and the uncertainty associated with the seep flow rate and <br />quality, Stantec recommends the seep collection system be installed as soon as possible so flow, sulfate, <br />total/dissolved iron, oxidation/reduction potential (ORP), temperature, and dissolved oxygen monitoring <br />can be completed to better inform the design. <br />Carbon Source <br />The carbon source for the BSR can be a number of different feeds: wood chips, molasses, corn syrup, <br />whey, ethanol, or any other biologically degradable organic matter. The choice is dependent on cost, <br />ease of dosing control, and both short-term and long-term reliability. Ethanol has been used in both <br />studies and similar settings as the optimal combination of these factors and it likely will be for this <br />application as well; therefore, ethanol has been assumed for this preliminary cost estimate. In addition, <br />locally available and lower cost carbon sources, such as wood mulch or other forms of biomass can be <br />evaluated as part of a value engineering effort if this option is chosen. Solid biomass carbon sources can <br />be difficult to control from a dosing perspective, resulting in variable ORP as the plant matter degrades. <br />ORP control is critical to proper bioreactor performance and preferential flow paths (i.e. short-circuiting) is <br />common; therefore, use of biomass would likely be used only in an initial stage to obtain conditions close <br />to those required and to provide nutrients for microbial growth. <br />Nutrients <br />Often the macro and micro -nutrients that are required for healthy microbial growth are present in <br />seep/spring groundwater; however, mine drainage can lack one or more of these. Therefore, testing of <br />the mine water for micro -nutrients will be required. As discussed, organic matter may be incorporated into <br />the BSR media and may provide any missing nutrients. Regardless, because the nutrients are recycled <br />as the microbes grow and die, additional nutrients that are required after initial establishment can be <br />added at nominal cost. <br />Iron Source <br />Sulfide (S2-) is present as hydrogen sulfide (H2S) in water, an odiferous and toxic gas. To ensure it does <br />not re -oxidize to SO4, it will be reacted with ferrous iron (Fe2+) to precipitate out of solution as solid iron <br />sulfide (FeS) or as solid iron disulfide (FeS2), also known as pyrite. For this cost estimate, sufficient total <br />iron for 30 years of operation was assumed to be added to the bioreactor, either as iron shavings during <br />dpg \\us0321-ppfss011shared_projectsV33001407\reports\4_tre lines of investigationitre lines of investigation_20200320_ifra.docx 2.8 <br />