2021-01-04_GENERAL DOCUMENTS - C1981019

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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B TOXICITY REDUCTION EVALUATION — LINES OF INVESTIGATION Design Basis construction, or in the event of inerting (coating), additional powered iron could be fluidized and added to the reactor periodically during operation through the upper cell lateral pipes that are discussed later. However, iron as Fe3+ is present in many ores, and iron reducing bacteria (IRB) can reduce and dissolve it as Fez+. In addition to testing of the seep water for total and dissolved iron, locally available sources of rock fill will also be investigated. The more promising local site materials will be bench and field tested, possibly reducing or eliminating the need for supplemental iron. This could reduce both the construction and operating cost. Sizing At neutral pH and at 33°C (91 °F), an average of 0.17 grams (170 mg) per liter per hour of sulfate to sulfide conversion is possible based on prior studies (Table 8, Brahmacharimayum 2018). This means under warm water conditions, as little as 9 hours of hydraulic retention time may be required to convert 1,000 mg/L of sulfate to sulfide. The effect of lower temperatures on this rate are significant and will be evaluated during bench and site testing. On average, reaction rates halve (required treatment time doubles) for every 10°C decrease. For the purposes of this estimate, it is assumed a 30°C temperature decrease to a worst case water temperature of 3°C (37°F) will occur, resulting in a required retention time of 72 hours or 3 days. Including safety factors, up to 6 days of retention time was assumed for the purposes of this preliminary estimate. Assuming a maximum 364 gpm flow and 25% rock porosity, a bioreactor (and rock) volume of nearly 70,000 cubic yards (cy) would be required. Collecting data on actual temperatures and BSR rates will be used to optimize the final system size. Configuration The outfall from the East Taylor Pond flows through a graded creek channel that will be lined with an impermeable geomembrane to design elevations, several feet below the access road. (All geomembranes mentioned in this section would be sandwiched between geotextile for protection). This will create a liner bioreactor nearly 1,000 feet long, which enhances mixing. Cells are included to maintain fluid levels under variable flows, to promote mixing of the water, and to allow for more precise monitoring and carbon dosing. Cells: Rock fill placement would likely begin on the downgradient end, gradually sloping from the existing creek grade up to the bioreactor height, estimated to be between 10 to 15 feet. After the first section has been filled, a geomembrane will be placed across the creek channel against the rock angle of repose. The sides may be welded or tape -sealed to the adjacent geomembrane - some leakage between cells (i.e. several gallons a minute) would be acceptable given the anticipated high system flows. This fill and geomembrane installation process would be repeated up the channel creating a series of cells (assumed to be 6). Laterals: Lateral water conveyance piping would be placed across the bottom (influent) and near the top (effluent) of the rock in each cell. This pipe may be similar to leach -field piping for septic systems. Laterals would run parallel to the channel alignment, with several laterals in each cell as sizing and flow rates require. No pumping would be required, flow would be driven by gravity head differences between each dpg \\us0321-ppfss011shared_projectsV33001407\reports\4_tre lines of investigationitre lines of investigation_20200320_ifra.docx 2.9