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 Controls Each inter -cell conveyance pipe would be equipped with small diameter (i.e. '/-inch) tubing for collection of a water sample or dosing of ethanol. All of these lines would be brought to a nearby sampling building. Sample water would be by a low flow pump and analyzed for ORP, temperature, and DO. This information is indicative of system performance within each cell and could be remotely monitored. The ORP data would primarily be combined by the local programmable logic controller (PLC) with system flow rate and water temperature data to control the dosing rate of the ethanol chemical feed pump. The water could also be periodically sampled and analyzed for parameters such as sulfate, sulfide, dissolved and total iron, etc. It is possible ethanol addition to only Cell 2 may be required and controlling parameters may be reduced as operational experience is gained. The ultimate design configuration should allow for optimal monitoring and operational flexibility, if needed, and address the potential for freezing during winter operation. Proving Testing The bioreactor system requires proving testing to ensure that it meets the WET criteria like the membrane treatment that was performed by SeaCrest. Stantec recommends a two -stage approach to ensure that the design chemistry and physical design are viable and then optimized for implementation. Ethanol usage rates in this system represent the primary long-term O&M costs. Literature data for the process shows potential variability of between 0.67 and 2 gram COD (provided by ethanol) per gram of sulfate treated. A value of 1 gram of COD has been chosen for this design as it is the most prevalent number within the literature on the subject. Bench and pilot testing will be tailored to identify the optimal range and minimize the operational costs of this system over the remaining life of the treatment and remediation. Bench Tests: The bioreactor performance variability is associated with the specific site water characteristics, system configuration, and media fill, and iron source. This combined with the potential construction cost and possible operational savings, mean bench testing is warranted prior to design. The objectives of the bench testing would be to better define the required hydraulic retention time (bioreactor size), test various rock sources as suitable media and iron source, collect data on optimal ethanol dosage and ORPs, and identify other operational considerations. The laboratory bench testing would attempt to scale the conceptual system as closely as possible. Site mine water would be fed through a series of columns with all components maintained at seep water temperatures. If multiple viable fill rock sources are identified, parallel tests would be run with each rock type to assess effects of any natural mineral -based iron on the bioreactor performance as well as potential reductive dissolution of other contaminants. Each column in series could be constructed to mirror a cell in the full-scale system, including one with wood chips, two final cells with aeration, and the ability to dose in powered iron if/as needed. The bench tests could also be operated similarly to the full- scale system cells, including hydraulic retention time, ethanol dosage, parameters measurement, and sample collection. SRB bacteria from a supplier or local anaerobic sediments could be used to inoculate the system to initiate bacterial activity. The test would be completed over the course of one or two months to develop the necessary information for design the larger -scale field test. dpg \\us0321-ppfss011shared_projectsV33001407\reports\4_tre lines of investigationitre lines of investigation_20200320_ifra.docx 2.11