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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B <br />TOXICITY REDUCTION EVALUATION — LINES OF INVESTIGATION <br />Design Basis <br />cell, flowing downstream. Ethanol dosing and water quality sampling tubing would be connected to the <br />laterals and routed to a control building/shed. <br />Surface Completion: The top of the rock media would be graded to create a stream channel, then <br />covered with a geomembrane. The previously excavated native materials would be placed around the <br />edges, and rip -rap would be placed in the main channel to withstand flash flood flow rates. The final <br />surface would be flatter than current; hence after flow passes through the existing roadway culvert, it <br />would be dissipated across a larger area. <br />Cell 1 — Mulch: If a low-cost mulch or similar carbon source is available and testing shows value in <br />incorporating it, the first cell would be filled with this material to supply an initial carbon source and <br />nutrients. This material would biodegrade and require replenishment overtime; hence this cell may be <br />located between the creek and the East Tylor Pond berm, so a removable cover can be used for this <br />periodic refurbishing. <br />Cells 2 thru 4 — Primary Bioreactor: The next cells would be the primary bioreactor cells. Based on flow <br />and ORP monitoring, ethanol would be dosed into the laterals to maintain the optimal ORP for sulfate <br />reducing bacteria (SRB), which is in the range of -50 to -250 mV. Any lower and methanogenesis can <br />dominate, competing with the SRB for the carbon source with no treatment benefit. <br />Cell 5 — Supplemental Iron: If required, a supplemental iron source would be blended into this cell's rock <br />fill. This could be iron shavings or rock/ore with high iron content. ORP would be increased which the <br />addition of oxygen, if necessary, through small aerators. This would consume any remaining ethanol <br />(dissolved organic carbon), shifting into the zone more ideal of iron reducing bacteria growth. <br />Cell 6 — Contingency Aeration: Bubbler aerators supplied by a blower would also be installed in the final <br />small bioreactor stage. These would be operated only as needed. Aeration would add dissolved oxygen <br />(DO), effectively stopping biological activity and stabilizing the BSR effluent by oxidizing sulfur <br />compounds and iron. The discharge permit may also require a minimum DO concentration that could be <br />readily meet with aeration. <br />Bypass: A weir -based system will likely be incorporated to bypass excess flow around the bioreactor in <br />the event of unexpected future conditions. The feature could also be used to more thoroughly treat only <br />part of the total flow through the bioreactor, potentially reducing ethanol usage and increasing retention <br />time, and then blending to bring the total flow stream to below the sulfate target. <br />Wetlands: Water from Cell 6 could exit directly to the downstream creek channel. It is proposed to grade <br />the area at the end of the bioreactor to create a shallow pond that, given the continuous spring flow will <br />become natural wetlands that could provide some additional treatment and a wildlife habitat. Surface <br />stormwater flow would also be routed into the pond to improve overall water quality. Thus, the final point <br />of compliance could be at the wetland pond effluent, depending on regulatory requirements related to <br />maximum discharge flows and stormwater mixing allowances. <br />dpg \\us0321-ppfss011shared_projectsV33001407\reports\4_tre lines of investigationitre lines of investigation_20200320_ifra.docx 2.10 <br />