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DocuSign Envelope ID: D07924E6-3D94-4398-AO66-B683COBAB33E February 24,2021 Angela Aalbers,Chantell Johnson Page 12 of 12 Reference: Biological Sulfate Reduction Bench-Scale Test Results-Revision 1 a reduction in overall TDS. The biological sulfate reduction process essentially exchanges sulfate ions for carbonate ions. Observations during testing of white sediment in the treated effluents further support calcite supersaturation as a potential toxicant. The calcite oversaturation toxicity theory is based on Langelier Saturation Index(LSI) calculations which indicate that calcite will precipitate from BSR effluent overtime. Calcite precipitation is known to be toxic in WET testing due to scale formation on the test species. Toxicity due to calcite precipitation is the subject of research currently being conducted in western Canada. However, despite identifying these two potential toxicity sources, the two supplemental treatment approaches to reduce the overall calcite saturation level have not produced satisfactory results. The complete Whole Effluent Toxicity Testing Report performed by GEI Consultants is included as Attachment 3. RECOMMENDATIONS It is evident from these bench-scale test results that the passive BSR treatment is not sufficient to treat the mine water to an acceptable level. It is reasonable to conclude that the reduction of sulfate concentrations and/or calcite saturation does not adequately reduce toxicity. Reduction of TDS requires an active treatment approach. At present, the only treatment technology evaluated that passes WET testing is membrane separation (reverse osmosis or nanofiltration) as indicated by the TIE. In addition, deep-well injection, which would not discharge to the permitted outfall, is another approach that has been independently evaluated. Blending of these two approaches, membrane separation and brine discharge by deep-well injection would allow for a 75% recovery of the treated water(to be returned to the environment), thus requiring a smaller discharge well. This approach would eliminate the brine evaporation pond which was a requirement in the previous THE evaluation. The evaporation pond for brine management is considered to be a labor-intensive activity to maintain and operate. If the deep well injection is approved, either of these approaches can be pursued without the requirement of an additional WET test evaluation. An alternative approach for TDS reduction would be ettringite precipitation. This approach has been extensively evaluated by Freeport McMoRan in conjunction with a technology company located in Pueblo, CO. This process uses pH adjustments and elemental aluminum metal as the primary treatment method with published results indicating reduction between 60-90% depending on the rate of aluminum dosing. This approach is still considered experimental and is also an active treatment system. Desic th community in mir dpg llus0321-ppfss01lshared_projects\233001407\reports\6_bench_scale_report\rpLbench_scale_report_24feb2O20_final_rl.docx