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DocuSign Envelope ID: D07924E6-3D94-4398-AO66-B683COBAB33E <br /> ® Stantec Technical Memorandum <br /> To: Angela Aalbers, Chantell Johnson From: David Grills, Jensen Ricks <br /> Elk Ridge Colowyo Mine Stantec- Colorado <br /> Reviewers: Daniel Dupon, Paul Kos Date: February 24, 2021 <br /> Stantec- Colorado <br /> Reference: Biological Sulfate Reduction Bench-Scale Test Results -Revision 1 <br /> OVERVIEW <br /> This document presents the results for bench-scale testing of biological sulfate reduction (BSR) conducted by <br /> Stantec Consulting Services (Stantec) from October 2020 to January 2021. Elk Ridge Mining and <br /> Reclamation (Colowyo) is investigating viable treatment technologies for their Colowyo Coal Mine property <br /> located near Craig, CO to provide water quality improvements at Outfall 010 to meet Whole Effluent Toxicity <br /> (WET) requirements. <br /> Previous evaluations conducted by SeaCrest Group focused on lime treatment and reverse osmosis. Stantec <br /> was contracted to further investigate BSR as a potential treatment technology to meet the WET requirements <br /> of the discharge permit. Several evaluations of the water quality and toxicity indicated that an overall <br /> reduction in sulfate/total dissolved solids (TDS)will provide a more favorable environment for aquatic life. <br /> Table 1 presents a representative water quality for Outfall 010. <br /> Table 1 -Outfall 010 Water Quality Basis <br /> TIE (July) Parameter Units <br /> Flow GPM 1 364 SAR 1 17.2 <br /> pH S.u. 8.29 Arsenic µg/L 1 <br /> Conductivity µS/cm 3,890 Iron µg/L 54 <br /> Bicarbonate mg/L 1,146 Zinc µg/L 5.2 <br /> TDS mg/L 3,330 Silver µg/L <0.5 <br /> Sulfate mg/L 1,500 Copper µg/L 1.2 <br /> TSS mg/L <0.1 Cadmium µg/L <0.5 <br /> Calcium mg/L 140 Chromium µg/L <20 <br /> Magnesium mg/L 120 Manganese µg/L 210 <br /> Sodium mg/L 810 Selenium µg/L <1 <br /> Table 1 Notes <br /> TIE is toxicity identification evaluation <br /> GPM is gallons per minute <br /> mg/L is milligrams per liter <br /> uS/cm is microSiemens per centimeter <br /> ,ug/L is micrograms per liter. <br /> Outfall 010 has elevated levels of sodium, sulfate, and TDS. BSR treatment processes can lower sulfate but <br /> is not expected to decrease the TDS; however, this process is unproven with Outfall 010 water. The purpose <br /> of this bench-scale test is to demonstrate that this treatment technology is viable with Outfall 010 source <br /> water and can achieve passing WET test results. <br /> n community in mi <br /> dpg llus0321-ppfss01lshared_projects\233001407\reports\6_bench_scale_report\rpt_bench_scale_report_24feb2O20_final_rl.docx <br />