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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B <br />TOXICITY REDUCTION EVALUATION — LINES OF INVESTIGATION <br />Introduction <br />1.0 INTRODUCTION <br />1.1 PURPOSE <br />Elk Ridge Mining and Reclamation (Colowyo) engaged Stantec Consulting Services Inc. (Stantec) to <br />provide professional engineering services related to water treatment options at their Colowyo Mine in <br />northwestern Colorado, near the town of Meeker. This report evaluates the technologies identified in the <br />Toxicity Reduction Evaluation (TRE) as having the best opportunity for successfully addressing the Whole <br />Effluent Toxicity (WET) in water discharged at Outfall 010. <br />This report outlines the conceptual design considerations for three technologies: membrane separation, <br />biological sulfate reduction, and enhanced evaporation (as a supplemental addition). WET testing and <br />total dissolved solids (TDS) reduction testing are used to provide a quantitative basis for implementation <br />of the long-term treatment scenarios outlined within this document. <br />Following the design concept discussion, the engineering opinion of probable costs is prepared and <br />detailed for the options. This includes a capital expenditure based on the expected implementation of the <br />option itself as well as the operational costs based on the design. <br />1.2 PROJECT BACKGROUND <br />Elk Ridge Mining and Reclamation operates the Colowyo Mine, a coal mine in the Moffat and Rio Blanco <br />Counties of northwestern Colorado. The mine is located approximately 20 miles north of Meeker. Some of <br />the water from operational areas reports to East Taylor Creek, located in the central portion of the mine <br />property along Taylor Creek. Taylor Creek flows south to north through a watershed that includes <br />undisturbed areas, areas disturbed by mining operations, and reclaimed areas. <br />Colowyo Mine has permitted discharges (generally stormwater runoff and springs) from the surface coal <br />mine areas per the CDPS Permit No. CO-0045161. Starting in October 2018, the mine was required to <br />commence chronic WET testing at several locations that receive "process waters" such as springs in <br />contact with backfilled overburden (spoil) and other processes. There is a "pattern" of WET in the Outfall <br />010 samples with sublethal impacts to Ceriodaphnia dubia. Outfall 010 is associated with the East Taylor <br />Pond sediment control pond. <br />Flow behavior for Outfall 010 has shown changes in the past 24 months of monitoring. The cause of this <br />increase in flow has not yet been fully investigated to determine if any flow reduction is possible. The <br />compiled data for this increased flow and the corresponding water chemistry is incorporated into the <br />design basis for the evaluations of these technology options. <br />dpg \\us0321-ppfss011shared_projectsV33001407\reports\4_tre lines of investigationitre lines of investigation_20200320_ifra.docx 1.1 <br />