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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B <br />® Stantec Memo <br />To: Angela Aalbers <br />Colowyo Coal <br />File: East Taylor Seep Investigation <br />Reference: East Taylor Seep Investigation <br />From: Paul Kos <br />Denver, CO <br />Date: December 22, 2020 <br />Elk Ridge Mining and Reclamation (Colowyo) engaged Stantec Consulting Services Inc. (Stantec) to provide <br />professional engineering services related to water treatment options at their Colowyo Mine in northwestern <br />Colorado, near the town of Meeker. Outfall 010 has been identified as requiring improvements to meet Whole <br />Effluent Toxicity (WET) requirements. Outfall 010 is a collection of seepage and stormwater runoff that flows <br />into the East Taylor pond, which discharges to Taylor Creek. The collected water has elevated levels of <br />sodium, sulfate, and total dissolved solids (TDS). Flow from the East Taylor Seep was suspected of being the <br />primary contributor to the discharges and water chemistry at Outfall 010, but the seepage water source had <br />not been investigated. Stantec recommended conducting a water balance and evaluating flows and water <br />quality at the seep and in Taylor Creek to determine potential seep water sources. This memorandum <br />summarizes the water balance calculations and seep and stream investigations. <br />Water balance calculations are a useful tool in determining likely inflows and outflows from a system to <br />identify variables that are not within anticipated values. The water balance method assumes that all water <br />inflows will equal water outflows for a closed system. Water inflows occur primarily from precipitation in the <br />form of rain and snow but may include groundwater inflows from external (outside the system) sources. Water <br />outflows occur as runoff, evaporation, transpiration, and infiltration. Additional losses may occur as <br />groundwater outflows. For this water balance calculation, Stantec initially assumed a closed system that does <br />not include any inflows or outflows from groundwater. Variables that are not within expected values may <br />suggest groundwater inflows or outfalls are affecting the water balance calculations. <br />Climate Data <br />Precipitation is the key input for the water balance, considering all flows originate as rain or snow. Colowyo <br />provided Stantec with site precipitation data, and the average monthly precipitation from January 2010 to <br />December 2019 was used for the water balance calculations. Stantec supplemented the site data with data <br />from 2020 collected at a weather station in Craig, Colorado (WUnderground 2020). The precipitation data <br />show that the site climate has wetter spring and fall months with drier summer and winter months. The <br />precipitation data are included in Table 1, below. <br />Evaporation is also a key factor for a water balance, and publicly available data are available from the <br />Western Regional Climate Center (WRCC). Stantec evaluated the 30 stations in Colorado that collect <br />evaporation data and determined that the Green Mountain Dam station best represented the site conditions <br />based on annual precipitation, elevation, and proximity to the site. There are two stations in Grand Junction, <br />Colorado that are closer than the Green Mountain Dam station, but these locations have approximately one- <br />half the precipitation that has been recorded at the site and were considered to not be representative of the <br />conditions at the Colowyo Mine. The evaporation data show that there is no evaporation during the six <br />months from mid -fall to mid -spring. Evaporation exceeds precipitation during the other six months. The <br />monthly net evaporation was calculated from the average site precipitation and the evaporation. The <br />evaporation data are included in Table 1, below. <br />Design with community in mind <br />pk c:\users\pkos\documents\_projects\colowyo\easttaylorseepwaterbalance_22dec2020_final.docx <br />