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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B <br />December 22, 2020 <br />Angela Aalbers <br />Page 5 of 12 <br />Reference: East Taylor Seep Investigation <br />Water Balance <br />For the water balance calculations, Stantec assumed that all infiltrated precipitation is discharged at the seep <br />location. The total seep discharge volume should equal the infiltrated precipitation volume unless there are <br />losses to groundwater or inflows from external sources. For water balance calculations, actual evaporation <br />and transpiration values are extremely difficult to calculate, and Stantec used a simplified approach to <br />calculate the amount of infiltration required to supply the seep water and determine the percentage of <br />precipitation that the seep volume represents. Knowing the seep flow rates allows for this approach. <br />Stantec identified several sources for infiltration rates in northwest Colorado. Hydrogeologic analyses <br />performed for the Colowyo Mine permit documents indicate that, based on stream flow measurements, <br />approximately 1.0% to 2.6% of annual precipitation infiltrates to groundwater and leaves the system in the <br />form of stream flow (Leonard Rice 1979). Assuming a closed model, all infiltrated precipitation would be <br />discharged by the stream. Similar water balance calculations performed for the nearby Piceance Creek <br />basin, suggest a net annual infiltration value of approximately 4.2% for native lands (USGS 1981). An <br />updated model for the Piceance Basin calculated a net infiltration of 6.5% (Day 2010). While all these values <br />are for native conditions, once mine lands have been reclaimed for more than five years, native infiltration <br />values are expected to occur (Sanchez 1988 and OSMRE 2016). Therefore, these values provide <br />approximate to lower -bound estimates of the percentage of precipitation that infiltrates and then daylights as <br />seep water. <br />Stantec used the monthly average precipitation values and multiplied them by the watershed area and various <br />infiltration percentages to obtain a range of seep flow curves. These curves are included in Figure 3 below. <br />The infiltration percentages ranged from 2.6% to 20% to include the values identified from literature and <br />assumed upper -bound estimates. Each of these calculations results in flow rates that are lower than the <br />discharge rates measured at Outfall 010 or at the seep. Only the October value for the 20% infiltration curve <br />matches the measured flow rates. <br />Stantec also calculated current (2020) discharges assuming 10% infiltration from native and reclaimed areas <br />and 50% infiltration from disturbed areas. These flows were calculated using the actual 2020 monthly <br />precipitation values from the weather station in Craig, Colorado (WUnderground 2020). The disturbed <br />percentage was adjusted until the calculated discharged mimicked the measured discharges. This value of <br />50% exceeds expected values since most precipitation runs off disturbed lands and is not available for <br />infiltration. This increased runoff response can be observed in the peak flow spikes in Figures 4 and 5. While <br />the discharge rates for the spring months and October mimic those calculated, the November and December <br />values diverge, likely the result of prolonged exceptional drought conditions (NMDC 2020). Precipitation in the <br />fall of 2020 has been significantly below average values, and there is less water available in the system to be <br />discharged. To assess future conditions, Stantec also calculated seep flows for the time when the entire <br />watershed has been reclaimed for several years. This calculation assumed 10% infiltration from all reclaimed <br />lands, and these flows are shown as the "2030" curve in Figure 3. <br />Design with community in mind <br />pk c:\users\pkos\documents\_projects\colowyo\easttaylorseepwaterbalance_22dec2020_final.docx <br />