2020-06-25_REVISION - C1981019 (6)

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RULE 2 PERMITS Good Spring Creek (3000 feet) will be used. The gradient would approximate the dip of the lithology in the area. Assuming an effective porosity of 0.15,with an average hydraulic conductivity of 1 ft/d for the Williams Fork Formation,then: vs=(1 ft/d) (500 ft)/(0.15) (3,000 ft) vs= 1.11 ft/d The average groundwater velocity of outflow from the South Taylor pit is calculated to be 1.11 ft/d, with the flow presumed to be predominantly in a southeasterly direction following the dip of the southeast dipping leg of the small anticline (refer to Map 7A). Thus,the first pit outflow through the bedrock strata would take about 2700 days or about 7 years to flow from the pit to the creek. Potential Surface Water Quantity Impacts As described above, diminishment of flow into Good Spring Creek appears to be probable during and for a period after mining and reclamation of the South Taylor pit is finished. The reduction can be estimated by assuming no meteoric water infiltrating into the reclaimed pit will reach the creek from a pit aquifer for approximately 45 years after the end of operations (the time to saturate the pit - see above) or that springs located downgradient from the mine will cease flowing during and for a time after mining. The area of the South Taylor pit is approximately 1,000 acres. Assuming that 1.8 inches of precipitation infiltrates, the pit will receive approximately 150 ac-ft per year, or 92 gpm or 0.21 cfs of recharge from infiltration as shown in the preceding paragraphs. Much of this infiltration may eventually surface at springs, likely in West Fork Good Spring Creek. The actual resultant spring discharge will likely vary from high flow to low flow periods by an order of magnitude, as measured in the surface water features. Thus, the discharge of groundwater originating as pit infiltration used in the following calculations is assumed to range from 0.06 to 0.6 cfs, which gives a geometric mean of approximately 0.21 cfs (calculated infiltration rate from above). Assuming that 0.06 cfs enters Good Spring Creek during low flow and 0.6 cfs enters Good Spring Creek during peak flow, the pit contribution would be approximately 7% of the base flow and 5% of the peak flow to Good Spring Creek at the NUGSC measuring point or about 3% of both base and peak flows at the LGSC measuring point. This is a maximum value, since the calculated contribution from the pit spoil aquifer is greater than the average measured flow from the potentially affected springs. Thus, the probable reduction in flow will be up to 7%of base flow for 45 years after mining ceases. Potential Surface Water Quality Impacts Potential impacts to the surface water quality from the South Taylor pit operations are considered here. The water quality would be impacted by meteoric water that enters the hydrologic cycle being impacted by contact with the overburden fill. To estimate the impact to surface water quality, existing geochemical and flow data for Good Spring Creek were modified by changing the flow entering from the pit (described above)to have water quality similar to that found in the Streeter Well (completed in backfill in the Streeter Fill) and Streeter Pond discharge. The Streeter Well is located in the Streeter Fill of the existing East Pit, and would appear to represent water quality in direct contact with Colowyo Mine spoils. The Streeter Pond accepts primarily groundwater from the Streeter Fill. Assumptions used include: South Taylor/Lower Wilson—Rule 2,Page 82 Revision Date: 6/22/20 Revision No.: MR-220