2013-04-10_PERMIT FILE - C1981019 (82)

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RULE 2 PERMITS approximately 0.002 ft/day (H sandstone) to 0.008 ft /day (F sandstone), with a geometric mean of 0.006 ft/day. Thus, calculating a single hydraulic conductivity (K) for pit wall seepage is difficult. For further discussion, a K of 0.01 (1.0x10 -2) ft per day will be used. The length of the northern pitwall is approximately 5000 ft. The thickness of the sandstones and coals along this highwall (using actual core measurements and geophysical logs from three drill holes near the proposed highwall) varies from a high of 84 feet thick to a low of 31 feet thick. The geometric average is 50.3 feet. Thus, the cross - sectional area (A) for the northern pitwall sandstones and coals is 2.52x105 square feet. With Q= KA(h/1), K = 1.0x10.2 ft /day A = 2.52x105 ftZ h /1= 0.1 This equals a minimum pit seepage volume of 252 cfd (0.005 acre -ft /day) over a height of 125 feet. However, this pitwall seepage will only occur if a spoil aquifer is developed. Based on the above, a spoil spring flow of up to 0.45 cfs from the Collom Lite Pit into the Little Collom Gulch valley fill is possible. This is calculated as follows: the total surface infiltration into spoil is calculated at 39,137 cfd. This number minus the calculated pitwall seepage of 252 cfd then equals a possible spoil spring discharge of 38,885 cfd, which equals 0.45 cfs. This water would then flow down Little Collom Gulch to Collom Gulch in Section 13, T. 4 N., R. 94W., if the spoil aquifer develops. To determine possible impacts of this spoil water on Collom Gulch, data from valley fill testing needs to be examined. Well testing of the monitoring wells in Collom and Little Collom Gulches showed a highly permeable material. No hydraulic conductivity was calculated due to insufficient data (WMC, 2005). Therefore, in determining probable ground water velocity for this spoil spring water, an average hydraulic conductivity from the WMC report (2005) for valley fill material of 3.3 ft/day is used, an effective porosity of 20% and a hydraulic gradient of 0.03 ft/ft (actual gradient in Collom Gulch and Little Collom Gulch). This calculates a ground water velocity of approximately 0.5 feet /day. At this rate of travel, the front edge of any spoil spring flow water transiting the Little Collom Gulch after exiting the Collom Lite reclaimed pit would take approximately 95 years to reach Collom Gulch in Section 13, T. 4 N., R. 94W. During this flow time, dilution of the water will occur due to infiltration through the valley fill materials from precipitation, runoff and inflow from the bedrock units. Prior to reaching Collom Gulch, the spoil spring flow water will encounter shallow ground water in the valley fill in Little Collom Gulch. The amount of valley fill ground water currently transiting Little Collom Gulch can be calculated using data from the WMC report. Using the data from MLC- 04 -01, the average water level is 46.9 ft. below ground surface, a depth to bedrock of 66 ft., a valley width of 225 ft., and a 20% effective porosity, calculates to a saturated area of approximately 430 square feet. Using Q= KA(h /1), the minimum volume of water transiting Little Collom Gulch in section 13 is approximately 43 cfd, which equals 5x10 -4 cfs. To determine the amount of valley fill water in Collom Gulch in Section 13, the probable valley fill water amount at MC -04 -02 needs to be calculated. This Collom Gulch valley fill well is the closest to Section 13. Using an average water level of 11.25 feet below ground surface (from WMC data), a depth to bedrock of approximately 85 feet, and a valley width of 325 feet, equals a cross sectional area of 11985 square feet. Using a 20% effective porosity, the saturated area would equal 2396 square feet. Using Q= KA(h/1), the minimum volume of water transiting Collom Gulch in section 13 is 237 cfd or 0.003 cfs. Collom — Rule 2, Page 139 Revision Date: 9/28/11 Revision No.: PR -03