2017-08-16_PERMIT FILE - C2010089A (3)

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methodologies were utilized. First, the monitoring data from the old Peabody Nucla Mine were analyzed to determine the annual volume of flow into the pit from actual observations and to also determine the distance of drawdown (cone of depression) observed as the old Peabody Nucla Mine advanced to the north (see Map 2.04.7-1, "Old Peabody Nucla Mine Highwall"). The second method involved the determination of pit inflow rates and drawdown in the adjacent overburden and coal zones as a result of groundwater drainage from the bedrock zones into the pit utilizing a version of the groundwater flow model MODFLOW. A description of the modeling procedures and the assumptions used are provided in Appendix 2.05.6(3)-2. Observation and Analysis Flow from the old mine backfill (SS#1, #2, and #3, see New Horizon 1 Mine Permit) indicate that total seepage from that high -wall varied from a low of about 17 gpm (during periods of no irrigation) to a high of about 52 gpm during irrigation season. The high flow rates average about 42 gpm and the irrigation season is about 165 days long. The low flow rates average about 17 gpm and represent no irrigation for about 200 days per year. The average annual flow rate is therefore on the order of about 28 gpm or about 5390 ft3/day. This flow rate compares well with the flow rate of about 27 gpm calculated by subtracting the flow in Nygren Draw (SW -N6) from the flow rate at NPDES 001, (see Table 2.04.7-5 in Section 2.04.7). This further suggests that once the irrigation water is diverted into the HDPE pipeline that highwall seepage should be more on the order of about 17gpm (< 3300 ft3/day) and then decrease as the bedrock zones dry out. The bedrock zones are tight, with low transmissivity (measured OB and Coal Zone transmissivity and hydraulic conductivity at 58 ft2/day and 2.1 ft/day respectively at GW -N9) which results in limited but steep cones of depression. The old Peabody Nucla Mine (New Horizon #1 Area) is again instructive as the hydrograph of GW -N8 (see pg. 7-1-75 of New Horizon 1 Mine Area Permit) shows the water level was first affected by the mining in November 1980. At that time the coal face was about 760 ft from hole GW -N8. Total draw down at GW -N8 after 2 years and 9 months was 12.24 ft. Monitor hole GW -N9 at a distance of 854 ft from the final high -wall has never been affected, (see GW -N8 and GW -N9 hydrographs in Appendix 2.05.6(3)-2; Figures 2.05.6(3)-2a and 2.05.6(3)-2b). Based on the evaluation of these observations, water level drawdown in the bedrock zones is expected to only extend about 590 feet beyond the permit boundary. NHN's hydrologic consultant, Bishop -Brogden Associates, Inc. (BBA), prepared Figures 2.05.6(3)-2f and 2.05.6(3)-2g in order to illustrate the observations at GW's N8 and N9. These two figures are contained in Appendix 2.05.6(3)-2. Figure 2.05.6(3)- 2c in Appendix 2.05.6(3)-2, is an illustration of the projected draw down from an ideal pit in a stationary position based on the evaluation of the GW -8 and GW -9 hydrographs. Water level measurements were suspended at GW's-N8 and N9 from 1987 through 2007 and the "old" highwall was reclaimed in 1992. Water level monitoring at these two holes was resumed during 2008 and 2009 as part of the 2 year "close out" monitoring for New Horizon #1 Area Permit. Water levels in GW -N8 have recovered about 80% of the maximum draw down since the Section 2.05.6(3) Page 12 April 2016 (PR -01)