2014-05-12_PERMIT FILE - C2010089 (5)

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1) Interruption of Groundwater Flow and Drawdown. The bedrock zones, overburden and coal, will be exposed or daylighted by the NHN excavation. There are no users or beneficial uses for the relatively small amount of ground water that is currently moving through the bedrock zones toward the cropline to the southwest. Recharge from irrigation (the major source of recharge) will continue to the east, north and west of the NHN permit area as it currently does. Irrigation water that now infiltrates into the bedrock zones at NHN will be diverted around the mining area in a HDPE pipeline. Seepage out of the northward advancing high -wall will be diminished from what is currently being experienced from the old Peabody high -wall because the annual recharge from irrigation water flowing across the permit area will be eliminated. In order to develop the impact assessment for groundwater flow and drawdown, two different 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 Section 2.05.6(3) Page 12 October 2013 (TR -05) E: \New Horizon \DRMS \NHN Permit \04_Technical Revisions TR \TR -05 \Documents from Jason \2.05.6(3) Protection of Hydrologic Balance TR- OS.doc