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Twentymile's mining and subsiding of Fish Creek and its alluvial valley Floor will not impact those AVF's located <br />downstream of the operation. The subsiding of the valley floor above panels 13 Right through 17 Right will result <br />in the formation of pools, and these should be similar to those formed over panel 9 Right in the Eastern Mining <br />District. <br />Twentymile has gained experience in creek channel response to subsidence through its work on the segment of <br />Foidel Creek which overlies panels 9 and 8 Right. The subsiding of the creek resulted in a temporary disruption of <br />flow in the creek. The disruption lasted for approximately 24 hours and then it took approximately 6l days for the <br />inflow and outflow from the panel to equilibrate. The panel had a limited section of the Twentymile Sandstone <br />outcrop in contact with Foidel Creek. As noted on DMG inspections forms, a crack was noted in the sandstone in <br />the creek bed. One of two mechanisms or a combination of the two could have resulted in reestablishment of the <br />flow in Foidel Creek. As presented in PR-03, it is believed that the majority of the creek length is composed of <br />fine grained sediments that will fill in any cracks that develop or due to the thickness of the sediment layer the <br />water will be sealed from entering the fractures in the bedrock. The other alternative for those areas that are <br />exposed in the creek bottom is to have a combination of sediment filling in the cracks or water from the creek <br />entering the bedrock fractures and filling them so that the flow regime returns to its pre-subsidence condition. <br />Given the existence of fracturing in the bedrock in the creek bottom it is postulated that the additional fracture <br />volume was filled and then flow reestablished below the subsided area. <br />Table 72A presents the volume of water that entered the Twentymile Sandstone during the period of August 26 <br />through October 24, 1996. As is shown on the table 13.23 ac-ft of water entered the sandstone. A review of Map <br />23 indicates that approximately 2200 feet of channel traversed the mine area. Assuming a worst case analysis this <br />provides an estimate of 0.006 ac-ft/ft of channel. <br />It is anticipated that the Fish Creek channel will respond in the same manner as the Foidel Creek channel for the <br />entire length it goes over panels l3 Right through 17 Right. The entire length of Fish Creek overlying the panels <br />• was surveyed to look for outcrop areas. The survey showed that approximately 1260 feet of channel in section 10, <br />TSN, R86W has a rock outcrop (Fish Creek Sandstone) exposed. This would be an area that could experience <br />surficial cracking and be a potential site recharging the bedrock. However, the cover is greater over these panels <br />and have additional shale between the mine and the surface. This shale is the unit over which the alluvial materials <br />were deposited. The combination of greater depth and thicker shale deposits will minimize the potential for <br />su~cial cracking to occur during and after mining. Twentymile does not anticipate any loss of flow in Fish Creek <br />as a result of its mining operations. <br />If one is to look at the worst case of Flow disruption in Fish Creek it would be prudent to assume that the same loss <br />as experienced in Foidel Creek would occur in it. Also, it should be understood that the loss would be <br />experienced over each of the panels and not all at once, thus limiting the volume of water that will enter the <br />bedrock system. <br />As presently shown on Map 23, mining will progress from the south to the north in the Northern Mining District. <br />Panel 13 Right is the first panel that will result in subsidence potentially impacting Fish Creek and its alluvial <br />valley floor. Fish Creek meanders over the panel for approximately 16078 feet. Using the above referenced <br />relationship of stream length to recharge volume results in approximately 96 ac-ft of water potentially recharging <br />the bedrock. In reference to panel l4 Right, Fish Creek meanders over the panel for approximately 7303 feet. <br />This will result in approximately 44 ac-ft of water potentially recharging the bedrock. Panel IS Right is overlain <br />by approximately 5741 feet of Fish Creek, resulting in 34 ac-ft of water recharging the bedrock. Panel 16 Right is <br />overlain by approximately 6269 feet of Fish Creek, resulting in 38 ac-ft of water recharging the bedrock. Finally, <br />Panel 17 Right is overlain by approximately 2091 feet of Fish Creek, resulting in 13 ac-ft of water recharging the <br />bedrock. Panel ] 3 Right is projected to be mined in 2000, panels 14 and 15 Right in the year 2001, and panels l6 <br />.and 17 Right in 2002. Given the volume of water that passes through the Fish Creek system any loss during the <br />year is not significant to the irrigation systems on lower Fish or Trout Creeks. <br />.~P"~nt'l~'.C:'? `,,`~~i J .~ 2000 <br />PR 99-OS 2.06-25.3 2/24/99 <br />