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RULE 2 PERMITS <br />The Trout Creek Sandstone aquifer is separated from the lowest coal seam to be mined by approximately <br />400 feet in the Collom pit area. Between this coal seam and the Trout Creek Sandstone is a <br />mudstone/shale, sandstone, siltstone, and coal sequence of the Williams Fork Formation. About 200 feet <br />above the Trout Creek Sandstone, a laterally continuous, smectite clay layer known as the KM bed exists. <br />This layer has very low permeability and, therefore, is an effective barrier to vertical groundwater flow. <br />No impacts from mining or mine dewatering activities are anticipated to the quantity of groundwater in <br />the Williams Fork Formation or the Trout Creek Sandstone of the Iles Formation. <br />Potential effect of miningon n the groundwater flow system <br />The bedrock groundwater system intersected by the Collom Pit will be affected by mining and backfilling <br />activities. The existing bedrock groundwater system is highly anisotropic because of the alternating <br />layers in the bedrock that have permeabilities varying over many orders of magnitude. The coal seams <br />generally comprise the higher permeability layers, the sandstones have a lower permeability and the <br />siltstone and mudstone units have a very low permeability. The hydraulic conductivity values of the <br />bedrock units are reported to average about 0.14 ft/d for the coal seams and about 0.006 ft/d for the <br />sandstone units. The hydraulic conductivity value for the mudstone and siltstone units is expected to be <br />less than 0.0001 ft/d (WMC, 2005). Mining will displace these layers within the mine footprint and <br />replace them with a more uniform and isotropic backfill material. <br />The permeability of the backfill will be higher than the bedrock units and will be more similar the <br />permeability of an valley fill material. The hydraulic conductivity of the backfill is expected to be in the <br />range of 1 to 200 fl/d. The geometric mean value of hydraulic conductivity for valley fill is about 33 ft/d <br />(WMC, 2005) so this value is considered a reasonable estimate of the hydraulic conductivity of the <br />backfill. <br />The capacity of the backfill to transmit groundwater will be much greater than the capacity of the un - <br />mined bedrock as a result of the higher hydraulic conductivity. This means that the saturated thickness of <br />the spoil backfill necessary to provide the same quantity of groundwater flow under a similar hydraulic <br />gradient will be much less than the saturated thickness of the un -mined bedrock. Thus, it is likely that the <br />groundwater level in most parts of the backfilled pit area will be lower than the current groundwater level <br />in the bedrock. Conceptually, this means that the groundwater levels in bedrock around the backfilled <br />areas up -dip of the highwall will re -adjust to lower groundwater levels in the backfill itself. The exception <br />will be near the north highwall of the pit where the quantity of groundwater flow to the north from the <br />backfill will be limited by the permeability of the bedrock units to the north. In this area, groundwater <br />levels are expected to re-establish to the pre -mining elevation of about 7150 ft or higher. <br />Re -saturation of the pit backfill during the post -mining period <br />During mining the Collom pit will be progressively backfilled with spoil material once the initial boxcut <br />is established. The mine advances from north to the south, which is the up -dip direction for the bedrock <br />layers, so as the deeper portions of the pit are backfilled with spoil, water accumulating in the pit can flow <br />down -dip along the pit bottom into the backfill. The mining activity will not cause any decrease in the <br />hydraulic conductivity or transmissivity of the un -mined bedrock units located down -dip (north) of the <br />pit, and the capacity of the bedrock units to transmit groundwater will not diminish. Consequently, the <br />recharge and upgradient inflow entering the pit area will re-enter the bedrock units on the down dip side <br />of the pit. While the highwall dewatering wells to the north of the boxcut are operating, they will collect <br />this seepage. Once they are turned off, the seepage will continue to flow to the north in the bedrock <br />groundwater system in the same way that groundwater flow occurs prior to mining. <br />Collom — Rule 2, Page 119 Revision Date: 1/6/17 <br />Revision No.: TR -109 <br />