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COMPLIANCE FOR THE COLOWYO MINE. <br />Backfill and Excess Spoil Groundwater <br />The Colowyo coal mine is a multi-seam, open-pit, operation. The pits are excavated to up to <br />approximately 450 feet deep, and are then backfilled to approximate original contour. The <br />original stratification of geologic units is not reconstructed during this backfill process. Placed <br />backfill materials are not as dense as in situ geologic formations (materials are blast cast, back- <br />cast by dragline, end-dumped from trucks). Being less dense, they have more voids, are more <br />permeable, and have higher transmissivity values. In addition, the up to 450-foot backfill section <br />becomes a more homogenous section. As precipitation penetrates these areas it becomes <br />groundwater. <br />Such groundwater would develop as a result of the ruining rneinod, and is not considered a pre- <br />existing groundwater aquifer/saturated zone thought would be impacted by the mining. These <br />zones of developed groundwater are not deemed aquifers and the water is known to be of <br />degraded quality. Thus the belief of the Division is that points of groundwater compliance are <br />not required in areas of mine pit backfill. <br />During the open-pit mining process, the replacement volume of mined materials expands by <br />approximately 20%, requiring the development of excess spoil fills for the excess material. <br />These fills approximate the pit depths in thickness, and are similar to characteristics of backfilled <br />areas. However, they include underdrain systems designed to minimize groundwater reservoirs. <br />In the event that groundwater reservoirs do develop, it is the belief of the Division, as with <br />backfilled areas, groundwater points of compliance are not required in excess spoil fills. Spoil <br />groundwater is of known degraded quality and is not intended for use as an aquifer. As <br />described below, backfill seepage and spoil water discharge may have an impact on surface <br />water or alluvial groundwater. <br />Alluvial Groundwater <br />Mining activities at Colowyo have the potential to impact alluvial groundwater in those areas of <br />Taylor Creek, Wilson Creek, and Good Spring Creek that are hydrologically down-gradient of <br />mining activity. Sources of impact include discharges of runoff from surface disturbed areas; <br />discharges or seepage from within backfill and excess spoil areas; and from surface and <br />subsurface flows from the coal stockpiles at the Gossard Loadout. <br />The findings by the Division, for alluvial groundwater as described above, is substantiated by the <br />information in the application. On page 88 of Volume 12 in the permit application package, <br />Colowyo indicates that the alluvial aquifer associated with Good Spring Creek has a high <br />transmissivity and is unconfined. Possible impacts to this aquifer would be associated with the <br />infiltration of water from the pit and water quality deviations caused by infiltration of runoff <br />water. Colowyo further indicates, "meteoric water infiltrating into the reclaimed pit should enter <br />the bedrock aquifer and eventually contribute to seeps and springs tributary to Good Spring <br />Proposed Decision and Findings of Compliance 14 September 2009 <br />Permit Renewal 05 Page 45