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INCLUDE A WRITTEN DETERMINATION OF THE NEED FOR GROUNDWATER POINTS <br />OF COMPLIANCE AT THE MINE. IF DEEMED APPROPRIATE, BASED ON THIS <br />ANALYSIS, COLOWYO SHALL ESTABLISH ONE OR MORE POINTS OF COMPLIANCE <br />FOR THE COLOWYO MINE. <br />Backfill and Excess Snoil Groundwater <br />The Colowyo coal mine is a multi-seam, open-pit, operation. The pits are excavated to <br />approximately 330 feet deep, and are then backfilled to approximate original contour. The original <br />stratification of geologic units is not reconstructed during this backfill process. Placed backfill <br />materials are not as dense as in situ geologic formations (materials aze blast cast, back-cast by <br />dragline, end-dumped from trucks). Being less dense, they have more voids, are more permeable, <br />and have higher transmissivity values. In addition, the 300-foot backfill section becomes a more <br />homogenous section. As precipitation penetrates these azeas it becomes groundwater. <br />Such groundwater would develop as a result of the mining method, and is not considered a pre- <br />existing groundwater aquifer/saturated zone thought would be impacted by the mining. These zones <br />of developed groundwater are not deemed aquifers and the water is known to be of degraded quality. <br />Thus the belief of the Division is that points of groundwater compliance are not required in azeas of <br />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. These <br />fills approximate the pit depths in thickness, and are similar to characteristics of backfilled azeas. <br />However, they include underdrain systems designed to minimize groundwater reservoirs. In the event <br />that groundwater reservoirs do develop, it is the belief of the Division, as with backfilled areas, <br />groundwater points of compliance are not required in excess spoil fills. Spoil groundwater is of <br />known degraded quality and is not intended for use as an aquifer. As described below, backfill <br />seepage and spoil water dischazge may have an impact on surface water or alluvial groundwater. <br />Alluvial Groundwater <br />Mining activities at Colowyo have the potential to impact alluvial groundwater in those azeas of <br />Taylor Creek, Wilson Creek, and Good Spring Creek that are hydrologically down-gradient of <br />mining activity. Sources of impact include dischazges of runoff from surface disturbed areas; <br />discharges or seepage from within backfill and excess spoil azeas; and from surface and subsurface <br />flows from the coal stockpiles at the Gossard Loadout. <br />The findings by Division, for alluvial groundwater as described above, is substantiated by the PR-02 <br />application. On page 88 of Volume 12 in the permit application package, Colowyo indicates that the <br />alluvial aquifer associated with Good Spring Creek has a high transmissivity and is unconfined. <br />Possible impacts to this aquifer would be associated with the infiltration of water from the pit and <br />Proposed Decision and Findings of Compliance 4 May 2007 <br />Permit Revision 02 Page 42 <br />