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Evaluation of Hydrologic Impacts <br />In accordance with Section 3.03.2(2), the Division has evaluated whether pollution of surface <br />or subsurface water is occurring, and the probability of future occurrence of such pollution. <br />The evaluation considered information in the following documents: the Probable Hydrologic <br />Consequences section of the permit application, Annual Hydrology Reports submitted by the <br />operator, the Division's Annual Hydrology Report review memoranda, and documents in the <br />SL-2 bond release application and file. The results of the evaluation are summarized below. <br />Ground Water Impacts: <br />Saturation of mine spoil and ash. The Keenesburg mine pits will be fully backfilled with coal <br />combustion ash and mine spoil (Figure 1) when reclamation is complete. This backfill material <br />will become saturated with meteoric water that infiltrates into the ground from the land surface. <br />Infiltration can be expected to occur at a rate on the order of at least 2 inches per year, based on <br />infiltration rates reported in a lysimeter study by the U. S. Geological Survey at a surface coal <br />mine in Routt County, Colorado. With an infiltration of 2 inches per year, the Keenesburg pits <br />would reach maximum saturation over a period of a few decades after backfilling is complete. <br />Formation of leachate in pit backfill. Meteoric water infiltrating from the land surface will <br />become mineralized, creating a leachate, as it migrates through backfill in the pits. The main <br />processes of leachate formation can be expected to be oxidation, dissolution, and cation <br />exchange. The resulting leachate will be high in total dissolved solids (TDS), mainly salts of <br />sulfate and sodium. The leachate will have a high sodium absorption ratio (SAR). Metals will <br />not be mobilized due to the near neutral pH of the leachate. This predicted leachate composition <br />has been verified from monitoring data from wells SMW-2 and AMW-2. <br />Potential for migration of spoil leachate into aquifers or seepage to land surface. The <br />Division's review of the Keenesburg Mine's 2004 Annual Hydrology Report included an <br />analysis of the migration potential of spoil leachate. The analysis concluded that leachate will <br />migrate in the subsurface from the pits. Dispersion and attenuation can be expected to limit <br />ground water degradation to an area within a few a hundred feet of the pits (Memo to Annual <br />Hydrology Report File dated July 6, 2005). The relatively flat topography will preclude <br />significant forlnatlon of leachate seeps or springs on the ground surface. <br />Potential for migration of coal combustion ash leachate into aquifers or seepage to land <br />surface. The overburden stratigraphic sequence at the Keenesburg Mine contained layers of <br />clay. During mining, this clay was stockpiled. After the pits were excavated, the clay was placed <br />on the bottoms and sides of those areas of the pits where ash was to be backfilled. The clay liner <br />is at least 3 feet thick in the B pit. These clay liners would act as permeability barriers that would <br />5