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Ground Water Impacts <br />Pit effluent could degrade the quality of ground water in the vicinity of the <br />Trapper Mine. Based on analogy with other northwestern Colorado coal <br />mining operations, the geochemical processes that can be expected to control <br />the chemical evolution of ground water in Trapper's coal spoils include: <br />recharge and percolation of water from snowmelt or precipitation that contains <br />cazbon dioxide and oxygen gases, dissolution of carbonate and sulfate minerals <br />in the spoils, oxidation of pyrite in the spoils, and cation-exchange reactions <br />(U.S. Geological Survey Water Resources Investingations Report 92-4187). <br />The water in a mined-out, spoils-filled pit flows downgradient (northward at <br />Trapper) through the spoils until it reaches the unmined lowwall at the north <br />end of the pit. When the pit ftlls with water a spoil spring may develop at the <br />ground surface at the low (north) end of the pit. <br />Beneath the ground surface, the water (effluent) passes through the unmined <br />lowwall and flows through the bedrock parallel to the bedding plane down the <br />hydrologic gradient which generally is northward. The polluting capability of <br />the effluent is gradually reduced as it flows down-gradient as it is diluted by <br />fresher ground waters in urunined areas and the chemicals in the effluent <br />become less concentrated as they are spread out (attenuated) over a wide area. <br />The only major aquifer that would be in the flow path of an advancing front of <br />effluent at Trapper is the Third White Sandstone. Page 4-226a of the permit <br />application explains that if contaminated ground water were to flow beyond the <br />permit azea, 2 bedrock wells could be affected: W-624-74 and 80998. <br />The dominant cations in spoil water are: calcium, magnesium, and sodium; the <br />dominant anions are: bicarbonate and sulfate.(analyses from wells GD-3, GD- <br />5, GD-7, and GF-11 reported in Appendix "H" of permit application). All of <br />these ions also dominate natural waters in the area. <br />Batch leaching tests performed by the applicant indicated that the metallic <br />constituents in coal spoils at Trapper which are most likely to form a leachate <br />are manganese, molybdenum, and vanadium (section 4.8.2.5 of permit <br />application). As of 1997, high levels of manganese have been detected in <br />samples from three of the four wells on Trapper that monitor water quality in <br />spoils-filled pits (wells GD-3, GD-5, GD-7, and GF-11) [Appendix "H" of <br />permit application]. Molybdenum and vanadium have not been detected in <br />samples from the four wells. <br />The permit's restrictions on waste placement should ensure that no significant <br />amount of leachate develops from water flowing into the pits from ground <br />water inflows or percolating surface waters (section 4.3.4.6 of permit <br />Trapper Mine 21 May 13, 1998 <br />