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Limited ground water also occurs within the discontinuous, lenticular <br />sandstones associated with coal seams and the coal seams themselves <br />within the Mesaverde Group. Twentymile Park operations mine the Wolf <br />Creek, Wadge and Lennox seams, and consequently interrupt the hydrologic <br />regimes associated with these coals. While ground water within Iles and <br />Williams Fork Formation is predominantly calcium and sodium-bicarbonate <br />types, water in contact with coals may be calcium sulfate type and <br />contain fluoride, iron, manganese, selenium and sulfate in excess of U. S. <br />Public Health Service drinking water standards (Brogden and Giles, <br />1977). TDS levels range from 76-185 mg/1 quality and limited yields from <br />water-bearing units associated with the coals, preclude significant use <br />of this water in the Twentymile Park Basin. <br />The Wadge coal overburden aquifer is composed of the coal and 75 feet of <br />mixed shale and interbedded and lanticular sandstones. The permeability <br />of this aquifer ranges from .001 ft/day to .5 ft/day. Generally, areas <br />of shallow overburden have the highest permeabilities which decrease with <br />increasing overburden thickness. Pump tests on monitoring well 021-87-26 <br />with a top of screen depth of 1518 feet and well 006-82-48B at 987 feet <br />have permeabilities of .008 ft/day and .09 ft/day respectively. <br />Shallower wells 008-77-85 (40 feet) and 008-74-6 (216 feet) have <br />permeabilities of .495 ft/day and .455 ft/day respectively. When <br />considering ground water flow through the entire basin the lower <br />permeabilities will be the limiting factor in predicting flow rates. <br />The flow net analysis for the Wadge overburden in the permit application <br />used an average permeability of .04 ft/day which yields a flow rate of 8 <br />gpm. Amore appropriate permeability of .1 ft/day would increase the <br />flow into the mine and into the basin to 20 gpm. <br />The Trout Creek sandstone underlies the Wadge coal seam and is separated <br />from the coal by 150 to 300 feet of interbedded shales, siltstones, <br />sandstones and coals within the permit and adjacent areas. The latter <br />acts as an aquitard for the artesian Trout Creek sandstone aquifer east <br />of the mine. The Trout Creek sandstone provides recharge to alluvial <br />aquifers and base flows to Trout Creek and its tributaries. The <br />sandstone is an important regional bedrock aquifer within the Twentymile <br />Park Synclinal Basin. This sodium bicarbonate-based water has TDS values <br />of approximately 900 mg/1. It is used for industrial (mining), domestic, <br />and irrigation ground water supplies. <br />Limited ground water yields have been obtained from the Lewis Shale but <br />use of water from this unit is considered insignificant in the Twentymile <br />Park Basin. Where the Lewis Shale is present in the area, it acts as an <br />impermeable confining layer creating artesian conditions within the <br />underlying aquifers of the Mesaverde Group. <br />The highest reported well yields of ground water aquifers in the basin <br />can be obtained from the unconsolidated alluvial deposits of the Yampa <br />River and its tributaries. Well yields range from 5 gal/min along the <br />upper tributaries to as much as 900 gal/min along the Yampa (Brogden and <br />-35- <br />