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<br />Limited ground water yields have been obtained from the Lewis Shale but use of <br />water from this unit is considered insignificant in the Twentymile Park <br />Basin. Where the Lewis Shale is present in the area, it acts as an <br />impermeable confining layer creating artesian conditions within the underlying <br />aquifers of the Mesaverde Group. <br />The highest reported well yields of ground water aquifers in the basin can be <br />obtained from the unconsolidated alluvial deposits of the Yampa River and its <br />tributaries. Well yields range from 5 gal/min to as much as 900 gal/min <br />(Brogden and Giles, 1977). The principal use of water from the alluvial <br />aquifers is for domestic and stock watering purposes, however, permeabilities <br />are sufficient in some areas to support long term, high yield wells for <br />municipal and industrial needs and irrigation, particularly in the Yampa River <br />alluvium. Water quality is variable, depending on the underlying rock and <br />source of alluvial material. TDS concentration ranges from 82 to 2,970 mg/1 <br />and the water may contain concentrations of arsenic, iron, manganese, nitrate, <br />selenium, and sulfate in excess of U. S. Public Health Service Drinking Water <br />Standards (Brogden and Giles, 1977). <br />Ground water flow in the Upper Yampa River Basin is controlled by the geologic <br />structure, stratigraphy and geomorphology of the general area. The flows of <br />ground water in the rock aquifers are controlled by the structural folding and <br />faulting and stratigraphy while the flows of ground water in the alluvial <br />aquifers are controlled by the geomorphology of the stream valleys. <br />As was earlier stated, it is believed that the ground water occurrence in the <br />general area is structurally separated from other mine areas in the region. <br />The rock strata in the Seneca II-W Mine area are recharged at the areas of <br />outcrop which parallel the Sage Creek Anticlinal axis, a minor structural <br />feature within the larger Sand Wash Basin. The rock strata dip steeply, with <br />the dip direction being slightly south of west. Ground water movement follows <br />this dip. Because of this steep dip aquifers are under atmospheric pressure <br />(water table conditions) near their recharge areas and under hydrostatic <br />pressure (artesian conditions) within the basin or at discharge points. <br />Artesian conditions are developed in aquifers which are confined by overlying <br />and underlying strata with low permeabilities. Wells drilled down dip adjacent <br />to the Seneca II-W Mine exhibit artesian flows. <br />Discharges from bedrock aquifers are primarily through springs to the surface <br />system. The flow characteristics of the springs are related to: 1) the <br />lateral extents and thicknesses of the associated aquifers; 2) the distance <br />between the springs and the aquifer recharge areas; and, 3) the permeabilities <br />and ground water storage characteristics of the associated aquifers. Springs <br />issuing from laterally discontinuous, thin sandstones (lenticular and <br />interbedded sandstones) which occur close to the aquifer outcrops will flow <br />intermittently and their flows will fluctuate seasonally; springs issuing from <br />regionally extensive, thick sandstones or coals which occur at a distance from <br />recharge areas will flow perennially and their flows will not significantly <br />fluctuate with time. <br />_ 2g_ <br />