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<br />TAB 7 <br />HYDROLOGIC DESCRIPTION <br />Regional Hydrogeol ogy <br />Introduction. Topography, lithol ogy and structural trends each play a~ principal role in <br />controlling the quality, quantity and flow characteristics of the regional ground water <br />regime in the direct vicinity of Peabody Coal Company's Nucla Mine. As seen in the <br />geology discussion (Tab 6) of the Permit Application Package (PAP), the geology of this <br />region is complex. Consequently, the ground water system is difficult to interpret. To <br />understand the extent the Nucla Mine may affect or has affected the local and regional <br />ground water supply, one must delineate the regional and local ground water systems. <br />Topography. Topography is the principal element governing shallow aquifers ( 100 feet) in <br />the Canyon Land Section of the Colorado Plateau Physiogr aphic Province. Topographic highs <br />act as recharge areas, and lows along streams serve as discharge areas during periods of <br />high water table. In alluvial and other unconsolidated material, ground water moves for <br />• the most part from high topography to low. This also appears to be the case for the <br />unconfined portions of the Dakota sandstone and Burro Canyon formations. <br />Li thol ogy. Li thology governs the water yielding ability, the aquifer characteristics and <br />the chemical quality of the ground water. As seen in the geology section of the PAP (Tab <br />6), the lith of ogy of the Nucla area is extremely variable. Shallow aquifers of the area <br />consist of unconsolidated valley deposits of the quaternary system and the <br />str atigra phical ly variable sandstone, shales and coals of Cretaceous Age (Dakota, and <br />Burro Canyon Formations). Deeper aquifers consist of the varicolored Morrison Formation <br />and the Entrada Formation both of Jurassic Age. The variability of lithol ogy within each <br />of the aquifers creates a complex hydrologic system. <br />• <br />Table 7-1 is a summary ehart adapted from Boettcher (1972) describing the physical and <br />hydrologic character of the water bearing formations mentioned above. Included in Table <br />7-1 is water quality which is controlled by the reaction of the ground water and the <br />aquifer and the ground water retention time (Hem, 1970). In addition, ground water yields <br />and uses are given. It must be noted that these are generalizations, and these hydrologic <br />properties can vary greatly within the formations. <br />7-1 Revised 04/11/88 <br />