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HYDROLOGIC DESCRIPTION <br />SECTION 7.1 - SENECA II-W BASELINE HYDROLOGIC DESCRIPTION <br />Regional Hydrogeology <br />Introduction. Topography, lithologies, and geologic structures are important factors affecting <br />the occurrence, movement, and quality of ground waters near Peabody's Seneca II-W Mine. <br />Each of these factors have been examined in order to characterize the local and regional ground <br />water systems so that the effects of mining lif anyl at the Seneca II-W Mine can be <br />determined. <br />Topography. The Seneca 11-W Mine permit area is located in the rugged Williams Fork <br />Mountains of northwestern Colorado. Topography is the primary factor controlling the <br />movement of ground water within shallow, unconfined aquifers; recharge occurs in areas of <br />higher elevation, and discharge occurs in lower areas along drainages. Unconfined ground <br />water flow directions generally coincide with topographic slopes. The movement of confined <br />ground waters is controlled by geologic structures, although the attitudes of topographic slopes <br />and geologic structures may be similar li.e. dip slopes are often presentl. <br />• Lithology. Aquifer characteristics and water quality ere primarily determined by lithology. The <br />permit area is underlain by structurally deformed sedimentary rock strata that range in age from <br />late Cretaceous to Quaternary. <br />A summary of the physical and hydrological character of the lithologies affected or potentially <br />affected by mining activities at Seneca II-W is presented in Table 7-1. Sources of information <br />for this table are Bass et al. 119551, Steele et al. (19791, Brogden and Giles (19771, Stewart <br />119831, and the results of Peabody's hydrological monitoring and testing at Seneca II-W. <br />Although the Trout Creek sandstone will not be directly disturbed by mining, a well completed <br />in this strata will provide water far the mine facility. Detailed geologic descriptions of these <br />lithologies can be found in Tab 6 ("Geology") of this permit application package (PAPI. <br />It should be noted that the characteristics presented in Table 7-1 are generalizations. The <br />physical and hydrological properties of these lithologies can vary significantly both laterally and <br />vertically. <br />r1 <br />f~ <br />1 Revised 9/96 <br />