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State Reg. 2.04.7(1) A. Regional • Regional ground water occurrence in the general area of the Seneca II permit is controlled by a complex interrelationship between lithology, structural trends, and topography. In general, water table conditions exist in alluvial aquifers and in consolidated aquifers at outcrops. Arte- sian (confined) conditions exist in aquifers which are overlain by im- permeable shales and are located downdip of recharge areas. Aquifers under artesian conditions may flow at the surface in valley bottoms where the potentiometric surface is above ground level (Brogden and Giles, 1977). 2.04.7(Ixa) Table?-I summarizes the hydrologic properties of aquifers in the (iii) vicinity of the Seneca II Mine permit. As previously mentioned in fhe section on geology (see Tab 6), mining operations will disturb the lower portion of the Williams Fork formation which lies between the Twenty- mile and Trout Creek Sandstone members. These lithologic units are . "thick ( 100 feet), massive, fine to medium-grained sandstones which form prominent cliffs where they outcrop. Yields as high as 100 gallons per minute have been reported from these formations (Brogden and Giles, 1977). Deeper major aquifers including the Dakota sandstone and older rocks are hydrologically separated from the Mesaverde Group rocks by the 5,000 foot thick Mancos shale. Numerous minor aquifers occur in the Williams Fork and Iles Forma- tions. These aquifers consist primarily of thinly interbedded, very fine to fine rained sandstones, siltstones, and coals. The low yield ( 10 gallons per minute) of these units reported by Brogden and Giles (1977) may vary greatly throughout the formation as a function of saturated thickness, porosity, and permeability (including fractures) and artesian pressure. Unconsolidated stream laid deposits in the Seneca II permit area and general area range from narrow bodies of sandy silt and clay deposits along the upper reaches of intermittent and perennial streams such as ~y` Grassy and Fish Creeks, to coarser sand and gravel size deposits of the 7-2 Revised 5/27/81