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. 2.04.7 Hvdroloev Information <br />Groundwater <br />Occurrence of Groundwater <br />General Area <br />Colowyo Coal Company's general area of operation is defined by the area bounded by <br />Goodspring and Wilson Creeks, both of which are tributaries to the Yampa River. The mine <br />permit area and its relationship to Goodspring and Wilson Creeks are shown on Map 32. <br />Colowyo Coal Company's, mining operations are in an area of limited hydrology. <br />Precipitation averages 21.5 inches for the drainages in the "general area" and runoff is <br />limited. Flows in Wilson, Taylor and Goodspring Creeks are mostly from snowmelt or <br />thunderstorm events. Groundwater resources on and adjacent to the mine property are also <br />limited. Base flows observed in Goodspring and Wilson Creeks are mostly groundwater <br />discharged from the thin and limited alluvial/colluvial aquifers that can be found in the <br />valleys of both creeks. There are no known places of groundwater discharge from bedrock <br />units to Goodspring, Taylor, or Wilson Creeks from the sandstones between the coals to be <br />mined. <br />Rocks on the property are part of the Mesa Verde Group. These rock units are almost entirely <br />interbeds of sandstones and shales. The uppermost geologic unit in the permit area is the <br />• Williams Fork Formation. The coals to be mined on the property are part of the Fairfield <br />Group, the principle coal-bearing zone in the area. The sandstones are predominately fine to <br />medium-grained, poorly sorted and calcareous. [n the upper part of the formation they are <br />generally channel sandstones and exhibit unilateral and vertical continuity. In the lower part <br />of the formation the sandstones are more continuous than in the upper part. The thickness of <br />the Williams Fork Formation, in the general area, is estimated to he about 1600 feet. <br />Underlying the Williams Fork Formation is the Iles Formation. Little data are available to <br />describe the characteristics of the Iles Formation, principally because of its great depth. <br />However, based on information from adjacent areas, the general <br /> <br />z.o4.7-I <br />