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<br />CHAPTERTHREE <br />Affected Environment <br />3.4 GROUNDWATER <br />Groundwater resources aze summarized below on a regional basis followed by asite-specific <br />presentation. <br />3.4.1 Regional Hydrogeology <br />The discussion of regional bedrock hydrogeology focuses primazily on the Piceance Site since no <br />mining or other intrusions to bedrock would occur along the pipeline corridor or at the Parachute <br />Site. <br />3.4.1.! Aquifer Description <br />The principal water-bearing bedrock units within the Piceance Creek Basin include the Uinta and <br />Green River Formations which aze both of Tertiary age. The underlying Wasatch Formation <br />consists oflow-permeability clays, shales, and lenticular sandstone and forms an aquitazd in the <br />study azea. Alluvial sediments found within major drainages can provide a locally important <br />source of groundwater. Groundwater reserves in the Piceance Creek Basin aze large in both areal <br />extent and volume. The aquifer systems described below extend over 700 square miles (derived <br />from Robson and Saulnier 1980). The total estimated groundwater reserves aze estimated at 25 <br />million acre-feet (BLM 1983), with over 2.5 million acre-feet in the Lower Aquifer (Coffm et al. <br />1971). Other estimates of groundwater storage volume are even higher. <br />The groundwater system within the basin is typically divided into three aquifers: (1) Alluvial <br />Aquifer, (2) Upper Aquifer, and (3) Lower Aquifer (Weeks and Welder 1974). This subdivision <br />is based on hydraulic and water chemistry characteristics. Each of the aquifers, along with the <br />semi-confining Mahogany Zone, which separates the Upper and Lower Aquifers, is discussed <br />below. <br />The Alluvial Aquifer, comprised of unconsolidated sand, gravel, silt, and clay derived from <br />' weathering of the Uinta Formation, occurs in valley bottoms along the creeks. The saturated <br />thickness of this aquifer is variable, ranging from a few feet up to 100 feet as reported by Weeks <br />and Welder (1974) for a well location along the lower reaches of Piceance Creek. Weeks and <br />Welder (1974) reported that high pumping rates (e.g., 1,500 gpm) can initially be obtained from <br />alluvial aquifer wells at some locations within the basin; however, the high rates can be sustained <br />for only a short duration because of the limited extent of the aquifer. Groundwater can occur <br />under unconfined or semi-confined conditions within the Alluvial Aquifer. <br />The Upper Aquifer consists of fractured, lean oil shale of the Parachute Creek Member of the <br />Green River Formation above the Mahogany Zone and sandstone, siltstone and fractured <br />mazlstone of the saturated portion of the overlying Uinta Formation (Figure 3.1-4). <br />Primary porosity of materials within the Upper Aquifer is generally low, but permeability is <br />enhanced by secondary features such as fractures, faults, joints, and solution cavities. Weeks and <br />Welder (1974) reported that the primary porosity of sandstone and siltstone beds within the Uinta <br />Formation is typically very low due to precipitation of cements from percolating water. <br />However, the mazlstones within the formation typically conduct water much more readily due to <br />Groundwater 3-15 <br />i <br />