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<br />• Mahogany Zone is considered a leaky semi-confining layer, while the Saline Zone is <br />a confining layer s <br />Parachute Creek Basin <br />Because no mining or other intrusions to bedrock will occur at the Parachute Site, <br />only the groundwater of the local Parachute Basin Alluvial Aquifer will be described <br />here (see Section 7.4.2 and Section 7.4.4). <br />7.4.2 Aquifers and Confining Layers <br />Piceance Creek Basin <br />Groundwater reserves in the Piceance Creek Basin are large in both areal extent and <br />volume. The aquifer systems described below extend over 700 square miles (derived <br />from Robson and Saulnier 1980). The total estimated groundwater reserves are <br />estimated at 25 million acre-feet (BLM 1983), with over 2.5 million acre-feet in the <br />Lower Aquifer (Coffin et al. 1971). Other estimates of groundwater storage volume <br />are even higher. <br />Although the aquifer systems are frequently considered as separate units, it is <br />generally accepted that the Upper and Lower Aquifers have some degree of <br />• communication. The Mahogany Zone separates the Upper and Lower Aquifers and <br />is asemi-confining layer. Vertical movement is downward through the Mahogany <br />Zone in areas near the Piceance Site. <br />7.4.2.1 Alluvial Aquifers <br />Local alluvial aquifers near the Piceance Site are restricted to broad, low-lying <br />valleys such as those of Yellow Creek and Piceance Creek. These shallow aquifers <br />are primarily confined to Quaternary alluvium valley till consisting of sand, gravel, <br />silt, and clay weathered from Uinta Formation sandstones and marlstones. The <br />alluvial material is generally permeable; however, clay lenses may create areas of <br />low permeability or even impermeable intervals. Groundwater in the alluvium <br />occurs in both unconfined and confined conditions and is dependent on the <br />existence of clay beds (Weeks et al. 1974). Alluvium thickness in Piceance Creek has <br />been estimated at less than 50 feet (Duncan 1976). <br />Other models have included afive-aquifer/layer model developed by the USGS for computer <br />modeling, in which the Mahogany Zone and Saline Zone confining layers were considered <br />individual layers and the Upper Aquifer was divided into hvo aquifers, the Uinta Formation <br />• and [he upper Parachute Creek Member (Taylor 1982). Similarly, Robson and Saulnier in 1980 <br />developed afour-layer model that consisted of two layers in the Upper Aquifer and two in the <br />Lower Aquifer divided by the Mahogany Zone (Robson and Saulnier 1980). <br />American Soda, L.L.P. '7_18 <br />Commemal Mine Plan <br />Augusl 18, 1998 <br />