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s • <br />Aquifer at the U.S. Bureau of Mines (USBM) Horse Draw Mine Research Facility <br />yielded an average discharge of 280 gpm for 900 minutes, with an approximate 10- <br />foot drawdown (Dale and Weeks 1978). <br />7.4.2.3 Mahogany Zone <br />The Mahogany Zone is a kerogen-rich oil shale zone that separates the Upper <br />Aquifer from the Lower Aquifer spatially, but not entirely hydrologically. The <br />Mahogany Zone is more resistant to fracturing than leaner shales and is, therefore, <br />less permeable than the rock units immediately above and below it (Weeks et al. <br />1974). The Mahogany Zone is considered asemi-confining layer. <br />The vertical hydraulic conductivity, which is the vertical exchange of water between <br />the Upper Aquifer and the Lower Aquifer, varies throughout the rock formation. <br />Calculated values of vertical hydraulic conductivity of the Mahogany Zone ranged <br />from 2.2 x 10 a to 2.2 x 10' feet per day directly west of the Piceance Site. At that site, <br />the hydraulic head of the Lower Aquifer is approximately 10 feet lower than the <br />head of the Upper Aquifer and the vertical conductivity has a downward hydraulic <br />gradient. Potentiometric surface data developed by the USGS for the Upper and <br />Lower Aquifers are inconclusive but indicate that there may be no vertical <br />movement of groundwater in the vicinity of the Piceance Site (Robson and Saulnier <br />1980). Pressure records and water quality records for solution mining well 20-3 <br />• indicate that vertical movement of groundwater at the Piceance Site is either nil or <br />upward (Steigers 1997c). <br />The Mahogany Zone ranges in thickness from 100 to 225 feet in the Piceance Creek <br />Basin (Figure 7-14) (Donnell and Blair 1970); it is 180 feet thick at monitoring well <br />20-1 (Dyni 1974). <br />7.4.2.4 Lower Aquifer <br />The Lower Aquifer consists of the Leached Zone of the Parachute Creek Member of <br />the Green River Formation. The Leached Zone is generally located below the <br />Mahogany Zone and above the Dissolution Surface. The saturated thickness of the <br />Leached Zone in most of the Piceance Creek Basin varies from 125 and 250 feet <br />(Figure 7-15). Daub has demonstrated that, in the depositional center of the Piceance <br />Creek Basin, the Leached Zone extends up through the Mahogany Zone into the <br />upper Parachute Creek Member (Daub et al. 1985). The Leached Zone is <br />characterized by interstratified solution openings (vugs, brecciated zones, rubble <br />zones, and cavities) caused by the removal of saline minerals from the oil shale by <br />percolating groundwater. The water in the Lower Aquifer is contained within these <br />solution features. The Lower Aquifer is considered the principal bedrock aquifer in <br />the northern part of Piceance Creek Basin. <br />• The porosity and permeability of the Lower Aquifer are generally low, but the Lower <br />Aquifer is more porous and permeable than either the underlying zone or the <br />American Soda, L.L.P. '7_20 <br />Commercial Mine Plan <br />August t8, 1998 <br />