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r~, ~I CNAPTERTHREE nttecteld Emtironmem fracture-enhanced porosity and permeability similaz to that found in mazlstones of the Parachute , Creek Member portion of the Upper Aquifer (Weeks and Welder 1974). Groundwater in the Upper Aquifer usually exists under confined conditions but may be unconfined in some locations, particularly near outcrop areas. , Marlstone beds within the upper Uinta Formation may contain perched groundwater that is not considered to be part of the Upper Aquifer. These perched zones can occur in the ridges between , surface water drainages and may be manifested as springs or seeps above the valley floor in outcrop azeas (Weeks and Welder 1974). The estimated saturated thickness of the Upper Aquifer in the Project Area is approximately , 800 feet (Robson and Saulnier 1980). Pumping rates for water wells completed in the Uinta Formation are usually in the 100 gallons per minute (gpm) range, while wells completed in the Pazachute Creek Member average 200 to 400 gpm (Weeks and Welder 1974). The Mahogany Zone sepazates the Upper Aquifer from the Lower Aquifer and is considered to be a leaky, semi-confining layer. The Mahogany Zone persists throughout the btusin and is composed of kerogen-rich oil shale that is more resistant to fracturing than the rock units ' immediately above and below it. The estimated thickness of the Mahogany Zone in the Project Area is approximately 200 feet (Robson and Saulnier 1980). , The Lower Aquifer is composed of fractured oil shale and mazlstone of the Parachute Creek Member of the Green River Formation. The Lower Aquifer extends from the ba::e of the Mahogany Zone to the top of the Saline Zone, which is noted for its high resistivity on geophysical logs. The Lower Aquifer is also referred to as the Leached Zone because secondary porosity and permeability have been enhanced by the dissolution of minerals, mainly nahcolite and sodium bicazbonate, by percolating groundwater. The boundary between the Leached Zone and the urileached Saline Zone is described as the Dissolution Surface (Figure 3.1-4). The saturated thickness of the Lower Aquifer in the Project Area is estimated to be approximately 375 feet (Robson 1980). Groundwater within the Lower Aquifer occurs under confined conditions except neaz outcrop azeas. Yields from water wells completed within the Lower Aquifer typically range from 200 to 400 gpm (Weeks and Welder 1974). The Lower Aquifer is underlain by low permeability deposits of the Pazachute Cr~:ek and Gazden Gulch Members of the Green River Formation. These low permeability units are unleashed and form a confining layer several hundred feet thick in the central part of the basin. ]t is within the , unleashed zones of the Parachute Creek Member that nahcolite occurs and commercial mining using in situ methods is proposed (Figure 3.1-4). ' 3.4.1.2 Groundwater Flow Groundwater elevations in the Upper Aquifer range from approximately 7,400 feet msl in the southern part of the basin to 5,800 feet msl in the northern portion of the basin net~r the confluences of Piceance Creek and Yellow Creek with the White River. In the Project Area, ' groundwater generally flows from the southwest to the northeast. Robson and Saztlnier (1980) estimated horizontal hydraulic gradients ranging from 20 to as much as 120 feet p~;r mile within the basin, with a gradient of approximately 35 feet per mile evident in the Project Area (Robson , and Saulnier 1980). 3-16 Groundwater ,