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Thus, there exists a series of alternating permeable and impermeable layers (CEI 1983). Basin wide, the transmissivity of the Upper Aquifer ranges from 8 to 1,000 ft2/day, based on data from 26 wells (Weeks et al. 1974). Recharge to the Upper Aquifer occurs principally from snowmelt during the spring, especially at altitudes above 7,000 feet (Weeks et al. 1974). The Upper Aquifer is in excess of 1,000 feet thick in the Lease area, with a saturated thickness in the range of 400 to ti00 feet (CEI 1983). G.2.3.2.2.2 Mahogany Zone The Mahogany Zone is a high-grade oil shale zone ranging from 100 to 200 feet thick that lies between the Upper and Lower Aquifers. Within the Mahogany Zone there ~is a 3- to 10-feet thick interval of very high grade oil shale known as the Mahogany Bed. This is the principal confining layer separating the Upper and Lower Aquifers (Weeks et al. 1974). Oil shale that is kerogen rich appears to be much more resistant to fracturing than lean shale, thus the Mahogany Zone is less permeable than the rocks above or below it. The Mahogany Zone persists throughout the Piceance Creek Basin and, except in areas of recharge or discharge, generally separates the Upper and Lower Aquifers. G.2.3.2.2.3 Lower Aquifer The Lower Aquifer System consists of fractured oil shales that extend from the base of the Mahogany Zone to the dissolution surface located at the top of the Saline Zone. Fracturing, faulting, and solution breccia have created the majority of then permeable zones. Permeabilities have been greatly enhanced by the dissolution of saline minerals (such as nahcolite) resulting in solution vugs and cavities. Throughout the Piceance Creek Basin, transmissivities as high as 20,000 gallons per day per foot (gpd/ft) have been reported (Coffin et al. 1971) although values in the range of 5,000 to 14,000 gpd/ft are more common (Weeks et al. 1914). ~• G-29