2012-09-06_REVISION - M2008070 (29)

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3.2 Basin Hydrology and USDW Identification The Piceance Basin contains both alluvial and bedrock aquifers. Unconsolidated alluvial aquifers are the most productive aquifers in the Piceance Basin. These alluvial deposits are narrow, and thin deposits of sand and gravel formed primarily along stream courses. The City of Meeker, Colorado is supplied by wells tapping these deposits where they are over 100 feet thick in the White River Valley (Taylor, 1987). The most important bedrock aquifers are known as the upper and lower Piceance Basin aquifer systems. These consolidated rock aquifers are lower Tertiary Eocene in age and occur within and above the large oil shale reserves. The upper and lower aquifers are separated by the Mahogany Zone of the Parachute Creek Member (Figure A3 -6). The Mahogany Zone is a poorly permeable oil shale, which retards water movement but does not stop it. Both bedrock aquifers overlie the older Cretaceous Mesaverde Group where the coal and coalbed methane are located. The upper aquifer system is about 700 feet thick and consists of several permeable zones in the Eocene Uinta Formation and the upper part of the Parachute Creek Member of the Eocene Green River Formation. Sub - aquifers of the Uinta Formations are silty sandstone and siltstone, while those of the Parachute Creek Member of the Green River Formation are fractured dolomite marlstone. There is some primary porosity (i.e., the porosity preserved from during or shortly after sediment deposition, such as the spaces between grains) in the sandstone and the permeability of the sub - aquifers has been enhanced by natural fracturing that occurred during post- deposition deformation. Layers between the individual sub - aquifers are less permeable than the sub - aquifers themselves, but they do not prevent water movement between the sub - aquifers. The lower aquifer system is about 900 feet thick and consists of a fractured dolomitic marlstone of part of the lower Parachute Creek Member of the Green River Formation. It is semi - confined below the Mahogany Zone and above the Garden Gulch Member of the Green River Formation and a high resistivity zone just above it (USGS, 1984 and Taylor, 1987) (Figure A3 -6). Fracturing during deformation of the rocks and subsequent solution enlargement owing to dissolution of soluble evaporite minerals has increased permeability of this lower aquifer system. Groundwater is recharged from snowmelt on high ground from where it travels down through the upper aquifer system, the Mahogany Zone, and into the lower aquifer system. The groundwater then moves laterally and/or upward discharging from both the upper and lower aquifer systems into streams (Figure A3 -7). The minerals nahcolite (NaHCO3), dawsonite (NaAI(OH)2CO and halite (NaC1) are present in the groundwater, and the circulation of the groundwater (with these minerals in solution) has caused enlargement of the natural fractures (Taylor, 1987). Water in the lower aquifer is reported to contain several hundred milligrams per liter (mg/L) of chloride (Taylor, 1987). Evaluation of Impacts to Underground Sources of Drinking Water by Hydraulic Fracturing of Coalbed Methane Reservoirs June 2004 A3 -3