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which range up to 140 feet thick, have a highly variable composition both <br />within a given valley and from drainage to drainage. <br />The degree of groundwater saturation within the alluvium is seasonally <br />variable, having increased quantities in storage during the spring and early <br />summer months. Ground water generally occurs under unconfined conditions, <br />although locally semi-confined conditions may exist where deposits are <br />comprised of stratified silts and clays. Generally, the transmissivities <br />and potential volumes of groundwater are highest in the larger valleys due <br />to the greater percentage of coarse-grained deposits. <br />Aquifers within the alluvial deposits are predominantly recharged by direct <br />infiltration of precipitation, contribution from stream flows, and discharge <br />from adjacent bedrock strata. Discharge from the alluvial aquifer system <br />occurs via springs along valley bottoms and by withdrawal for domestic and <br />irrigation usage. Ground water movement generally parallels the direction <br />of stream flows. There are no important alluvial aquifers found in the <br />project area. <br />G.2.3.2.2 Bedrock Aquifers <br />As previously stated, the aquifer system can be divided into Upper and Lower <br />Aquifers, separated by the Mahogany Zone. A schematic diagram of these <br />three units is shown in Figure G-19, and are described below. <br />G.2.3.2.2.1 Upper Aquifer <br />The Upper Aquifer System extends from the top of the Uinta Formation to the <br />top of the Mahogany Zone in the Parachute Creek Member of the Green River <br />Formation. It consists of sandstones, siltstones, and marlstones of the <br />Uinta Formation and lean oil shales (marlstone) of the Parachute Creek <br />Member. The permeability of this zone is mostly secondary and is created <br />mainly by fracturing and solution cavities. The stratigraphy of the Upper <br />Aquifer is complex Niue to interfingering of the relatively impermeable <br />• sandstones and silt stones of the Uinta Formation and the fractured <br />marlstones of the Parachute Creek Member of the Green River Formation. <br />G-27 <br />