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<br />. <br /> <br />N <br />J- <br />CD <br />o <br /> <br />Fresh water usually occurs in shallow aquifers where the land <br />surface is above 7.000 feet in altitude, or in permeable sandstones <br />such as the Navajo and Dakota formations (Price and Arnow, 1974). <br />Alluvial aquifers generally yield the best quality water, but some <br />alluvial ground water may be highly saline as a result of hydrologic <br />interconnections with saline rocks. In many cases, fresh water <br />found in the Region is separated from saline formations by relatively <br />impermeable rock structures such as the Mahogany Zone in the Piceance <br />Creek Basin.2 <br /> <br />Relatively little is y~~O~~ about the areal extent of ground <br />water in the Region compared to what is known about resources in the <br />High Plains of Colorado, New Mexico, and Wyoming and in the Basin <br />and Ra~ge province of Utah and Arizona. Generally, ground water <br />yields are a function of rock permeability and aquifer storage. The <br />most permeable rocks are unconsolidated fluvial or glaciofluvial <br />deposits of sand and gravel which yield flows up to 500 gallons <br />per minute (gpm). These are followed by, in crder of inc~easing <br />consolidation (and decreasing yields): <br /> <br />. volcanic rocks (yielding up to 500 gpm) , <br /> <br />. <br /> <br />. sedimentar; rocks (usually yielding less than 10 gpm but <br />sometimes several thousand gpm), and <br /> <br />. igneous or meta~orphic rocks such as granite, schist, gneiss, <br />and quartzite (also yielding less than 10 gpm). <br /> <br />Major recharge areas in the Region lie primarily along and <br />below high mountainous zones where the greatest. precipitation <br />occurs (Fig. 0.2). Therefore, ~t f~ll~ws that ground water rich <br />areas (yielding 1.3 to 9.6 x 10 AFjQi of recoverable water in the <br />upper 100 feet) are found where relatively unconsolidated rocks are <br />located in either areas of high precipitation (more than 20 inches <br />annu31ly) O2 along streams. Areas of moderate yield (0.6 to 1.3 <br />x 10 AF/mi of recoverable water in the upper 100 feet) are found <br />where sedimentary rocks of continental origin, such as sandstones, <br />lie farther from recharge zones. Because of the extensive area <br />sedimenrary formations cover, these rocks contain about 85 percent <br /> <br />1... Hereinafter, IIPiceance Creek Basin" shall m.ean the geologic basin <br />and "Piceance Basin" shall mean the river drainage basin, unless noted <br />othervise. <br /> <br />. <br /> <br />D-3 <br />