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<br />. <br /> <br />. <br /> <br />~ <br />~ <br /> <br />. <br /> <br />-.n~-' <br />U~JU <br /> <br />GROUND WATER <br /> <br />Ground Water Occurrence <br /> <br /> <br />In the South Platte River Valley and tributary stream valleys, <br /> <br /> <br />the principal aquifer is the alluvium. The alluvial aquifer contains <br /> <br /> <br />most of the recoverable ground water in the study area and is the <br /> <br /> <br />principal ground-water source for irrigation wells. The Cretaceous and <br /> <br /> <br />Tertiary deposits contain relatively small amounts of recoverable ground <br /> <br /> <br />water and are, therefore, not considered as aquifers. The lithologic <br /> <br /> <br />characteristics of these formations not only make, them ansuitable for <br /> <br /> <br />sources of water but also unsuitable for use as possible recharge sites. <br /> <br /> <br />The Ogallala formation which is adjacent to the lower reaches of the study <br /> <br /> <br />area, however, may have limited possibilities for use as an underground <br /> <br /> <br />storage reservoir but has not been evaluated in this study, <br /> <br /> <br />The mantling deposits of dune sand and loess are not considered as <br /> <br /> <br />aquifers since these deposits generally lie above the water table and do <br /> <br /> <br />not yield significant quantities of water to wells. It is on these <br /> <br /> <br />deposits, however, that most of the recharge sites would be located. Such <br /> <br /> <br />deposits will have considerable effect on infiltration rates. <br /> <br />The alluvial aquifer consists of rounded to subrounded, well to <br /> <br /> <br />poorly sorted silts, sands and gravels. These materials occur in varying <br /> <br /> <br />percentages as interbedded lens-shaped bodies typical of aggrading river <br /> <br /> <br />systems. Boundaries of the alluvial aquifer, as delineated by the USGS <br /> <br /> <br />(1977, unpublished report), have been defined as the physical limit of the <br /> <br /> <br />alluvial deposits or a line of zero saturation (see.Plate 1). Since the <br /> <br /> <br />limits of the aquifer as mapped may not be the exact geologic limits of the <br /> <br /> <br />aquifer, estimates of ground-water storage and potential storage capacity <br /> <br /> <br />may be underestimated. <br /> <br />Ground water in the alluvial aquifer occurs in an unconfined state. <br /> <br /> <br />Localized confining and perching conditions may be observed due to the <br /> <br /> <br />presence of clay lenses. The water table slopes to the South Platte River <br /> <br /> <br />from both the north and south sides of the river. The shape of the water <br />