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<br />UlilHfI <br /> <br />1 <br /> <br />To date, the withdrawals of ground. water have not. been <br />reat enough to cause a widespread declUleJ altl,1ough. w a <br />~e.., small intensively developed areas, the declwe has been <br />measurable. Wat.er-level measurements in parts of the area <br />are being made periodically, and additional measurements are <br />lanned to give roore widespreau coverage. 'l'his pr09ram ~ill <br />k me Il'Ore and more valuable as development increaaes7 lot <br />wi~~ show toe status and trends of depletion. <br /> <br />cooperative ground-water investigations by the u.s. <br />Geol gica1 survey, and the Colorado ~later Conservation Board, <br />oisin9 the ground-water situation in the High Plains of <br />~~l~~ado, are near1n9 cOlllPletion. Field studies are complete <br />exee t for one area--eastern Cheyenne and Riowa CountiesJ <br />t;;ni: study was started in 1959 and is scheduled for cOll1Pletion <br /> <br />in 1962. <br /> <br />consolidated formations (Artesian aauifers).--The <br />consolidated formations in Colorado generally are capable of <br />~.l.~ldin9 IlIJIch leas water per well than the unconsolidated <br />.forD:lationS. li'a'" areas are underlain by rocks capable of <br />yieldiD4 IllOre than 300 gpm (gallons per minute) per well. <br />A E_ local areas have wells that yield more than 1,000 gpm, <br />but throughout most of the State well yields generally range <br />from 5 to 50 gpm. At least three-fourths of the State is <br />underlain by materials c~le of supplying sufficient water <br />to wells for domestic and stock use--although in some places <br />the water is of poor quality. llhere thick. alluvial deposits <br />overlie the consolidated formations, the alluvium generally <br />is the most productive aquifer. <br /> <br />The principal consolidated water-bearing material is <br />sandstone7 however, some conglomerate. limestone, and volcanic <br />deposits yield water locally to wells. 'l'he most widespread <br />sandstone deposits are of Cretaceous agel although older <br />water-bearing deposits of Jurassic and Triassic age underlie <br />large areas in the western part of the State, and major water- <br />bearing deposits of Tertiary age underlie fairly large areas <br />in the eastern part. Rocks older than 'l'riaosic generally do <br />not contain fresh water except in a few small areas where they <br />are near the land surface. <br /> <br />In eastern Colorado. the oldeat major water-bearing unit <br />is the Cheyenne sandstone member of the Purgatoire formation <br />of Early Cretaceous age. The next younger water-bearing unit <br />is the Dakota sandstone, which overlies the Purgatoire <br />formation. TWo other still younger formations of Cretaceous <br />age, the Fox Bills sandstone and the Laramie formation, are <br />separated from the older aquifersby thick deposits of relatively <br />impermeable material, which include the pierre shale. Younger <br />consolidated fox'mations that are good aquifers in places are <br />the Dawson arkOSe and Denver formation, both of Late Cretaceous <br />and Paleocene ages. <br />