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<br />MWSI Project <br />Phase II Conjunctive Use Summary Report - DRAFT <br /> <br />August 23, 1995 <br /> <br />BACKGROUND INVESTIGATIONS <br /> <br />Resource Assessment of Denver Basin Aquifers <br /> <br />General Description of Aquifers and Geographic Extent <br /> <br />In ascending order the Denver Basin aquifers include the Laramie-Fox Hills, Arapahoe, <br />Denver and Dawson aquifers. The Laramie Formation is between the Arapahoe and Laramie-Fox <br />Hills aquifers and the Pierre Formation lies below the Laramie-Fox Hills aquifer. These four <br />sedimentary aquifers were developed through the formation of the structural Denver Basin from <br />late Cretaceous to mid-Tertiary time periods and were deposited through sea regressions and <br />mountain building, The Denver groundwater basin underlies approximately 6,700 square miles, <br />It extends from Greeley in the north to Colorado Springs in the south and from the Front Range <br />in the west to the high plains in the east. Figure C I shows the general location of the Denver <br />Basin aquifers and their outcrop areas. Figure C2 is a north-south cross section and Figures C3 <br />, <br />and C4 are east-west cross section of the Denver Basin aquifers, The cross-section locations are <br />shown on Figure Cl. <br /> <br />The Laramie-Fox Hills aquifer consists of a sequence of sandy shales interbedded with <br />thin sandstone layers near its base and thick sandstone layers near the top, Thickness of the <br />sequence ranges from thin outcrop areas to about 200 feet, Sediment color ranges from yellow <br />and olive brown to dark gray, The Arapahoe aquifer can be 500 to 600 feet thick within the <br />central portions of the Basin. The aquifer material consists of tan to light brown and yellow to <br />reddish-brown conglomerate and sandstone interbedded with clay beds, The upper portion of the <br />Arapahoe aquifer is finer grained, Through the Denver Basin Rules (I985) the Colorado <br />Division of Water Resources divided the Arapahoe aquifer into Upper and Lower aquifers north <br />of Township 6 South. The Denver aquifer consists of a series of poorly-bedded to cross- <br />bedded, predominantly light-gray to dark-brown silty claystones and shales, and tan through light <br />brown and greenish, andesitic and basaltic sandstones and conglomerates. The thickness of the <br />Denver aquifer ranges from 400 to 800 feet throughout most of the basin, The bulk of the <br />Dawson aquifer consists of a thick sequence of light-gray to yellowish-gray and orange to pale- <br />reddish and moderate-brown conglomerate, sandstone, and clayey shale with local beds of pale- <br />green shale and lenses of lignitic coal. The thickness of the Dawson ranges between 800 and <br />1000 feet in the deeper parts of the Denver Basin (Romero, 1976), Because of the large number <br />of domestic wells completed in the Dawson aquifer and the effect a large nearby Dawson aquifer <br />municipal well field would have on the domestic wells, this study does not investigate the <br />development of Dawson aquifer ground water. <br /> <br />Aquifer Characteristics <br /> <br />Recharge to the Denver Basin aquifers occurs through precipitation and historically, <br />groundwater discharge occurs to the rivers and creeks which traverse the Basin, In some area <br />