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Upper and Lower Dawson aquifer units. The Dawson aquifer is separated from the underlying <br />Denver aquifer by a clay shale sequence ranging from 25 to 50 feet thick. <br />DENVER AQUIFER <br />The Denver aquifer consists of a series of interbedded clay shale, clay, claystone, <br />siltstone and sandstone lenses. Carbonaceous material and coal beds axe common within the <br />hydrgeologic unit. The aquifer underlies an area of approximately 3,500 miles and varies in <br />thickness from a feather edge up to 1��Q feet. Distinctive characteristics of the aquifer are the <br />fine-grained nature of the material composing the aquifer and sandstone lenses of limited areal <br />extent. <br />Although the aquifer is utilized extensi�ely for domesti� water supplies, relatively low <br />well yields �generally 20 to 50 gallons per minute (gpm)) limit i�s use primarily to individual <br />residential wells; however, a number of municipal wells have been developed with yields up to <br />144 gpm. The higher yield we11s are not indicative of the aquifer's potential in most areas. <br />The Den�er aquifer is separated from the underlying Arapahoe aquifer by an extensive <br />clay shale unit approximately 54 feet thick. The clay shale interval serves as a hydraulic barrier <br />between the aquifers. <br />ARAPAHUE AQUIFER <br />The Arapahoe aquifer is composed of interbedded cong�omerates, sandstones, siltstones, <br />and clay shales. The aquifer underlies an area of almost 4,70� square miles and is generally <br />about 404 feet thick. The northern portion of the hydrogeologic unit can be subdi�ided into an <br />upper and lower aquifer that is separated by a shale sequence ranging from 50 to 100 feet thick. <br />High capacity wells (300+ gpm} are common in the Arapahoe and the aquifer is used <br />extensively to supply municipal water systems. Water quality is generally good in the aquifer as <br />evidenced by its use by Deep Rock Water to obtain water for bottling. <br />LARAMIE FURMATION <br />The major part of the Laramie Formation forms the aquatard or hydraulic barrier that <br />separates the Arapahoe aquifer from the underlying Laramie-Fox Hills aquifer. The formation <br />consists of approximately 440 feet of clay shales with minor interbeds of sandstone and siltstone. <br />Coa1 beds are cvmmon within the formation and have been mined commercially along the <br />eastern margin of the basin. Minor amounts of water are present within the sandstones, but are <br />rarely developed for water supplies. Water in the sandstone units is of generally poor quality due <br />to the presence of sulfides and biogenic gas. <br />4 <br />