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DENVER AQUIFER The Denver aquifer consists of a series of interbedded clay shale, clay, claystone, siltstone and sandstone lenses. Carbonaceous material and coal beds are common within the hydrogeologic unit. The aquifer underlies an area of approximately 3,500 miles and varies in thickness from a feather edge up to 1,000 feet. Distinctive characteristics of the aquifer are the fine-grained nature of the material composing the aquifer and sandstone lenses of limited areal extent. Although the aquifer is utilized extensively for domestic water supplies, relatively low well yields (generally 20 to 50 gallons per minute (gpm)) limit its use primarily to individual residential wells; however, a number of municipal wells have been developed with yields up to 100 gpm. The higher yield wells are not indicative of the aquifer’s potential in most areas. The Denver aquifer is separated from the underlying Arapahoe aquifer by an extensive clay shale unit approximately 50 feet thick. The clay shale interval serves as a hydraulic barrier between the aquifers. ARAPAHOE AQUIFER The Arapahoe aquifer is composed of interbedded conglomerates, sandstones, siltstones, and clay shales. The aquifer underlies an area of almost 4,700 square miles and is generally about 400 feet thick. The northern portion of the hydrogeologic unit can be subdivided into an upper and lower aquifer that is separated by a shale sequence ranging from 50 to 100 feet thick. High capacity wells (300+ gpm) are common in the Arapahoe and the aquifer is used extensively to supply municipal water systems. Water quality is generally good in the aquifer as evidenced by its use by Deep Rock Water to obtain water for bottling. LARAMIE FORMATION The major part of the Laramie Formation forms the aquitard or hydraulic barrier that separates the Arapahoe aquifer from the underlying Laramie-Fox Hills aquifer. The formation consists of approximately 400 feet of clay shales with minor interbeds of sandstone and siltstone. Coal beds are common within the formation and have been mined commercially along the eastern margin of the basin. Minor amounts of water are present within the sandstones, but are rarely developed for water supplies. Water in the sandstone units is of generally poor quality due to the presence of sulfides and biogenic gas. The basal portion of the Laramie Formation contains two relatively thick sandstone units (the A and B sandstones) which are hydraulically isolated from the upper part of the formation. These sandstone units, combined with the underlying Fox Hills Sandstone, compose the hydrogeologic unit of the Laramie-Fox Hills aquifer.