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DAWSON AQUIFER The Dawson aquifer is the uppermost aquifer in the Denver Basin and covers an area of approximately 1,400 square miles. The aquifer varies in thickness, depending on location, and attains a maximum thickness of almost 1,200 feet in the vicinity of Monument. The Dawson aquifer is at or near the land surface throughout its entire areal extent. The aquifer is composed of predominately conglomeratic, coarse-grained arkosic sandstones with minor amounts of interbedded clay and clay shale. Typical well yields in the Dawson are up to 300 gallons per minute. The northern portion of the aquifer has been subdivided into two hydrogeologic units for administrative purposes. These units are known as the Upper and Lower Dawson aquifers. This separation is based on the presence of an approximately 50 -foot thick shale layer that hydraulically isolates the Upper and Lower Dawson aquifer units. The Dawson aquifer is separated from the underlying Denver aquifer by a clay shale sequence ranging from 25 to 50 feet thick. DENVER AQUIFER Underlying the Denver aquifer covers an area of approximately 3,500 miles and varies in thickness from a thin eroded edge at its outcrop/subcrop up to 1,000 feet. The Denver aquifer consists of a series of interbedded clay shale, clay, claystone, siltstone and sandstone lenses. The sandstone and pebble compositions are typically volcanic. Carbonaceous material and coal beds are also common. Due to its finer grained composition, wells commonly have yields of 50-150 gallons per minute. The aquifer is typically developed for domestic uses, though a number of municipal wells have been drilled on the western side of the basin where yields are higher. 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. ►e1 Z�. ,ff 7 IcZilllU19.0 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. For administrative purposes, the northern portion of the hydrogeologic unit is subdivided into an upper and tower aquifer that is separated by a shale sequence ranging from 50 to 100 feet thick. The Arapahoe is the most prolific of the four Denver Basin aquifers with wells yielding up to 800 gallons per minute. Consequently, 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.