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and clay shale. In the northern portion of the basin, the aquifer can be subdivided into two aquifers <br />known as the Upper and Lower Dawson Aquifers. This separation is based on the presence of an <br />approximately 50 foot thick shale bed which hydraulically isolates the Upper and Lower Aquifer units. <br />At the base of the aquifer, or rather at the top of the Denver Aquifer, another shale unit averaging about <br />25 to 50 feet in thickness separates the Dawson from the underlying Denver Aquifer. <br />DENVER AQUIFER <br />The Denver aquifer consists of a series of interbedded clay shale, claystone, siltstone and <br />sandstone lenses. Organic remains and coal are common within the unit. The aquifer underlies an area <br />of approximately 3500 square miles and ranges in thickness from 800 to 1000 feet. Distinctive <br />characteristics of the aquifer are the fine-grained nature and the fact that sandstone lenses with the unit <br />do not extend over long distances. <br />Although the aquifer is utilized extensively for domestic water supplies, the fact that well yield <br />are relatively low (generally 20 to 50 gallons per minute (gpm)) limit its use to primarily individual wells <br />although a number of municipal wells yielding 100 gpm have been developed. These higher yield wells <br />are the exception rather than the norm. <br />At the base of the aquifer is an extensive clay shale unit approximately 50 feet in thickness which <br />separates the Denver from the underlying Arapahoe Aquifer. <br />ARAPAHOE AQUIFER <br />The Arapahoe Aquifer consists of a series of interbedded conglomerates, sandstones, siltstones <br />and clay shales. It underlies an area of almost 4700 square miles. The aquifer is generally about 400 <br />feet in thickness. In the northern portion of the basin the aquifer can be subdivided into an upper and <br />lower aquifer owing to the presence of between 50 and 100 feet of shale. <br />F• <br />High capacity wells (300+ gpm) are common in the Arapahoe aquifer and it is extensively used <br />to supply municipal water systems. Water quality is good and in fact "Deep Rock Water" obtains bottled <br />water from this source. <br />LARAMIE FORMATION <br />Below the Arapahoe is the Laramie Formation which consists of approximately 400 feet of clay <br />