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GEULUGY 4F THE DENVER BASYN <br />The Den�er Basin is a structural basin bounded by the Front Range on the west and extends <br />eastward to near Limon, Colorado. The basin's margin extends southward from near Greeley on the north <br />to a location several miles to the south and east of Colorado Springs. In total, the structura.l Denver Basin <br />includes an area of more than 6,704 square miles. The basin is generally defined by the lower boundary of <br />the Laramie-Fox Hills aquifer which is the lowermosi of the fowr principal aquifers found in the basin. <br />Figure 1 is a map of the Denver Basin that identifies the lower boundary of each of the Denver Basin <br />aquifers. <br />The formations present in the basin that comprise the principal aquifers are composed of <br />sedimentary rocks that are up to 3,4�0 feet thick in the d�epest part of the basin. A generalized cross-section <br />is attached as Figure 3. In descending order, the hydrogeologic units are identified as the Dawson, Denver, <br />Arapahoe, and Laramie-Fox Hills aquifers, <br />Extensive studies of these aquifers ha�e been conducted over a period of about 100 years. Water <br />from the aquifers has been utilized in the Denver area for domestic supplies since tbe 1880's. The most <br />recent and most detailed study of the aquifer system was completed by the Colorado Division of Water <br />Resources in 1985 as part of a legislatively mandated study commonly known as Senate Bi115. During the <br />study, the staff of the Division collected extensive information on the nature of the aquifers, the occurrence <br />of ground water in the aquifers, and the effects of pumping ground water from the aquifers on surface <br />streams throughout the basin. The estimated amount of water contained in the aquifers is presented in <br />Figure 2. <br />The Coloradv Division of Wa�er Resources' study resulted in re-defining the aquifer system into the <br />four major aquifers currently recognized. For the reader tv better understand the aquifer system, a brief <br />discussion of each hydrogeologic unit is presented below. It should be noted that the Division's work in this <br />area is recognized as the authoritative study of the Denver Basin aquifers, and is utilized by the consulting <br />community and govemmentallregulatory agencies. <br />DAWSUN AQUIFER <br />The Dawson aquifer is the uppermost aquifer in the basin, covering an area of approximately 1,4�0 <br />square miles. The aquifer varies in thickness, dependi�ng upon location, and attains a maximum thickness of <br />almost 1,200 feet in the �icinity of Monument. The Dawson is at or near the land surface throughout its <br />entire areal extent. The aquifer is compvsed of predominately conglomeratic, coarse-grained arkos�c <br />sandstones with minor amounts of interbedded clay and clay shale. The northern portion of the aquifer can <br />be subdivided into two hydrogeologic uruts known as the Upper and Lower Dawson aquifers. This <br />separation is based vn the presence of an approximately 50-foot thick shale layer that hydraulically isola�es <br />the Upper atld Lowex Dawson aquifer units. The Dawson aquifer is separated from the underlying Denver <br />aquifer by a clay shale sequence ranging from 25 to 50 feet #hick. <br />1 <br />