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GEOLOGY OF THE DENVER BASIN AQUIFERS The Denver-Julesburg Basin is a structural sedimentary basin bounded by the Front Range on the west that extends northward into southern Wyoming. The administrative Denver Basin is a portion of the structural basin. The ground water resources administered for water rights within the basin extend east near Limon, Colorado, southward from near Greeley on the north, to a location several miles to the south and east of Colorado Springs. In total, the administrative Denver Basin includes an area of more than 6,700 square miles. The basin is generally defined by the lower boundary of the Laramie-Fox Hills aquifer, which is the lowermost of the four principal aquifers found in the basin. Figure 1 is a map of the Denver Basin that identifies the lower boundary of each of the Denver Basin aquifers. The formations present in the basin that comprise the principal aquifers are composed of sedimentary rocks that are up to 3,000 feet thick in the deepest part of the basin. A generalized cross-section is attached as Figure 2. In descending order, the hydrogeologic units are identified as the Dawson, Denver, Arapahoe, and Laramie-Fox Hills aquifers. Extensive studies of these aquifers have been conducted over a period of about 100 years. Water from the aquifers has been utilized in the Denver area for domestic supplies since the 1880’s. The most recent and most detailed study of the aquifer system was completed by the Colorado Division of Water Resources in 1985 as part of a legislatively mandated study commonly known as Senate Bill 5. During the study, the staff of the Division collected extensive information on the nature of the aquifers, the occurrence of ground water in the aquifers, and the effects of pumping ground water from the aquifers on surface streams throughout the basin. The estimated amount of water contained in the aquifers is presented in Table 1. The Colorado Division of Water Resources’ study resulted in re-defining the aquifer system into the four major aquifers currently recognized. For the reader to better understand the aquifer system, a brief discussion of each hydrogeologic unit is presented below. It should be noted that the Division’s work in this area is recognized as the authoritative study of the Denver Basin aquifers, and is utilized by the consulting community and governmental/regulatory agencies. DAWSON AQUIFER The Dawson aquifer is the uppermost aquifer in the basin and covers an area of approximately 1,400 square miles. The aquifer varies in thickness, depending upon location, and attains a maximum thickness of almost 1,200 feet in the vicinity of Monument. The Dawson 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. The northern portion of the aquifer can be subdivided into two hydrogeologic units 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.