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GEOLOGY OF THE DENVER BASIN <br />The Denver basin is a structural basin which covers the area from the Front Range on the West <br />to a point near Limon on the east. And from Greeley on the North to a point several mites to the south <br />and east of Colorado Springs to the south. In total, this area is over 6,700 square miles. The basin is <br />generally defined by the area of outcrop of the Laramie -Fox Hills Aquifer which is the lowest of the four <br />principal aquifers found in the basin. Figure 1 is a map of the Denver Basin area with the outcrop of <br />the base of the aquifers identified. <br />Sedimentary rocks make up the formations present in the basin and range up to 3,000 feet in <br />thickness in the deepest part of the basin. A generalized cross section is attached as Figure 3. In <br />descending order, the sedimentary sequence is comprised of the Dawson, Denver, Arapahoe, Laramie <br />and Fox Hills formations. Within these formations are four aquifers or water bearing portions which are <br />named the Dawson, Denver, Arapahoe and Laramie -Fox Hills Aquifers. <br />Extensive studies of these aquifers has been done over a period of about 100 years. Water from <br />the aquifers has been utilized in the Denver area for domestic water supplies since about the turn of the <br />century. The most recent and the most detailed study of the aquifer system was done by the Colorado <br />Division of Water Resources in 1985 as part of a legislatively mandated study commonly known as Senate <br />Bill 5. During this study, the staff of the Division collected extensive information on the nature of the <br />aquifers, the occurrence of ground water in the aquifers and the affects of pumping of ground water on <br />surface streams throughout the basin. The amount of water available from each of the aquifers is <br />presented in figure 2. <br />The study resulted in re -defining the aquifer system into the four major aquifers which are now <br />recognized. For the reader to better understand the aquifer system, a brief discussion of each of each <br />of the aquifers will be presented. It should be noted that the Divisions work in this area is recognized <br />as the authoritative study of the Denver Basin and is utilized extensively by the consulting community <br />as well as governmental and regulatory agencies. <br />DAWSON AQUIFER 38 WCk S <br />The Dawson Aquifer is the upper most aquifer in the basin and covers an area of approximately <br />1400 square miles. It ranges in thickness from 0 to almost 1200 feet in the vicinity of Monument. The <br />Dawson is at or near the land surface throughout its entire areal extent. The aquifer is made up of <br />predominately conglomeratic, coarse-grained arkosic sandstones with minor amounts of interbedded clay <br />