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Glenn Graham and George 1Vanslyke INTRODUCTION The Denver basin as administered by the CDWR con- sists of about 6,700 square miles located entirely within the structural Denver basin (Fig. 1). The boundary of the administered area is defined by the base of the Fox Hills Sandstone. Four aquifer intervals are recognized and defined by the Denver Basin Rules and Regulations. These aquifer designations agree only generally with present and past stratigraphic nomenclature (Fig. 2). The current regu- latory framework for the Denver Basin bedrock aquifers, commonly known as the Denver Basin Rules, also known as Senate Bill 5 (SB -5) became effective July 1, 1986. The Denver Basin Rules are supplemented by the Statewide Non - Tributary Ground Water Rules which were effective March 3, 1986. HYDROGEOLOGIC SETTING The structural Denver Basin was recognized in an early geological survey of the West. The Hayden Survey of the 1870s mapped the basin and speculated on undergroundwa- ter contained in the sedimentary sandstones (Hayden, 1881). Hand -dug wells were being constructed into the shallow weathered sandstones of the Basin as early as the 1860s, with the water being pumped by hand or windmill for use on farms. The first recorded flowing artesian bedrock aquifer well was constructed in the downtown Denver area in 1883. Additional deep wells were constructed to take advantage of the flowing artesian character of the aquifer. These wells were several hundred feet deep and encoun- tered good quality water under high artesian pressures in the sands of the Arapahoe aquifer. The initial pressure was sufficient to bring the water to the surface where it was uti- lized not only for commercial purposes, but also for decora- tive fountains (Union Station), power assist for elevators (Brown Palace Hotel), and to operate the organ bellows in the Trinity Methodist Church. Rapidly increasing develop- ment of this resource resulted in the lowering of the artesian pressure to the level that, by the mid 1890s, pumping was needed to bring the water to the surface. As early as 1891 there was talk that administration of this groundwater resource was needed to provide orderly development of the groundwater (Van Diest, 1891). During the early twentieth century, the development of well construction and pumping technology allowed for the drilling of wells throughout the Basin and into deeper aquifers. Exploitation of groundwater in the bedrock aquifers gradually increased into the late 1970s, when pop- ulation growth outside the Denver metropolitan area began to rely more heavily on the bedrock aquifers as a source of potable water. This increased demand resulted in significant water level declines in wells located in and around pump- ing centers. These declines, as well as projected future demands for groundwater, and Denver Water's reluctance to expand their service area, caused some individuals and organizations to call for a quantification of the volume of water that might be available in the Denver Basin aquifers, acknowledging finite nature of this resource. The Rocky Mountain Association of Geologists 154 Figure 1. Index map of the administrative Denver Basin.