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Ralj Topper LOCATION AND GEOLOGIC SEITING The Denver Basin is a structural sedimentary basin that underlies the Denver metropolitan area within the Great plains physiographic province. The Basin is rich in mineral fuels, with active production of oil and gas, and is a his- toric coal producer. It is also an important non-renewable source of groundwater for municipal, industrial, agricul- tural, and domestic uses. This layered, multi-aquifer systcm is recognized nationally as a major aquifer from a water resource perspective. The administrative groundwater por- tion of the Basin, for water rights considerations, underlies a 6700-mi2 area extending into Weld County on the north; EI Paso County on the south; Jefferson County on the west; and the eastern parts of Adams, Arapahoe, and Elbert counties on the east (Fig. 1). This corresponds to the area of outcrop/sub crop of the Laramie-Fox Hills aquifer south of the Greeley Arch. The 8-county Denver metropolitan area contains 56% of Colorado's population, slightly over 2.4 million people according to the 2000 census. In addition to housing a large population, this area represents the largest commer- cial and industrial region in the state. Major industries include agriculture, communications, oil and gas, utilities, and transportation. Faced with a paucity of surface-water supplies and accelerated urban growth, water districts and water suppliers are relying on and extensively developing the Denver Basin aquifers as both primary and supplemen- tal sources of water supply. The Denver Basin aquifer system consists of water- yielding strata (predominantly sandstones and siltstones) in Tertiary and Cretaceous sedimentary rocks. The northern part of the Denver Basin aquifer system underlies the allu- vial aquifer of the South Platte River, and is hydraulically connected to that unconsolidated aquifer over part of the area. In descending order, the geologic formations that contain the Denver Basin aquifers are Tertiary and Creta- ceous sandstone, conglomerate, and shale of the Dawson, Denver, Arapahoe, Laramie, and Fox Hills formations (Romero, 1976). The four statutory aquifers contained in these formations are named the Dawson, Denver, Arapa- hoe, and Laramie-Fox Hills (Table 1). Underlying the Fox I-Hils sandstone is as much as 7000 ft of nearly imperme- able shale known as the Pierre Shale. Although the Denver structural basin extends north into Wyoming and contains permeable Cretaceous rocks, they are little utilized as a SOurce of water north of Greeley (Fig. 2). In cross-section, the Denver Basin's asymmetrical bowl shape is expressed by low-angle dips of the uppermost rock units along the northern, eastern, and southern mar- gins, and high-angle dips along the western margin. The outcrop area of each successively deeper aquifer becomes larger while maintaining the Basin's overall kidney shape. The center of the basin lies just west of the town of Parker, The Rock)' Mountain Association of Geologists where the Laramie-Fox Hills aquifer is approximately 3000 ft deep. Economic considerations have generally limited water well depths to 2500 ft. HYDROGEOLOGIC UNITS In a cooperative effort, scientists with the U.S. Geologi- cal Survey (USGS) and the Colorado Division of Water Resources (CDWR) have conducted some of the most detailed investigations available of this aquifer system cul- minating in the publication of a USGS Hydrogeologic Atlas series: HA-643 (Dawson Aquifer); HA-646 (Denver Aquifer); HA-647 (Arapahoe Aquifer); and HA-650 (Laramie-Fox Hills Aquifer) in 1981 (Robson and Romero, 1981a,b; Robson et al., 1981a,b). This series of atlases was expanded upon and refined as a consequence of the passage of Colorado Sen- ate Bill 5, which required the promulgation of rules and regulations applying exclusively to the withdrawal of groundwater from the Denver Basin (VanSlyke et al., 1988). Robson (987) prepared a quantitative water resource appraisal of the bedrock aquifers in the Denver Basin that included the first groundwater flow model analysis. Table 1 provides a summary of both the geologic and hydrologic characteristics of the Denver Basin aquifer sys- tem. A brief synopsis of the four main aquifer units fol- lows. The Dawson aquifer, the shallowest aquifer in the basin, is at or near the land surface throughout its entire areal extent. It covers an area of approximately 1400 mi2. The Dawson aquifer consists primarily of conglomeratic, coarse-grained arkosic sandstones with minor amounts of interbedded clay and clay shale (VanSlyke, 2001). The Dawson Formation ranges in thickness from a few feet to 1200 ft. The saturated portion of the aquifer may be up to 400 ft thick in the deeper portion of the basin (Fig. 3). The Denver aquifer underlies an area of approximately 3500 mi2. The sediments comprising the aquifer consist of interbedded shale, claystone, siltstone, lignitic coals, and sandstone lenses. The aquifer ranges in thickness from 800 to 1000 ft. Because of the interbedded shale and claystone, the thickness of saturated water yielding materials gener- ally ranges from 100 to 350 ft (Fig. 3). Distinguishing char- acteristics of this formation include its olive, green-gray, and brown color; the presence of organic matter and coal; and the fine-grained and discontinuous nature of the sand- stone lenses. The Arapahoe aquifer underlies an area of approximately 4700 mi2 and consists of an interbedded sequence of con- glomerate, sandstone, siltstone, and shale. The formation is 400 to 700 ft thick and is up to 2200 ft deep in the center of the Basin. The saturated thickness of the water-yielding materials in the aquifer ranges from 0 along the margins to a maximum of 400 ft in the basin center (Fig. 3). The Arapa- hoe aquifer is the most prolific of the Denver Basin aquifers, and it is extensively used as a municipal water supply. 146