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VERTICAL HYDRAULIC CONDUC77VITY MEASUREMENTS LN THE DENVER BASIN, COLORADO to the aquifers on the whole and less to the individual layers that comprise them. A comprehensive understanding of groundwater flow in the Denver Basin will require hydrogeologic characteriza- tion of individual layers within the aquifers. Deciphering the characteristics should not be limited to the water -bear- ing layers, but should also be extended to the less- perme- able shale intervals, which restrict groundwater flow throughout the entire system. The interbedded nature of the sedimentary package and geometric shape of the water - bearing intervals imply that the hydraulic characteris- tics of the shale intervals are critical to the behavior of the entire sequence. Ultimately, long -term yields from wells tapping the resource will depend to a certain extent on how quickly water moves vertically from one water -bear- ing sand interval to another. Yet, empirical data from sam- ples of the shale layers are available from only five locations across the Basin. THE DENVER BASIN AQUIFER SYSTEM Consisting of a thick sequence of Paleogene and Upper Cretaceous (about 49 to 69 Ma [Raynolds, 20021) interbed- ded sandstone, conglomerate, and shale, the Denver Basin aquifer system has been subdivided into four principal aquifers named in ascending order, the Laramie -Fox Hills, Arapahoe, Denver, and Dawson aquifers (Fig. 2A). This nomenclature has been fixed by statute for the purposes of allocating water within the Basin, and many legal decrees granting water rights using this nomenclature have been granted to date. Shale, herein used to include shale, claystone, mud - stone, and muddy siltstone as variously described in the lit- erature, is present throughout the entire sequence. Specific shale intervals identified in geophysical logs and correlated between boreholes have been used to separate the princi- pal aquifers (VanSlyke, 2001). The Laramie -Fox Hills aquifer is at the base of the aquifer system and consists of the 150- to 200 -ft thick fine - grained Fox Hills Sandstone, and a 50- to 100 -ft thick fine - to medium - grained sandstone in the overlying Laramie For- mation (Robson, 1987). A thin (5 -20 ft) shale separates the Fox Hills Sandstone from sandstones in the Laramie For- mation. The upper 400 to 500 ft of the Laramie Formation consists of shale with coal seams and minor amounts of siltstone and sandstone. The sedimentary rocks forming the Fox Hills Sandstone and Laramie Formation were deposited by the regression of the Cretaceous Western Interior Seaway prior to, or at the inception of, subsidence of the Denver Basin (Raynolds, 2002). The Laramie Forma- tion forms a confining layer between the Laramie -Fox Hills aquifer and the overlying Arapahoe aquifer. Next is the Arapahoe aquifer, which is perhaps the most important aquifer in use due to its greater saturated thick- ness over an extended area combined with generally higher hydraulic conductivity values, and, hence, higher overall transmissivity. This aquifer consists of a 400- to 700 -ft thick sequence of Upper Cretaceous interbedded conglomerate, sandstone, siltstone, and shale (Robson, 1987). Individual sandstone bodies within the Arapahoe aquifer are believed to be lens- shaped and range in thick- ness from less than a foot to 40 ft or more. The sandstone lenses can be closely spaced and hydraulically connected, forming a relatively uniform hydraulic unit. The net thick- ness of the water - bearing sandstone and conglomerate generally ranges from 200 -300 ft, although the net sand thickness can exceed 400 ft. These sedimentary rocks were deposited in a synorogenic fluvial environment wherein sediments were being shed from the emerging Laramide Front Range uplift to the west and deposited by rivers and streams in the subsiding Denver structural basin (Raynolds, 2002). A layer of shale up to 50 ft thick generally separates the Arapahoe aquifer from the overlying Denver aquifer (VanSlyke, 2001). The Denver aquifer occurs in Upper Cretaceous and Paleogene interbedded shale, claystone, siltstone, lignitic coal, and sandstone (Robson, 1987) with a total thickness approaching 1000 ft. As with the Arapahoe aquifer, these rocks were deposited in a synorogenic fluvial environment as the Laramide Front Range continued to rise and the Denver structural basin subsided. Individual sandstone bodies are also lens- shaped, however, shale is more preva- lent and the sandstone bodies are less likely to be inter- connected. The total thickness of the saturated sandstone within this interval generally ranges from 100 to 350 ft. A shale layer averaging 25 to 50 ft thick generally separates the Denver aquifer from the overlying Dawson aquifer (VanSlyke, 2001). At the top of the Denver Basin aquifer system is the Dawson aquifer, consisting of Paleogene conglomeratic to coarse - grained arkosic sandstone interbedded with clay- stone and shale (Robson, 1987). These sediments were deposited in fluvial environments in the subsiding Denver Basin with sand coming from the rising Front Range to the west. They reach a total thickness of over 1000 ft in the center of the Basin. The water - bearing sandstone and con- glomerate of the Dawson is up to 400 ft thick. The subdivision of the sedimentary sequence holding the Denver Basin aquifer system is simplistic in its layer - cake concept, but it allows an orderly allocation of the water resource. However, it belies the complexity of the geology, much of which has come to light with the grow- ing body of subsurface data made available as the resource is being developed. Since the synorogenic basin was being filled with clastic sediments derived from the rising Front Range to the west, there is considerable horizontal variability 171 The Rocky Mountain Association of Geologists