The Effect of River Valleys and the Upper Cretaceous Aquitard on Regional Flow in the Dakota Aquifer in the Central Great Plains of Kansas and Southeastern Colorado

In his reports on the regional hydrogeology of the central Great Plains, in particular southeastern Colorado and southwestern and central Kansas, Darton considered the Dakota aquifer to be a classic example of an artesian system. Computer simulations of the flow system in this study, however, suggest that the Dakota is not a regional artesian aquifer in the classic sense. Sensitivity analysis of a steady-state vertical profile flow model demonstrates that the flow system in the upper Dakota in western Kansas is heavily influenced by the Upper Cretaceous aquitard, the Arkansas River in southeastern Colorado, and rivers in central Kansas, such as the Saline, that have eroded through the aquitard and into the Dakota to the west of the main outcrop area of the aquifer. The model shows that local flow systems and the vertical hydraulic conductivity of the Upper Cretaceous aquitard heavily influence the water budget and the flow patterns. The aquitard restricts recharge from the overlying water table to underlying aquifers in western Kansas because of its considerable thickness and low vertical hydraulic conductivity. The Arkansas River intercepts ground-water flow moving toward western Kansas from recharge areas south of the river and further isolates the upper Dakota from sources of freshwater recharge. In central Kansas, the Saline River has reduced the distance between confined portions of the aquifer and its discharge area. In essence, this has improved the hydraulic connection between the confined aquifer and its discharge area, thus helping to generate subhydrostatic conditions in the upper Dakota upgradient of the river. elevated recharge areas south of the river and the maintenance of artesian pressure in the Dakota from recharge to discharge areas by overlying Upper Cretaceous shales, referred to here as the Upper Cretaceous aquitard. He described the Dakota as a classic example of an artesian aquifer. More recent, regional hydrogeologic investigations have not supported Darton’s conceptualization of flow in the Dakota. In a review of the literature, Helgeson et al. (1982) questioned the applicability of Chamberlin’s (1885) concept to the Dakota on the basis of aquifer geometry and lateral hydraulic continuity. In the Denver basin and adjacent areas of eastern Colorado and western and central Kansas, Belitz (1985), Belitz and Bredehoeft (1988), and Helgeson et al. (1994) reported that some heads in the Dakota and the deeper aquifers are more than 2,500 ft (760 m) lower than the elevation of the overlying water table. An aquifer is usually considered to be in good hydraulic communication with the overlying water table if there are only small head differences between them. Thus the Dakota, with its subhydrostatic heads—heads that are significantly lower than those of the overlying water table—is essentially isolated from the overlying water table. In the Denver basin the Dakota is overlain by the Upper Cretaceous aquitard, which is as much as 10,000 ft (3,050 m) thick. From regional flow models, Belitz (1985), P. Allen Macfarlane Kansas Geological Survey, 1930 Constant Avenue, Lawrence, Kansas 66047 The Dakota aquifer and its equivalents extend over much of the Great Plains of North America and are known as dependable sources of freshwater in much of this region. In the Arkansas River valley of southeastern Colorado and adjacent parts of southwestern Kansas, the Dakota was probably one of the first sources of water used by the early settlers and railroads because of its flowing well conditions and its shallow depth. Figure 1 shows the extent of the study area in Kansas and Colorado that is the subject of this paper. Darton (1905, 1906) described the results of the earliest regional reconnaissance investigations into the hydrogeology of the Dakota aquifer in southeastern Colorado and adjacent parts of western and central Kansas. Darton reported numerous flowing wells in the Arkansas River valley and its tributaries in southeastern Colorado and in parts of central Kansas. He demonstrated that water enters the Dakota where it crops out at the surface south of the river, flows northeastward, and eventually discharges in central Kansas, where again the aquifer crops out at the surface (fig. 1). Using Chamberlin’s (1885) concept of artesian aquifers, Darton believed that ground-water flow was controlled mostly by the head difference between recharge and discharge areas. Darton reasoned that the flowing wells in the Arkansas River valley and elsewhere in central Kansas could be accounted for only by the