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