USGS Publications Warehouse

Abstract

The Dakota Sandstone in South Dakota is one of the classic artesian aquifers; it was first studied by N. H. Darton at the turn of the century. Since then, hydrogeologists have debated the source of the large quantities of water which have been discharged by artesian flow from the Dakota. Among suggestions for the source of this water are (1) recharge of the aquifer at outcrops in the Black Hills, (2) compaction and compressive storage within the aquifer, (3)leakage through confining layers, and (4) upward flow from the underlying Madison Group limestones. A series of increasingly refined models of the aquifer system in South Dakota have been developed and used for numerical simulations of the ground-water flow. The simulations have provided estimates of leakage through the confining layers. The results indicate that, before development, most of the flow into and out of the Dakota Sandstone occurred as leakage through confining layers and, since development, most of the water released from storage has come from the confining layers. In situ and laboratory hydraulic conductivity measurements have been made for the Cretaceous shale confining layer which overlies the Dakota. These data indicate hydraulic conductivities which are one to three orders of magnitude lower than the conductivities indicated by the numerical analyses; this suggests that the leakage through the confining layer is largely through fractures. The fractures apparently did not influence the laboratory and in situ measurements. To test the conception of flow in the aquifer-confining layer system derived from our analyses, the transport of sulfate in the system was simulated. Simulations using a numerical ground-water transport model were reasonably successful in explaining the present distribution of sulfate in the system. This result increases confidence in the flow system implied by the flow simulations in which leakage through confining layers is dominant.