Aquifer/aquitard Units of the Dakota Aquifer System in Kansas: Methods of Delineation and Sedimentary Architecture Effects on Ground-water Flow and Flow Properties

Abstract

ABSTRACT In the near future, the demand for water in many parts of central and western Kansas will exceed the supply available from near-surface aquifers, forcing exploitation of the underlying Dakota aquifer. The Dakota aquifer framework consists of discontinuous sandstone aquifers confined by mudstone aquitards belonging to the Cretaceous Cheyenne Sandstone and the Kiowa and Dakota Formations. These strata were deposited in a variety of nonmarine through marine settings associated with several cycles of sea-level rise and fall. The aquifer comprises two entire and one partial unconformity-bounded sequences recognized in the Western Interior: Cheyenne-Kiowa, lower Dakota J sequence, and upper Dakota D sequence. Scale-dependent heterogeneity characterizes the Dakota aquifer, because of the inf uence of sedimentary processes on the arrangement and geometry of sandstone bodies and on the fabric of the sandstones. A multidisciplinary approach is being employed in the Dakota Aquifer Program of the Kansas Geological Survey to delineate aquifer/aquitard units and characterize scale-dependent heterogeneity using a variety of techniques. Colorized images of the subsurface from gamma-ray logs in cross sections reveal the complex arrangement of sandstone bodies within the Dakota due to deposition in fluvial and deltaic/estuarine to marine settings. At the more local scale, borehole geophysical logs are being used both to delineate local aquifer zones and to estimate overall ground-water quality. Pumping test results indicate that the contrast of hydraulic properties between a sandstone aquifer and its surrounding mudstone aquitard significantly channels the flow of ground water through the aquifer. he geometric mean hydraulic conductivity of fluvial sandstones is generally higher than in deltaic/estuarine sandstones in either the Dakota or Kiowa Formations. Overlapping ranges of hydraulic conductivity values in fluvial and deltaic/estuarine sandstones may be due to the uniformity of the sediment supplied from source areas combined with variations in the energy of transport and winnowing within each of these depositional systems. Permeameter tests of coreplugs and gamma-ray response on logs indicate that the better sorted, coarser, and less radioactive sandstones make the most permeable aquifer units in two cores of fluvial and deltaic/estuarine sandstones. Experimental variograms using coreplug data indicate that bedding is a significant control on the partitioning of hydraulic con uctivity within sandstone bodies. Succeeding efforts will focus on using the results presented in this paper combined with techniques, such as indicator kriging and geologic process modeling, to map the distribution of sandstone aquifers and their properties in the subsurface within small geographic areas of manageable size where detailed information is needed.