AQUIFER AQUITARD UNITS OF THE DAKOTA AQUIFER SYSTEM IN KANSAS - METHODS OF DELINEATION AND SEDIMENTARY ARCHITECTURE EFFECTS ON GROUNDWATER-FLOW AND FLOW PROPERTIES/

In the near future, the demand for water in many parts of central and western Kansas Hill exceed the supply available from nearsurface aquif ers, forcing exploitation of the underlying Dakota aquifer. The Dakota aquifer framework consists of discontinuous sandstone aquifers confin ed by mudstone aquitards belonging to the Cretaceous Cheyenne Sandston e 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 a nd one partial unconformity-bounded sequences recognized in the Wester n Interior: Cheyenne-Kiowa, lower Dakota J sequence, and upper Dakota D sequence. Scale-dependent heterogeneity characterizes the Dakota aqu ifer, because of the influence of sedimentary processes on the arrange ment and geometry of sandstone bodies and on the fabric of the sandsto nes. A multidisciplinary approach is being employed in the Dakota Aqui fer Program of the Kansas Geological Survey to delineate aquifer/aquit ard units and characterize scale-dependent heterogeneity using a varie ty of techniques. Colorized images of the subsurface from gamma-ray lo gs in cross sections reveal the complex arrangement of sandstone bodie s within the Dakota due to deposition in fluvial and deltaic/estuarine to marine settings. At the more local scale, borehole geophysical log s 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 s urrounding mudstone aquitard significantly ''channels'' the flow of gr ound water through the aquifer. The geometric mean hydraulic conductiv ity of fluvial sandstones is generally higher than in deltaic/estuarin e sandstones in either the Dakota or Kiowa Formations. Overlapping ran ges 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 w innowing within each of these depositional systems. Permeameter tests of coreplugs and gamma-ray response on logs indicate that the better s orted, coarser, and less radioactive sandstones make the most permeabl e aquifer units in two cores of fluvial and deltaic/estuarine sandston es. Experimental variograms using coreplug data indicate that bedding is a significant control on the partitioning of hydraulic conductivity within sandstone bodies. Succeeding efforts will focus on using the r esults presented in this paper combined with techniques, such as indic ator kriging and geologic process modeling, to map the distribution of sandstone aquifers and their properties in the subsurface within smal l geographic areas of manageable size where detailed information is ne eded.