THE DISTRIBUTION OF PERMEABLE ZONES WITHIN THE OGALLALA IN THE SOUTHWEST KANSAS GROUNDWATER MANAGEMENT DISTRICT 3

Ground-water availability and the potential for recharge moving downward from the surface to the water table are important factors to consider in the formulation of tailored management plans for the Ogallala aquifer of the Southwest Kansas Groundwater Management District 3. The Ogallala aquifer framework consists of a heterogeneous mix of Neogene and Quaternary gravel, sand, silt, and clay that were deposited in fluvial, lacustrine, and eolian settings. A project was undertaken to map distribution of permeable deposits in the District using more than 7,000 drillers' logs of water wells. Geologic interpretation of the logs required developing a consistent means to (1) translate the log entries into lithologic descriptions and (2) quantify the relative proportions of each lithology where more than one was mentioned in an entry. Rules based on the phrasing of the lithologic description were formulated to translate the interpreted descriptions into relative proportions of each lithology (clay/silt, sand, and sand & gravel) represented within each interval. Following careful screening, the rules were applied to translate the drillers' descriptions into relative proportions of permeable sediments in each interval.

The fraction of permeable deposits within the Ogallala varies from near 0 up to 100 percent with the mean and median values of approximately 50%. Three-dimensional models of the distribution of permeable deposits were formulated using the Rockware© software using the interpreted interval data from the logs. Comparison of the results with the drillers' logs indicates that the models poorly reflect the interval data on the individual logs. Closer inspection seems to indicate that the portrayed permeable fraction distribution may be measure of the probability of occurrence rather than actual. Taken in this light the models suggest that permeable zones are likely to follow lows incised into the bedrock surface and recur vertically upward in the Ogallala sequence. Recurrence suggests localization of the drainage within well-defined belts during the deposition of Ogallala sediments.