The Distribution of Permeable Zones within the Ogallala in the Southwest Kansas Groundwater Management District 3

P. Allen Macfarlane and Nicholas Schneider. Kansas Geological Survey, 1930 Constant Avenue, Lawrence, KS 66047 [email protected]

The Ogallala aquifer framework Southwest Kansas Groundwater Management District 3 consists of a heterogeneous mix of Neogene and Quaternary gravel, sand, silt, and clay that were deposited in fluvial, lacustrine, and eolian settings. Two projects were undertaken to update the map of the bedrock surface below the aquifer and to map the distribution of permeable deposits within it using more than 7,000 drillersÕ 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 were formulated to translate the interpreted descriptions into relative proportions of permeable deposits represented within each interval. Following careful screening, the rules were applied to translate the drillersÕ 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% with mean and median values of approximately 50%. Three-dimensional models of the distribution of permeable deposits were formulated with Rockware^© software and using the interpreted interval data from the logs. Comparison of the results with the drillersÕ 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 a 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.