PRACTICAL SATURATED THICKNESS: A MORE REALISTIC MEASURE OF GROUNDWATER AVAILABILITY IN HETEROGENEOUS AQUIFERS

Groundwater managers and planners use the saturated thickness (ST) as the measure of groundwater availability in unconfined aquifers. However, in using ST for this purpose, it is assumed that all water-saturated sediments contribute to pumping wells. The practical saturated thickness (PST) considers only the net thickness of saturated sediments that significantly contribute to well yield from the water table down to the bedrock surface. Thus, PST provides a more accurate picture of water availability and may also provide insight into future water-level trends at the scale of an individual well.

To demonstrate the utility of this concept a PST mapping project was conducted in the Ogallala aquifer in the Southwest Kansas Groundwater Management District using more than 7,000 drillers' logs of water wells. The aquifer consists of a heterogeneous mix of Neogene and Quaternary gravel, sand, silt, and clay that were deposited in fluvial, lacustrine, and eolian settings. 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 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 mean and median values of approximately 50%. Three-dimensional models of the distribution of permeable deposits were formulated with Rockware© software using the interpreted interval data from the logs. Comparison of the results with the drillers' logs indicates 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.