Hydrogeology of thin limestones: the Konza Prairie Long-Term Ecological Research Site, Northeastern Kansas

Abstract

The Konza Prairie Research Natural Area and National Science Foundation Long-Term Ecological Research (LTER) Site is a dissected tallgrass-prairie upland located in northeastern Kansas. It is underlain by nearly horizontal, alternating thin limestones and shales of Permian age and by discontinuous Quaternary alluvium. Shallow aquifers which interact with surface-water in this region are an important steady source of water to the biota because of sporadic precipitation. This study provides a summary of the shallow hydrogeology in a typical watershed on the Konza Prairie, including an overview of directions of ground-water movement inferred from water-level data and of ground-water chemistry. Measurements made every 4 to 6 weeks for over 2.5 years provide information on spatial and temporal variability in water chemistry and in directions of ground-water flow during the end of a drought (first year), during a period of average rainfall (second year) and through a period of exceptionally high summer rainfall (third year). Well yields from the limestones (each cut by the ephemeral stream draining the watershed) are variable and depend upon secondary-porosity development. Potential for vertical flow is downward in all units and lateral flow directions are sometimes toward and sometimes away from the stream. In addition, water levels are sometimes above and sometimes below the tops of the aquifers, indicating that aquifers behave sometimes as confined and sometimes as unconfined aquifers. The stratigraphically and topographically lowest unit monitored (Morrill Limestone) is the highest yield aquifer and appears to be hydraulically linked to the stream. The uppermost unit studied (upper Eiss Limestone) has water chemistry and well yield somewhat similar to the Morrill, although there is insufficient well control to establish that it, too, is closely linked to the stream. The middle unit (lower Eiss Limestone) is characterized by lower hydraulic conductivity, steady water chemistry, and higher total dissolved solids than the upper and lower units. Despite the differing hydraulic behavior of the limestones, concentration ranges of the major elements in water samples from the aquifers overlap, with the exception of consistently higher dissolved magnesium in the lower Eiss Limestone water. Variability in hardness, alkalinity, and sulfate, the principal components of the dissolved solids, do not clearly distinguish between high- and low-permeability aquifers or parts of aquifers. Occasional, easily identified sulfate spikes are the result of dissolution of pockets of gypsum or anhydrite in the limestones. Relatively long-term, progressive depression of sulfate concentrations is probably the result of steady recharge during relatively long et periods, and is a good indicator of flushing of the aquifers even though water levels are not elevated. Calcium and alkalinity variations at this site reflect variations in recharge as modified by seasonal variations in below-ground P-CO 2. Calcium/magnesium ratios are not constant, indicating that either there is a non-carbonate mineral source for magnesium or that more than one carbonate mineral (with different magnesium contents) contributes calcium to the water. The time-series sampling used in this study revealed information about the complicated chemical and hydraulic response of these shallow, thin limestone aquifers to variable recharge.