ROLE OF LITHOLOGY AND STRUCTURE ON REGIONAL GROUND-WATER FLOW TO LARGE SPRINGS OF THE OZARK PLATEAUS, MIDCONTINENT U.S

The regional karst aquifers of the Ozark Plateaus are unique in many ways for midcontinent U.S. aquifer systems. The Ozark region of Missouri and Arkansas contains many more large springs (> 1 cubic m/sec) than contiguous Paleozoic aquifers elsewhere, including 3 of the 10 largest first-magnitude springs in the county. Recent studies within the past 20 years that have evaluated regional aquifer systems (e.g. RASA and NAWQA) have tended to lump a thick stratigraphic interval (~400 m) of predominantly dolomite and sandstone into the Ozark aquifer, which for some regional assessments is appropriate; emerging data, however, indicate that within this thick interval there are strong, localized karst-flow distributions. Studies at less than regional scale utilizing the entire Ozark aquifer interval rather than distinct lithologic units tend to oversimplify the hydrogeology, with the concurrent loss of information and understanding. This paper evaluates the stratigraphic range and areal extent of flow distributions in the interval of the Ozark aquifer, and seeks to draw a coherent conceptual model that explains discharge and water quality in terms of processes and controls related to the evolution of the karst hydrogeology of the Potosi Formation.

Data from numerous municipal wells indicate that the widespread, vuggy nature of permeability and the predominant lithology of the Potosi Formation resulted from early dolomitization and geochemical alteration. Well yields and focused hydraulic testing of individual units likewise is consistent with the Potosi as the predominant, most permeable interval within the Ozark aquifer. The concentration of large springs along linear trends that coincide with geophysical anomalies suggests major tectonic reactivation of basement faults broke the overlying brittle, carbonate cover into systematic orthogonal joint sets that facilitated vertical flow. Surface faults that appear to truncate the entire hydrostratigraphic section are consistent with this interpretation, as are distribution of endangered organisms such as Amblyopsis and Cambarus. Dye tracing, water-level mapping, stream piracy, and geochemistry provide additional constraints that explain most of the anomalous hydrogeologic behavior that cannot be accounted for with a single-layer Ozark aquifer model.