INTERPRETATION OF THE HYDROGEOLOGY OF A LARGE KARST SPRING IN THE OZARK NATIONAL SCENIC RIVERWAYS, MISSOURI, FROM AUDIO-MAGNETOTELLURIC (AMT) SOUNDINGS AND GEOLOGIC MAPPING

The Ozark Plateaus of Missouri, in the central United States, contain a well-integrated karstic flow system feeding numerous large karst springs. Dye traces have been used to delineate the major groundwater basins; these yield an average flow velocity over a straight-line distance but provide little information on the actual path of groundwater flow. Alley Spring, located in the Ozark National Scenic Riverways (ONSR) in southern Missouri, is one of numerous large springs in the park and a major contributor of flow to the Jacks Fork River. The spring has an average discharge of 3.6 m³/sec fed by a single water-filled conduit with a horizontal extent of at least 600 m and an average depth of about 50 m below the spring surface that has been mapped by divers. This part of the conduit is in flat-lying Cambrian and Ordovician dolomites. Audio-magnetotelluric (AMT) data was collected in the vicinity of the spring in an attempt to identify the electromagnetic (EM) signature of the conduit, and to map its location. AMT soundings consist of electric and magnetic field measurements over a range of frequencies from 10- to 100,000-Hertz with fixed receiver locations. Since low-frequency signals penetrate to greater depths than high-frequency signals, measurements of the EM response at several frequencies contain information on the variation of resistivity at depth. Karst conduits may be identifiable as low resistivity areas in the processed AMT data.

Preliminary analysis of the data from Alley Spring indicates a high resistivity anomaly below the Alley Spring orifice and the mapped cave. A north-south cross section of the data about 2 km west of the spring shows a low resistivity anomaly that delineates the subsurface extent of the recently mapped Hartshorn fault. Data from a line of soundings orthogonal to the trends of two dye traces and collected 7.5 km WNW of the spring show a low resistivity anomaly that might indicate the main spring conduit or a major subsurface tributary. The AMT data, combined with other geologic data collected during geologic mapping of the Alley Spring 7.5 minute quadrangle by the U.S. Geological Survey in 2004-2005, support interpretations of groundwater flow through large conduits from sinkhole-rich areas tens of kilometers to the west of the spring.