QUANTITATIVE EVALUATION OF VERTICAL SHAFT EVOLUTION AND FUNCTION WITHIN THE MAMMOTH CAVE SYSTEM, KENTUCKY

A long-term project is underway within Kentucky's Mammoth Cave System to better understand the evolution and behavior of vertical shafts within karst aquifers. The first phase of the project involved an inventory of all relatively accessible vertical shafts near the cave's nearly twenty kilometers of tourist trails. In phase two, we measured fluctuations in water chemistry that allowed weekly calculation of CO₂ pressures along with calcite saturation indices and dissolution rates at Shower Bath Spring, a shaft in the cave's Frozen Niagara area. Results suggest that shaft water chemistry there is controlled by a mix of relatively high CO₂ storm waters and low CO₂ diffuse waters that have dissolved limestone in closed system regions of the vadose zone. The (combined) shaft water was undersaturated with respect to calcite at all times during the study. Storm events increased dissolution rates, with typical shaft dissolution rates over the year of approximately 1 mm/yr.

The third phase is currently underway at Edna’s Dome, within the cave's Cathedral Dome shaft complex. Carbonate water chemistry and flow parameters from both free falling and wall film water are being measured with two-minute resolution. Within a single bed around the shaft's perimeter, eight evenly spaced stations have electronic wetness indicators that indicate every two minutes whether dissolving fluids are in contact with that section of the wall. With limestone dissolution kinetics theory these data allow calculation of the predicted wall retreat rate for each of the shaft's eight sections. For a direct and independent measurement of wall retreat, micro-erosion pins are set next to each of the wetness indicators. Along with planned dye tracing and development of a surface meteorological station above the shaft complex, analysis of the conditions at these locations is also revealing information about vadose zone hydrology and chemistry.