VERTICAL SHAFT HYDROLOGY AND EVOLUTION 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, calcite saturation indices, and dissolution rates at Shower Bath Spring, a shaft in the cave's Frozen Niagara area. Shaft water chemistry there was found to be controlled by a mix of relatively high CO₂ storm water and low CO₂ diffuse waters (at times less than atmospheric background) 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.

The third phase is currently in the early stages at the Edna’s Dome shaft complex. Flow and water chemistry parameters from both free falling and wall film water are being measured with two-minute resolution, either directly (spC, pH, and temperature) or indirectly (relating spC to directly analyzed key ions). Within a single bed around the shaft's perimeter, eight evenly spaced stations have paired electronic wetness indicators and micro-erosion pins. With limestone dissolution kinetics rate expressions these data allow calculation of the predicted wall retreat rate for each of the shaft's eight sections, which will then be compared with the direct and independent measurements using the erosion pins.

Preliminary results indicate a good linear fit between spC and bicarbonate (r²=0.81 for the shaft waters) but much lower for the calcium (r²=0.27)—sulfate concentrations (x=10.8 mg/L) may indicate the presence of gypsym contributing an additional source of calcium adding complexity to the relationship. From directly measured samples, fluctuations in parameters (as measured by coefficients of variation(CV)) range from about 9% for spC to 19% for calcium. Water temperatures are also equilibrated to the aquifer temperature by the time they reach the shaft, with a CV of only 3% through the winter of 2001-02.