MICROBIAL ECOLOGY OF CONDUIT STREAM SEDIMENT INTERSTITIAL FLUIDS OF THE SOUTH CENTRAL KENTUCKY KARST AQUIFER

Previous study of interstitial fluid geochemistry within the sediments beneath Charon’s Cascade in the Echo River/River Styx area of the Mammoth Cave System found that carbon dioxide pressures were as much as an order of magnitude higher than the fluids of the cave stream itself. Modeling of both equilibrium chemistry and kinetics of limestone dissolution showed that fluids were undersaturated with respect to calcite, and that dissolution rates were correspondingly high. This was confirmed by a limestone weight loss experiment, in which samples readily dissolved at various levels (15, 30, 60 and 90 cm) below the streambed, in spite of the low fluid velocities there. The high CO₂ pressures and relatively low pH's of the sediment interstitial fluids thus appear to influence conduit dissolution rates. An obvious question relates to the source of the elevated CO₂ pressures, which presumably results from the microbial degradation of organic material within the sediments.

To explore the relationship between the geochemical environment of these fluids and microbial ecology, additional fluid and sediment samples were collected in February 2000 from the same location beneath Charon’s Cascade. Eight Coliform bacteria that had the ability to grow at 12^oC were identified to species level. Six were members of Enterobacteriaceae. Many bacteria, especially members of the family Enterobacteriaceae, carry on mixed acid fermentation which results in the excretion of a complex mixture of acids and the production of carbon dioxide and hydrogen gas, and thus may impact limestone dissolution and thus aquifer evolution. These 8 isolates were inoculated into 65 ml of thioglycollate broth with a known weight of calcite and incubated at 12^oC for 92 days. Due apparently to the fluid/rock interaction, the control sample dissolved at a rate of 45 gm^-2yr^-1. Calcite in the presence of five of the incubating bacteria species dissolved more than the control, ranging up to 181 gm^-2yr^-1 for Escherichia Coli. Although all media began the experiment with a pH of 7.0, final pH’s of all calcite and bacteria containing samples ranged from 8.5 to 8.8. Since the calcite/fluid systems were not stirred, we assume that dissolution rates are transport-rate controlled.

These preliminary results suggest that in typical southeastern U.S. cave environments acid-producing bacteria within cave sediments may influence limestone dissolution and thus cave and aquifer development. Further experiments are currently underway to better understand the relationships between microbial ecology and biochemistry, limestone dissolution kinetics, and karst aquifer evolution under geologically reasonable conditions. These include dissolution experiments with Ste. Genevieve Limestone, and culturing bacteria from other saturated stream sediments within the aquifer, including pristine locations.