KARST AQUIFER CONDUIT SEDIMENT INTERSTITIAL FLUIDS AND AQUIFER EVOLUTION

Rates and spatial patterns of carbonate rock dissolution control the single most important feature of carbonate karst landscape/aquifer systems: the capacity of an aquifer to transmit fluids within the subsurface. Influencing these rates and patterns, within many karst aquifers blankets of non-carbonate, clastic sediment cover the floors of active stream conduits within which dissolution is focused, masking the bedrock below. This is especially common the southeastern US, where major aquifers form within Mississippian limestones overlain by, and which drain, clastic rocks such as in the Cumberland and Mammoth Cave Plateaus. There has been virtually no quantitative study of the chemistry and flow of such substream interstitial fluids. The purpose of this research has been to quantitatively evaluate interstitial fluid chemistry and water/rock interactions within the stream sediments below a perennial cave stream within Kentucky's humid-subtropical Mammoth Cave System.

Carbonate chemistry was monitored within the stream (~5 m downstream from Charon's Cascade in the Historic section of the cave system), and within the sediment 15, 30, 60, and 90 cm below the streambed. Carbon dioxide pressures within the interstitial fluids were elevated as much as an order of magnitude above those in the stream waters, having levels as high as 31 times atmospheric background. Interstitial fluids were all undersaturated with respect to calcite (SI=-0.4 to -0.9), and 32 limestone blocks buried at the four levels in the sediment for 110 days all dissolved with rates ranging from 0.8 to 21.9 gm^-2yr^-1 (corresponding to wall retreat rates of 3x10^-4 to 8x10^-3 mm/yr). In the stream itself, although the waters tended to be closer to thermodynamic equilibrium (SI=-0.3 to -0.5), measured dissolution rates were higher (11.6 to 39.5 gm^-2yr^-1, or 4x10^-3 to 1x10^-2 mm/yr). A reasonable explanation is that different rate-limiting mechanisms associated with dissolution kinetics (surface reaction rates in the stream and boundary layer diffusion rates within the sediment), control overall dissolution rates.

Interstitial fluids are thus shown to be capable of dissolving limestone. This may impact both conduit growth rates and geometries, especially within vadose passages that have free-surface streams flowing across passage floors, since interstitial fluids are constantly in contact with the limestone at the sediment/bedrock interface, as well as buried breakdown blocks within the sediment. This is in contrast to the walls and ceilings of vadose passages which, while not masked by sediment, only dissolve under relatively infrequent flood conditions.

In order to understand controls on the geochemical environment of these interstitial fluids, our research group is now undertaking study of microbial ecology and function of the Charon's Cascade interstitial fluids using direct culturing methods and 16S rRNA gene sequencing. We are also expanding to other locations throughout the aquifer. These methods allow description of microbial ecology as well bacterial functions in the direct and indirect production of acids that appear to influence aquifer evolution.