Suspended Sediment Grain Size Distributions as Indicators of Sediment Sources and Karst Aquifer Hydrodynamics in Logsdon River, Mammoth Cave, Kentucky, USA

Flow rate, water chemistry and suspended sediment concentrations and particle size distributions were measured in Logsdon River, a base level karst conduit that drains approximately 25 km² within the Mammoth Cave system. These observations provided information on the dynamics and spatial pattern of storm period fine sediment inputs from the surface and on the controls on mobilization and transport of in-cave sediment.

Analyses of several flow events reveal that suspended sediment concentrations and particle size distributions display a complex but consistent relationship to precipitation inputs, conduit flow hydraulics, and hydrochemical variations. There is generally a peak in turbidity and suspended sediment concentration that corresponds to peak stage and flow rate, flowed by a secondary turbidity peak on the falling limb of the hydrograph. The magnitude of the initial peak in sediment concentration scales with peak flow rate, whereas the magnitude of the secondary turbidity peak is poorly correlated with flow rate, reflecting either return of conduit-adjacent storage of turbid storm water or inputs of finer suspended solids from distal surface inputs to the aquifer.

The concentration of sand sizes (>63 μm) tends to track flow rates closely, suggesting that coarse particle suspension is a function of local fluid turbulence and upstream supplies of sand in the conduit system. The finer fractions display single and multimodal size distributions that appear to reflect patterns of surface and in-cave fine sediment mobilization, variations in sediment source size distributions, and variations in suspension and sedimentation within the karst conduit network. It is possible that continuous monitoring of grain size distributions with laser diffraction or other technologies may provide new insights into the internal flow and sediment dynamics of karst aquifers. These techniques also have significance for understanding and predicting spatial and temporal patterns of contaminant transport in karst systems.