WATER AND SUSPENDED SOLIDS FLUX MONITORING IN A KARST CONDUIT, TURNHOLE SPRING BASIN MAMMOTH CAVE, KENTUCKY

Water and suspended solids fluxes were measured in Logsdon River, a karst conduit within the Turnhole Spring Basin. Logdson River drains approximately 25 km2. Turnhole Spring drains approximately 240 km2 and discharges into the Green River within Mammoth Cave National Park. River stage and flow velocity were monitored continuously with an upward-looking acoustic Doppler current profiler. Turbidity, water temperature, pH, and specific conductivity were recorded continuously with a multi-parameter water quality sonde. In addition, a laser-scattering suspended sediment sensor (LISST 25x, Sequoia Scientific, Inc.) provided measurements of suspended solids concentration and grain size. Suspended solids concentrations were estimated based on the LISST concentration estimates and on empirical correlations between turbidity and measured sediment concentrations.

Analysis of several flow events reveal that peaks in suspended solids concentration generally lag behind increases in stage and discharge, suggesting that much of the finer suspended material is derived from surface inputs to the system which require sufficient time to reach the observation point. Decreases in specific conductivity correlate with increases in turbidity and suspended solids concentration, supporting the interpretation that the peaks in suspended solids concentration correspond to the arrival of surface water at the monitoring site. This lag in suspended solids flux relative to discharge variation is consistent with relatively rapid transmission of increases in hydraulic head through phreatic portions of the karst aquifer, and contrasts with typical surface water catchment responses in which suspended solids concentration peaks precede peak flow rates. Variations in the concentration of sand sizes (>63 micron), tend to track flow rates more closely, suggesting that the transport of these coarser fractions is dependent on local hydraulics and local supplies of sand in the karst conduit.