WARM SEASON, STORM-SCALE HYDROCHEMICAL VARIATIONS IN LOGSDON RIVER, MAMMOTH CAVE SYSTEM, KENTUCKY

Logsdon River, a major underground stream within south-central Kentucky's Mammoth Cave System, drains the 25 km² Cave City Basin. High resolution (10 minute) data on temperature, pH, specific conductance (spC), stage, turbidity (multiparameter water quality sonde), discharge (acoustic Doppler profiler), and sediment flux (Laser In-Situ Scattering Transmissometry) are being collected from a new site one km from the river's downstream end. Here we focus on carbonate chemistry and dissolution kinetics signals from a storm in late May 2006, derived from the direct water chemistry data and previously developed relations between spC and appropriate ions (Ca, HCO₃, and Mg) from Logsdon River. Results show that storm responses create rapid changes in both equilibrium chemistry and kinetics that have in-phase peaks but varying relaxation rates. Pre-storm conditions of the chemical signatures (spC, PCO₂, calcite saturation index (SI_cal), and calcite dissolution rate) were not achieved by the time of the next storm event, about six days later. With the flush of storm water reaching the measurement site, spC dropped by almost 50% (375 to 200 μS/cm), PCO₂ increased from about 3 to 12 times the atmospheric background level, SI_cal levels dropped from +0.6 to -0.7, and associated with that drop, predicted dissolution rates changed from an oversaturated state of about 3.5 mm/yr of precipitation to about 1 mm/yr of dissolution during the signal peak. During the same period pH dropped from over 8.0 to below 7.3, associated with increased CO₂ pressures, and to a lesser degree, dilution. A lag in the chemical signature peak following the stage peak may be explained by totally water-filled segments of the conduit upstream from this location, previously explored by divers, which may propagate the stage signal almost instantaneously through the flooded sections, in a “u-tube” type effect, while the actual storm water takes longer to travel through the same flooded sections.