EXCHANGE OF WATER BETWEEN CONDUIT AND MATRIX POROSITY IN EOGENETIC KARST? EVIDENCE FROM A QUANTITATIVE DYE TRACE

In the eogenetic Upper Floridan Aquifer, high intergranular porosity (20-40%) and matrix permeability (10^-12 – 10^-14 m²) may contribute to the aquifer's hydraulic diffusivity (transmissivity/storativity), flow paths, and mass transport. To assess the exchange of water between intergranular and conduit porosity in the Upper Floridan Aquifer, we conducted a quantitative dye trace in the lower portion of the Santa Fe River Sink-Rise system in north-central Florida. At base-flow conditions, we injected 18.14 kg (40 lbs) of 20% Rhodamine WT solution (3.63 kg active ingredient) into a karst window called Sweetwater Lake, which connects to a first magnitude spring, the River Rise, via a single conduit previously mapped by cave divers. Fluorescence of river water, river stage, and water elevation of three monitoring wells were monitored continuously over an 88 hour period. The monitoring wells are located 30 to ~890 m from the conduit. River stage was monitored 70 m downstream of the spring and converted to discharge with a well-established rating curve. Monitoring of fluorescence began ~16 hours prior to release of the dye and showed average background values of ~0.125 ppb with a standard deviation of 0.02. The initial breakthrough of dye occurred 13 hours and 58 minutes, peak dye concentration of 6.97 ppb occurred 20 hours and 2 minutes, and a return to background of ~0.1 ppb occurred 48 hours following injection of dye. Shape of the breakthrough curve and rapid return to background fluorescence reflect conduit flow with minimal dispersion of dye. Measured fluorescence in the monitoring wells is likely background, although an increase from 0.07 to 0.380 ppb at one well down gradient from the conduit 2 days after injection of the dye may reflect loss of dye from the conduit to the matrix porosity. Calculated dye mass return at the River Rise was 5.33 kg, which is 46% more dye than injected. This discrepancy may result from analytical errors in measurements of discharge or dye concentrations, a source of discharge in addition to the River Rise or a combination of these factors. The magnitude of the excess dye suggests dye-free water may be included in the discharge measurements. These results suggest that there is little loss of water to the matrix from this single conduit at base-flow conditions.