WHY DO QUANTITATIVE GROUNDWATER TRACING? LESSONS AND EXAMPLES FROM THE WOODVILLE KARST PLAIN OF NORTH FLORIDA

Quite simply, quantitative groundwater tracing provides the only means of indirectly measuring hydraulic parameters that can be readily incorporated into groundwater models, which, ultimately provide the most widely used tools watershed characterization. Moreover, quantitative tracing also promises to bridge the gap between tracing and modeling because the resulting hydraulic data can significantly improve model calibrations. We chose the quantitative approach in the Woodville Karst Plain (WKP) for precisely that reason.

The WKP is a broad lowland that extends from just south of Tallahassee, Florida to the Gulf of Mexico, where the Floridan aquifer is either unconfined or poorly confined and contains several large fully saturated conduits, 10-80 m in diameter, approximately 80 m deep, and as much as 15 km long. Water in these conduits flows from local disappearing streams and upland groundwater recharge areas, through numerous karst windows to springs that discharge more than 15 m3/s on average.

Logistical problems with quantitative tracing in the WKP include: large but variable residence times in karst windows, dilution along convergent flow paths, extremely large spring discharges, tannic water in the disappearing streams, and numerous potential pathways. To address these problems, tubing was run into the actual conduit flow paths from the karst windows. The tubes were used for water sampling and, in some cases, injections. Estimated discharges and cave maps were used to calculate the minimum requisite tracer quantities such that breakthrough curves would be easily detectable yet well below the visible concentration.

Tracer breakthrough curves, from more than five tracer tests, have established minimum conduit groundwater velocities of between 800 and 6000 m/day over minimum flow paths of up to 16 km long. Data based on between 20% and 90% mass recoveries have yielded longitudinal dispersivities of between 12 and 75 m, Reynolds numbers of between 185,000 and 750,000, and Peclet numbers of around 250. These data represent the first quantitative description of conduit flow in the WKP and are now being used to construct and more precisely calibrate groundwater models that effectively address flow to and flow along the known saturated conduits in the basin. See www.hazlett-kincaid.com/FGS for further details.