REALISTIC NUMERICAL MODELING OF GROUND-WATER FLOW BASED ON QUANTITATIVE SITE CHARACTERIZATION IN THE WOODVILLE KARST PLAIN OF NORTH FLORIDA

Inadequacies of standard numerical modeling methods in karst aquifers are widely recognized. A less recognized but critical insight is that the fundamental modeling limitations can be overcome by incorporating sound quantitative site characterization data with proven finite element and probabilistic modeling techniques. Our work in the Woodville Karst Plain (WKP) of North Florida has shown that data obtained through quantitative groundwater tracing can be used to construct and more precisely calibrate finite element numerical groundwater models that effectively address flow to and flow along saturated conduits.

The WKP is one of the best documented karst basins in the world containing more than 40 km of mapped underwater caves that converge on springs with average discharges of more than 15 m3/s. Data obtained from quantitative groundwater tracing and hydrologic meters in the caves have established flow paths, minimum conduit groundwater velocities of between 800 and 6000 m/day, over flow paths up to 16 km long, longitudinal dispersivities of between 12 and 75 m, Reynolds numbers of between 185,000 and 750,000, and Peclet numbers of around 250.

Where flow paths that have not been confirmed through groundwater tracing, preferential pathways have been simulated with a statistical model (KARSTMOD) based on the gradient between inputs and spring discharges. The model further predicts spring hydrographs and ultimately, the break-through curve for contaminants, given precipitation records and the location and timing of contamination. The model is currently being calibrated against storm/discharge responses recorded by rain gauges in the basin and flow meters installed at various points in Wakulla cave.

Preliminary modeling results have shown significant improvements in calibration precision and average travel time estimates that are several orders of magnitude faster than those predicted by “standard” numerical models. Although this work would not be possible but for the exhaustive quantitative characterization and instrumentation efforts in the WKP, the underlying purpose of this effort is to develop streamlined characterization, estimation, and modeling protocols that can be applied in other karst basins throughout Florida and the world. See www.hazlett-kincaid.com/FGS for further details.