Numerical Modeling in Karst Aquifers: Examples from Florida and Pennsylvania

The combination of diffuse and conduit flow regimes make karst aquifers extremely dynamic flow systems that are difficult to numerically simulate. It is typically assumed that the fundamental problems associated with simulating karst aquifer flow stem from inadequacies of the available modeling codes and incomplete knowledge of the conduit networks. This paper presents the results of three numerical models constructed for extremely karstified sections of the Floridan aquifer in the Woodville Karst Plain and Santa Fe River Basin and of the Jacksonburg & Epler Formations of eastern Pennsylvania. The models were constructed with the commercially available finite element modeling software FEFLOW™, which provides for a dual-permeability description of flow using porous media equations for the matrix and pipe flow equations for explicitly defined conduits.

In each case, the numerical simulations of groundwater flow were calibrated to available data describing heads, spring discharges, stream flow gains and losses, and tracer-defined conduit flow velocities by modifying the extent, pattern, and size of conduits along with matrix permeabilities during model calibration. The broader significance of these results is three-fold. First, we learned that successful simulation of groundwater flow in karst aquifers is less limited by numerical capabilities or by direct knowledge of conduit patterns than by the explicit inclusion of and/or calibration to hydrologic data such as spring flows, swallet inputs, and tracer-defined velocities. Second, it is clear that where data is lacking, it is critical to include reasoned estimates of individual spring discharges and swallet inflow rates because omitting that data and/or estimates of those values will lead to a dramatically different depiction of groundwater flow patterns and velocities. Finally, the ability of these models to calibrate to multiple lines of hydrologic data demonstrates both the efficacy and utility of numerical modeling in karst aquifers if it is done correctly.