A Numerical Model of Conduit-Controled Groundwater Flow in the Floridan Aquifer

A numerical model of conduit-dominated groundwater flow through the western Santa Fe River Basin, Florida was developed using FEFLOW™ where conduits were described as discrete features embedded in a porous matrix. A 381 km network of saturated conduits was delineated through calibration to heads and spring discharges wherein the location, size, and roughness of the conduits were the main calibration variables.

Two calibration datasets were compiled from 30 years of data describing average high- and average low-water conditions based on heads at 188 wells and discharge from 18 springs. The model was iteratively calibrated to the two datasets wherein the location, size and roughness of the conduits were assigned as needed to accurately simulate observed heads and spring discharges while bounding simulated velocities by tracer test results. The final model calibrated to within 5% of the total head change across the model region at 99% and 94% of the 188 wells in the high-water low-water scenarios respectively. Simulated spring discharges fell within 13% and 100% of the observed range in the high-water and low-water scenarios respectively. Simulated velocities ranged from as low as 10^-4 m/day in the matrix to as high as 10⁺³ m/day in the largest conduits.

This model produces a more plausible simulation of groundwater flow than existing porous media models of the same region and is significantly better calibrated. These results indicate that conduit flow can be simulated through numerical modeling and that doing so produces more reliable simulations. Lessons learned included: 1) calibration datasets need to be expanded to include all data that can be shown to be indicative of similar water level conditions through well-well regressions; and 2) data synthesis efforts need to be expanded to include swallet inflows, spring discharges, and conduit patterns even if the information is based on estimates and anecdotal descriptions.