NUMERICAL SIMULATION OF THE MAMMOTH CAVE AQUIFER, KENTUCKY
A numerical simulation of flow in the aquifer was performed using the popular MODFLOW code. The first realization assumes that the aquifer behaves as a homogeneous EPM. Model calibration gives a hydraulic conductivity (K) of 1.1 x 10-3 m/s, with a mean absolute error of 12 m for heads. The model behaves poorly in several key aspects: discharge at the major spring is underestimated by 94% and tracer velocities are underestimated by two to three orders of magnitude. The second realization assumes that there are tributary conduit networks discharging at springs. The conduits are modeled as lines of high K cells. The calibrated non-conduit K of 4 x 10-5 m/s is midway between slug test and pumping test values, and conduit K ranges over four orders of magnitude. There is a good fit to water level data in both wells and cave streams and discharge to springs is accurately simulated. However, it is not possible to accurately model groundwater velocities.
The problem with using the EPM hypothesis for this aquifer is that the assumption that karst features are only of local scale is incorrect. Instead, an interconnected network of high-permeability channels extends over the scale of the aquifer, which consequently does not behave as an EPM.