| In karst regions, groundwater flow and pollutant transport occur in two modes: fast-response flow in cave passages and slow-response flow in the aquifer-matrix. This thesis presents a computationally efficient numerical model that simulates the behavior of the two flow and transport modes.; A finite-difference method is used to discretize all governing equations. First, the fast-response flow in a network of cave passages is mathematically modelled by means of the full unsteady hydrodynamic equations and solved numerically using the Preissmann method. The slow-response flow in the aquifer-matrix is modelled by an unsteady Darcy equation and solved numerically using a fractional-step approach. These two equations are iteratively coupled through an exchange term reflecting the water exchange between cave passages and the aquifer-matrix. Next, the pollutant transport equations in the network of cave passages is solved using a characteristic method and in the aquifer-matrix using a fractional-step approach. These two pollutant transport equations are solved iteratively through an exchange term reflecting the pollutant exchange between cave passages and the aquifer-matrix.; Sensitivity analysis on important parameters affecting groundwater flow and pollutant transport in karst regions shows that the equivalent diameters of the cave passages and their distribution in the aquifer-matrix are the most important parameters of the model, provided the topology of the cave passages is fixed.; The model is capable of simulating dye trace experiments performed in Iowa's Big Spring basin. Time travel of two dye trace experiments from two different sinkholes is simulated. The results show that, for a fixed topology and given boundary conditions, only a narrow range of equivalent diameters of cave passages, about 1.5 ft, can simulate the correct travel time of the dye.; At the Big Spring basin, for a nondiffusive system, i.e., karst formations with large equivalent diameters of cave passages, it is possible to predict the flow at Big Spring using a reservoir-type formulation, thereby avoiding the complexity of a detailed mathematical formulation that considers all water pathways. The pathways are important for pollutant transport and the reservoir formulation is not adequate to determine the concentration of pollutants in the Big Spring flow. |
