Groundwater Flow in the Edwards Aquifer: Comparison of Groundwater Modeling and Dye Trace Results

The Edwards Aquifer of central Texas is a karst aquifer developed in faulted and fractured Cretaceous-age limestones and dolomites. Numerous groundwater models have been developed for the three segments of the Edwards Aquifer since completion of the first major flow model in 1979. Groundwater models have helped refine our understanding of the relationships among flowpaths, recharge, groundwater pumping, and springflow. Tracer test studies have been performed on the Edwards Aquifer since 1982. These studies have also brought about a better understanding of aquifer flowpaths, yet the results of groundwater modeling and dye trace studies do not have a high level of agreement. A comparison of the two types of studies has indicated the strengths and weaknesses of each method. Groundwater models of the Edwards Aquifer have been effective in simulating spring discharge and determining water budgets. The models have simulated water levels reasonably well, but there are significant discrepancies between measured and simulated water levels in areas that are more strongly influenced by conduit flow. Tracer testing is the best method for measuring rates of flow from a recharge feature, or a well, to springs and other wells. However, tracer tests provide little useful information about water levels or water budgets. Traditional groundwater models are poor tools for simulating contaminant transport and delineating areas for source-water protection of mature karst aquifers with well-developed conduit networks.