During the past several years, groundwater has become a subject of great concern to the public. Unfortunately, this attention is the result of many serious subsurface pollution problems. The need to manage or clean up these pollution aquifers has created a great demand for methods or tools to accurately predict groundwater flow and pollutant transport, and how they will change in response to imposed stresses. Bear and Verruijt (p. xi) state, “The necessary information about the response of the system is provided by a model that describes the behavior of the considered system in response to excitation.”