The ground-water flow system in the Lower Susquehanna River Basin in Pennsylvania and Maryland can be considered as one complex unconfined aquifer in which secondary porosity and permeability are the dominant influences on the occurrence and flow of ground water. The degree of development of secondary porosity and permeability in the various lithologies of the lower basin determines the aquifer characteristics of each lithology. Based on qualitative evidence, the use of a porous-media model was assumed to be appropriate on a regional scale and a finite-difference ground-water flow model was constructed for the lower basin.

The conceptual model of ground-water flow in the lower basin incorporates the major features of the flow system. Through the use of two layers, 21 hydrogeologic units, and five topographic settings, the conceptual model was systematically reduced to arrive at a simplified conceptual model. Further reduction produced a numerical model representation of the conceptual model, in which the essential features of the lower-basin flow system were quantified for input into the numerical model.

The model was calibrated under both steady-state and transient conditions, and was used to evaluate the water-supply potential of the 21 hydrogeologic units. The carbonate units have the greatest potential for ground-water development and the Triassic sedimentary and crystalline units have the least potential. A total ground-water yield potential of about 900 million gallons per day could be obtained from the lower basin with a consequent 50-percent reduction of base flow in streams.