Improvements to SEAWAT and Applications of the Variable-Density Modeling Program in Southern Florida

By coupling two commonly used computer programs, MODFLOW (McDonald and Harbaugh, 1988) and MT3Dms (Zheng and Wang, 1998), the SEAWAT program was developed to simulate variable-density, transient ground-water flow in three dimensions. Freshwater head and salt concentration are used as the two principal dependent variables. Major improvements from the previous version of SEAWAT include (1) reformulation of the flow equation based on the conservation of mass, (2) modification of stress packages to allow variable-density sources and sinks, and (3) incorporation of an implicit coupling procedure between the flow and solutetransport equations. These improvements produce more accurate simulation results and extend the applicability of the code to a wider range of hydrogeologic problems. The SEAWAT program has been widely used in southern Florida by the U.S. Geological Survey and CDM-Missimer International to investigate coastal ground-water flow processes. Seven applications have been completed or are in progress and pertain to the following types of hydrogeologic problems: availability of brackish ground water, saltwater intrusion, surface water and ground-water interactions, submarine ground-water discharge, and effectiveness of salinity barriers. For each application, the SEAWAT program was successfully used to investigate the effects of fluid density variations on ground-water flow rates and patterns.