ABSTRACT: 

Variable-density ground water flow models are often used to understand the complex nature of coastal ground water systems.  Hydrodynamic dispersion is a difficult process to represent with a model because of spatially varying hydraulic conductivity and spatially and temporally varying hydrologic boundary conditions.  Temporal variations in water levels, for example, that occur in the field are often time averaged into boundary conditions in a model.  In this study, cross-sectional and three-dimensional models were developed to test the response of a simulated salt water interface to different methods for treating the temporal variability in the ocean boundary condition.  Results indicate that if tidal variations, with an amplitude of approximately 0.6 meters, are averaged over time, simulated concentrations within the interface can be as much as 10 ppt higher in a three-dimensional model, than for the same model that includes the tidal variations.  This observation indicates that for certain aquifers, it may not be appropriate to calibrate a variable-density model at one temporal scale and apply the model to a different temporal scale.