Home Introduction Hydrogeology Simulation of GW Discharge -Equations -SEAWAT -X-Sxn. Model -Regional-Scale Model -Model Limits Conclusions References Appendix 1 Appendix 2 Appendix 3 Appendix 4 Plates PDF Version

Simulation of Ground-Water Discharge to Biscayne Bay

SEAWAT Simulation Code

The original SEAWAT code was written by Guo and Bennet (1998) to simulate ground-water flow and saltwater intrusion in coastal environments. SEAWAT uses a modified version of MODFLOW (McDonald and Harbaugh, 1988) to solve the variable-density, ground-water flow equation and MT3D (Zheng, 1990) to solve the solute-transport equation. To minimize complexity and run times, the SEAWAT code uses a one-step lag between solutions of flow and transport. This means that MT3D runs for a timestep, and then MODFLOW runs for the same timestep using the last concentrations from MT3D to calculate the density terms in the flow equation. For the next timestep, velocities from the current MODFLOW solution are used by MT3D to solve the transport equation. For most simulations, the one-step lag does not introduce significant error (C.I. Voss, U.S. Geological Survey, oral commun., 1999), and the error can be reduced or evaluated by decreasing the length of the timestep.

One major reason the SEAWAT code was selected for this study is that it uses MT3D to solve the transport equation. MT3D contains a variety of methods for solving the transport equation including the method of characteristics (MOC), modified method of characteristics (MMOC), hybrid method of characteristics (HMOC), and a standard finite-difference method. During the simulation of solute transport, numerical dispersion and other problems often are encountered. Because MT3D has a variety of solution techniques, including MOC, which is ideal for reducing numerical dispersion, an acceptable solution can usually be obtained.

Another advantage for using SEAWAT is that it uses two widely accepted modeling codes: MODFLOW and MT3D. This means that SEAWAT is modular and contains the "package" approach for including various boundary conditions and transport options. Additionally, SEAWAT reads and writes standard MODFLOW and MT3D data files, which are easily manipulated with the commercially available pre- and post-processors. These pre- and post-processors can substantially reduce the length of time required to create input data sets and evaluate model results.

The original SEAWAT code (Guo and Bennet, 1998) was modified for this study. Documentation for the new version of the code (Version 2) is currently in preparation. A general description of the improvements is presented in Langevin and Guo (1999). The most significant modification is the incorporation of an updated flow equation that conserves mass (as written in eq. 12) instead of volume. Additionally, the boundary fluxes were updated to use variable-density forms of Darcy's law. This modification allows boundary conditions to contain variable-density source waters. SEAWAT also has an option to use a modified algorithm for evapotranspiration, which allows evapotranspiration to be withdrawn from the highest active layer.

Because SEAWAT is a relatively new code, it was verified by running three test problems and comparing the simulated results with results from other codes and analytical solutions. The comparisons are presented in appendix IV.

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