User's guide to SEAWAT; a computer program for simulation of three-dimensional variable-density ground-water flow

Abstract

The SEAWAT program was developed to simulate three-dimensional, variable-density, transient ground-water flow in porous media. The source code for SEAWAT was developed by combining MODFLOW and MT3DMS into a single program that solves the coupled flow and solute-transport equations. The SEAWAT code follows a modular structure, and thus, new capabilities can be added with only minor modifications to the main program. SEAWAT reads and writes standard MODFLOW and MT3DMS data sets, although some extra input may be required for some SEAWAT simulations. This means that many of the existing pre- and post-processors can be used to create input data sets and analyze simulation results. Users familiar with MODFLOW and MT3DMS should have little difficulty applying SEAWAT to problems of variable-density ground-water flow. MODFLOW was modified to solve the variable-density flow equation by reformulating the matrix equations in terms of fluid mass rather than fluid volume and by including the appropriate density terms. Fluid density is assumed to be solely a function of the concentration of dissolved constituents; the effects of temperature on fluid density are not considered. Temporally and spatially varying salt concentrations are simulated in SEAWAT using routines from the MT3DMS program. SEAWAT uses either an explicit or implicit procedure to couple the ground-water flow equation with the solute-transport equation. With the explicit procedure, the flow equation is solved first for each timestep, and the resulting advective velocity field is then used in the solution to the solute-transport equation. This procedure for alternately solving the flow and transport equations is repeated until the stress periods and simulation are complete. With the implicit procedure for coupling, the flow and transport equations are solved multiple times for the same timestep until the maximum difference in fluid density between consecutive iterations is less than a user-specified tolerance. The SEAWAT code was tested by simulating five benchmark problems involving variable-density ground-water flow. These problems include two box problems, the Henry problem, Elder problem, and HYDROCOIN problem. The purpose of the box problems is to verify that fluid velocities are properly calculated by SEAWAT. For each of the box problems, SEAWAT calculates the appropriate velocity distribution. SEAWAT also accurately simulates the Henry problem, and SEAWAT results compare well with those of SUTRA. The Elder problem is a complex flow system in which fluid flow is driven solely by density variations. Results from SEAWAT, for six different times, compare well with results from Elder's original solution and results from SUTRA. The HYDROCOIN problem consists of fresh ground water flowing over a salt dome. Simulated contours of salinity compare well for SEAWAT and MOCDENSE.