TWRI Book 6, Chapter A7


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Guo, Weixing, and Langevin, C.D., 2002, User's Guide to SEAWAT: A Computer Program for Simulation of Three-Dimensional Variable-Density Ground-Water Flow: Techniques of Water-Resources Investigations Book 6, Chapter A7, 77 p. (Supersedes OFR 01-434.)

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.

TABLE OF CONTENTS

Abstract
Chapter 1: Introduction
Purpose and Scope
Development of SEAWAT
Acknowledgments
Chapter 2: Mathematical Description of Variable-Density Ground-Water Flow
Basic Assumptions
Concept of Equivalent Freshwater Head
Governing Equation for Ground-Water Flow
Darcy's Law for Variable-Density Ground-Water Flow
General Form of Darcy's Law
Assumption of Axes Alignment with Principal Permeability Directions
Darcy's Law in Terms of Equivalent Freshwater Head
Governing Equation for Flow in Terms of Freshwater Head
Governing Equation for Solute Transport
Boundary and Initial Conditions
Dirichlet Boundary
Neumann Boundary
Cauchy Boundary
Initial Conditions
Sink and Source Terms
Concentration and Density
Chapter 3: Finite-Difference Approximation for the Variable-Density Ground-Water Flow Equation
Finite-Difference Approximation for the Flow Equation
Construction of System Equations
Chapter 4: Design and Structure of the SEAWAT Program
Temporal Discretization
Explicit Coupling of Flow and Transport
Implicit Coupling of Flow and Transport
Structure of the SEAWAT Program
Packages
Array Structure and Memory Allocation
Chapter 5: Modifications of MODFLOW and MT3DMS
Matrix and Vector Accumulators
Modifications of the Basic Flow Equation
Addition of Relative Density-Difference Term
Addition of Solute Mass Accumulation Term
Conversion from Volume Conservation to Mass Conservation
Conversion from Fluid Volume Storage to Fluid Mass Storage
Conversion Between Confined and Unconfined Conditions
Vertical Flow Calculation for Dewatered Conditions
Variable-Density Flow for Water-Table Case
Modifications of MODFLOW Stress Packages
Well (WEL) Package
River (RIV) Package
Drain (DRN) Package
Recharge (RCH) Package
Evapotranspiration (EVT) Package
General-Head Boundary (GHB) Package
Time-Varying Constant Head (CHD) Package
Modification of MODFLOW Solver Packages
MODFLOW-MT3DMS Link Package and Modifications to MT3DMS
Chapter 6: Instructions for Using SEAWAT
Preparation of MODFLOW Input Packages for SEAWAT
Basic (BAS) Package
Output Control (OC) Option
Block-Centered Flow (BCF) Package
Well (WEL) Package
Drain (DRN) Package
River (RIV) Package
Evapotranspiration (EVT) Package
General-Head Boundary (GHB) Package
Recharge (RCH) Package
Time-Varying Constant Head (CHD) Package
Solver (SIP, SOR, PCG) Packages
Preparation of MT3DMS Input Packages for SEAWAT
Basic Transport (BTN) Package
Advection (ADV) Package
Source/Sink Mixing (SSM) Package
Running SEAWAT
Output Files and Post Processing
Calculation of Equivalent Freshwater Head
Tips for Designing SEAWAT Models
Chapter 7: Benchmark Problems
Box Problems
Case 1
Case 2
Henry Problem
Elder Problem
HYDROCOIN Problem
References Cited

FIGURES:

1. Schmatic showing two piezometers, one filled with freshwater and the other with saline aquifer water, open to the same point in the aquifer
2. Diagram showing representative elementary volume in a porous medium
3. Schematic showing relation between a coordinate system aligned with the principal axes of permeability and the upward z-axis
4. Generalized flow chart of the SEAWAT program
5. Schematic showing example of the explicit scheme used to couple the flow and transport equations
6. Schematic showing example of the implicit scheme used to couple the flow and transport equations
7. Flow chart showing step-by-step procedures of the SEAWAT program
8. Schematic showing cell indices and variable definitions for the case of a partially dewatered cell underlying an active model cell
9. Schematic showing conceptual representation of flow between two cells for the water-table case
10. Diagram showing conceptual model and variable description for river leakage in MODFLOW and SEAWAT
11. Diagram showing conceptual model and variable description for drain leakage in MODFLOW and SEAWAT
12. Grid showing boundary conditions and model parameters for the Henry problem
13. Graphs showing comparison between SEAWAT and SUTRA for the Henry problem
14. Grid showing boundary conditions and model parameters for the Elder problem
15. Finite-difference grid used to simulate the Elder problem
16. Schematics showing comparison between SEAWAT, SUTRA, and Elder's solution for the Elder problem over time
17. Grid showing boundary conditions and model parameters for the HYDROCOIN problem
18. Graph showing comparison between SEAWAT and MOCDENSE for the HYDROCOIN problem

TABLE

1. MODFLOW and MT3DMS packages used in SEAWAT