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Order No: FOS 51
Version: 2.8d (July 1995)
Price: $250 **

The program 'FTWORK' is a block-centered finite difference model for simulating one-, two- and three-dimensional steady-state and transient flow and transient single species solute transport in saturated media under confined and unconfined conditions. The model supports both areal and cross-sectional twodimensional simulations. The flow equation is posed in terms of hydraulic head, the transport equation in terms of concentration. The model handles heterogeneities and anisotropy for flow (hydraulic conductivity tensor aligned with cartesian coordinate axes). The transport equation includes advection, hydrodynamic dispersion, first order decay (chemical, biological or radioactive), and linear and non-linear equilibrium adsorption. Cross product terms for dispersion can be included in the transport calculations. The model includes a parameter estimation option (semi-automatic history matching) of the steady-state flow equation, using a Gauss-Newton non-linear least-squares optimization technique with a Marquardt correction. This option may be used to estimate hydraulic conductivity and recharge.

The model supports variable grid block lengths in X-, Y-, and Z-direction and deformed coordinate approximation for variable thickness layers. Boundary conditions include prescribed head, prescribed concentration, prescribed flux of water (e.g. recharge) or solute mass, and head-dependent flux (e.g., for leakage to/from streams, to drains). It also handles single and multi-aquifer wells and chemical sources and sinks. The model can be used in a quasi-three dimensional mode.

FTWORK has an option to use either central or upstream weighting of the advection term and central or backwards weighting of the time derivative. Three different solvers are included for different types of problems based upon the Slice Successive Over-Relaxation (SSOR) method.

Currently, the program does not have a preprocessor. Input is developed by copying an example data set and creating the new input file with a text editor. The program has many output options, including the creation of a main output file containing a summary of the input data, convergence error, array data (head and/or concentration, Darcy velocity, and saturation index), and, if parameter estimation is performed, summary statistics and parameter multipliers, and residuals. Other (optional) output files include MODFLOW-type head and concentration distribution for graphic postprocessing, sensitivity coefficient generated in the parameter estimation procedure, computed heads and/or concentrations as function of time for selected nodes, head residual, head and/ or concentration at end of simulation to be used as initial conditions for a subsequent run, and a cell-by-cell flux file.

FTWORK is distributed on HD DOS formatted diskettes containing source code, executable image, and example data sets. Documentation include installation instructions, theory, user's manual, and test and example problems (complete with input and output).

**SYSTEM REQUIREMENTS**

Intel 80386/80486 based computer (i486 recommended); at least 11 Mb RAM to run program; math coprocessor; DOS 5.0 or higher; CGA (no graphics in program); at least 2 Mb free disk space to install program and example file; to run all examples requires about 17 Mb free disk space.

*Developers: C.R. Faust, et al (GeoTrans, Inc.)
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