Order No: FOS 13
Version: 1.1 (December 1996)
RANDOM WALK is a generalized FORTRAN computer code for simulation of two-dimensional ground-water flow and solute transport, written by T.A. Prickett, T.G. Naymik, and C.G. Lonnquist and released in 1981 by the Illinois State Water Survey (ISWS). Ground-water flow is simulated using either analytical solutions or a two-dimensional version of the PLASM finite difference model. The solute transport portion of the code is based on a particle-in-a cell technique for the convective mechanisms and a random-walk technique for the dispersion effects. The model also handles first-order decay, linear equilibrium sorption (retardation), and zero-order production.
RANDOM WALK is a DOS-based program that can simulate two-dimensional nonsteady/steady flow problems in heterogeneous aquifers under water table and/or artesian or leaky artesian conditions. Furthermore, the program covers time-varying pumpage or injection by wells, natural or artificial recharge, the flow relationships between surface water and ground-water, evapotranspiration, conversion of storage coefficients from artisan to water table conditions, and flow from springs. The program allows specification of a wide variety of water quality conditions through specification of chemical species concentrations and solute mass fluxes in any segment of the model domain, and along the model boundaries. This includes injection of solute by wells, leachate entering the aquifer from landfills or surface spills, location of a vertically averaged solute front representing salt water intrusion, leakage of water from overlying source beds with different water quality than the aquifer, and specification of concentrations along surface water boundaries to reflect their water quality. The model supports variable (rectangular) grid cells, various time stepping options, and elaborate output. The program runs in batch mode. Example data sets are provided, which can be copied and edited with a text editor for individual problems. Results and input are send to ASCII text files.
The IGWMC distribution of RANDOM WALK includes RW2D, a menu-driven, user-interactive DOS-based program written in BASIC. RW2D, based on the original version of RANDOM WALK, is a generalized computer code for the simulation of a large class of two-dimensional groundwater flow and solute transport problems. It solves in an uncoupled fashion both the equation for saturated two-dimensional horizontal flow and the advective-dispersive solute transport equation for a single non-conservative constituent. The flow model simulates areal steady-state or transient flow in a heterogeneous, isotropic or anisotropic aquifer under confined, leaky-confined or water-table conditions. The transport model includes terms for convection, dispersion, retardation and first-order decay. Longitudinal and transverse dispersivity and effective porosity are areally variable; halflife (for first-order decay) and retardation (for linear equilibrium sorption) are specified as a single value for the entire model domain. The flow and transport equations are solved using the same numerical approaches as in RANDOM WALK.
RW2D allows for time-varying pumpage/injection and contaminant release, areally-distributed natural or artificial recharge rates, water and contaminant exchange between groundwater and surface water (through boundary conditions, leakage through streambed sections, and mass balance calculations), evapotranspiration from groundwater, conversion from confined storage conditions to water-table storage conditions in time, and flow from springs.
The program also supports specification of solute concentrations in any part of the model, including injection well water, salt-water fronts, leachate from landfills, and leakage from overlying source beds (e.g., polluted upper-aquifer or stream-section). This can be accomplished by using either the initial concentration file option, by specifying the particles introduced by an injection well, or by using the areal particle source feature. In the latter case, the source may be instantaneous or continuous. Up to 2000 particles may be used.
The RW2D code features variable grid sizes (up to 35 *35 cells), various in- and output file options, including separate files for initial values of head and concentration, and time-dependent head and concentration boundary conditions (through the restart option). It calculates cell-averaged head-, velocity- and concentration-distributions, time-varying concentrations in sinks, and flow water and solute balances for total model area and individual boundary cells.
The RW2D program is and excellent educational tool and provides for a quick analysis of small to intermediate sized transport problems with complex geometries.. The program is run from within the RW2D DOS shell, together with the input processor PREWALK and various postprocessing utilities. It displays computational progress and program flow on screen. Output includes location- head/concentration and time-head/concentration files for graphic postprocessing with commercial software.
The distribution of the public domain RANDOM WALK and the proprietary RW2D programs include source codes, DOS-based runtime versions, example data sets, and documentation. The original program documentation as published by the ISWS in 1981 is included.
Intel 80i86 based computer with math coprocessor, 640 Kb RAM, DOS 3.0 or higher, hard drive with about 2 Mb free disk space for code and examples.