PHAST (PHREEQC And HST3D)

Criterion	Explanation
General Description	PHAST simulates multi-component, reactive solute transport in three-dimensional saturated groundwater flow systems.
Model Domain	Subsurface saturated zone
Developer	US Geological Survey
Hardware computing requirements	Can be run on most computer systems, including computers with Windows, Linus and Unix OS. PHAST may require large amounts of memory for execution (primarily dependent upon the number of nodes defined for the simulation grid). Run times may be long on single-processor machines. This is greatly reduced with the parallel (multiprocessor) version of PHAST, which requires libraries for MPI.
Code language	Written in FORTRAN-90, C, and C++
Original applications	PHAST is a versatile groundwater flow and transport simulator that models a wide range of equilibrium and kinetic geochemical reactions. Applicable from laboratory-scale experiments to regional field scales. Used for studies in: Migration of nutrients, inorganic, and organic contaminants, and radionuclides; Aquifer storage and recovery or engineered remediation; Natural rock-water interactions in aquifers. Not used for unsaturated-zone flow, multiphase flow, or density-dependent flow.
Public/proprietary and cost	Public, No cost
Physically or empirically based	Physically based
Mathematical methods used	The governing equations for a reactive solute-transport simulator are a set of partial differential equations describing groundwater flow and solute transport for each aqueous component, a set of nonlinear algebraic equations describing equilibrium chemical reactions, and a set of ordinary differential equations describing rates of kinetic chemical reactions. Flow and transport calculations are restricted to constant fluid density and constant temperature, based on a modified version of HST3D. The general saturated groundwater flow and component solute-transport equations solved by the PHAST simulator are based on those of the HST3D simulator. Combined flow, transport, and geochemical processes are simulated by three sequential calculations for each time step: Flow velocities calculated Chemical components transported Geochemical reactions calculated The definition of leaky and flux boundary conditions select only exterior cell faces with a specified volume of the model domain, and boundary conditions are restricted to the area of definition, which may include fractions of cell faces. Data can be defined in a combination of map and grid coordinate systems, independent of a specific model grid. Two-dimensional interpolation is used to define top and bottom surfaces for 3D regions. An area-weighted scheme (natural neighbor interpolation) assigns elevation to a target point based on elevations at the nearest of scattered X, Y points. Three-dimensional interpolation assigns porous-media properties, boundary condition properties, or initial conditions of the closest scattered point to a target point. The interpolation allows for groundwater-level elevation and chemical data to be saved at the end of one run and used as initial conditions for a subsequent run, even if the grid spacing has been changed. Three sets of equations are solved simultaneously; transport equations, the equilibrium reaction equations, and the kinetic reaction equations. The flow equation can be solved separately from the transport and reaction equations; the finite difference approach is used. Finite difference methods are also used to solve the transport equations. Chemical-reaction equations are solved independently from the flow and transport equations. In addition, chemical-reaction equations are solved independently for each node in the active grid region. In the case of only equilibrium reactions, a Newton-Raphson method is used to solve the nonlinear mass-action equations and mass-balance equations that describe equilibrium. If kinetic reactions are simulated then a set of ordinary differential equations must be integrated over the time step, in addition to solving the equilibrium equations. Two methods are available for integrating the rate equations: an explicit 5th-order Runge-Kutta algorithm and an implicit algorithm for stiff differential equations based on Gear's method.
Input data requirements	Node-by-node input is not required. Shape data can be imported from ArcInfo shapefiles and ASCII raster files, and from a simple X,Y,Z file format. Three required input files: Flow and transport date file Chemistry data file Thermodynamic database file
Outputs	Data may be saved for: Viewing with a text editor Exporting to spreadsheets and post-processing programs In hierarchical data format (HDF) – a compressed binary format. This HDF file be visualized on Windows Computers using Model Viewer and extracted with the PHASTHDF utility (both distributed with PHAST).
Pre-processing and post-processing tools	USGS Model Muse and Model Viewer
Representation of uncertainty	The model does not have built-in uncertainty representation. This can be assessed for each specific case using sensitivity analysis.
Prevalence	The PHAST model has not been widely used.
Ease of use for public entities	There are not barriers to public use.
Ease of obtaining information and availability of technical support	A discussion forum and a mailing archive for questions and answers are accessible via https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phast/ Example problems are included in the examples directory of the installation. Bug reports and other comments can be submitted to h2osoft@usgs.gov.
Source code availability	The source code is available in the /src directory of the application.
Status of model development	Fully developed and ready for use
Challenges for integration	Long run times when utilizing geochemical calculations would make it difficult to couple with surface water models. It has never been used in the Delta.

References

Parkhurst, D.L., Kipp, K.L., and Charlton, S.R., 2010, PHAST Version 2—A program for simulating groundwater flow, solute transport, and multicomponent geochemical reactions: U.S. Geological Survey Techniques and Methods 6–A35, 235 p. Available at: https://pubs.usgs.gov/tm/06A35/pdf/TM6-A35.pdf

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