PHREEQC

Criterion	Explanation
Model name/version	PHREEQC (pH-REdox-Equilibrium) version 3
General Description	Geochemical model based on speciation between physical phases (water, solids, gas). Simulates chemical and kinetic reactions and 1D transport (dispersion, diffusion, solute movement in porous media) of processes in water, laboratory and industrial environments. Applicable for rainwater, soil-water, groundwater and surface water quality. Capabilities include speciation (saturation indices; distribution of aqueous species; density and specific conductance of solution), batch reaction calculations (equilibrium distribution in aqueous phase, pure phases, solid solutions, gas phase, exchange sites, and surface sites; non-equilibrium processes such as aqueous phase mixing, kinetical reactions), sorption and desorption (surface complexation; ion exchange), inverse modeling to account for changes along flow path.
Model Domain	General
Developer	United States Geological Survey (USGS)
Hardware computing requirements	Dependent on version of program being used: 32-bit, 64-bit, or any processor.
Code language	Visual C and C++
Original application	Original model (PHREEQE) used for simulating geochemical reactions of the system, which included mixing of water, net irreversible reactions in solution, equilibrium dissolution and precipitation and temperature effects.
Public/proprietary and cost	Publicly available and modifiable with no fees. User rights can be found at https://water.usgs.gov/water-resources/software/PHREEQC/NOTICE.TXT.
Physically or empirically based	Model uses both mechanistic and empirical components.
Mathematical methods used	Speciation modeling: ion-association, DeBye-Huckel, Davies, Pitzer, SIT (Specific ion Interaction Theory) equations to estimate activity coefficients. Batch reaction modeling: pe (electron activity) and water mass using hydrogen and oxygen mole balance, Van't Hoff equation to account for temperature, Redlich type equations to account for pressure effects, Peng-Robinson equations to account for very high-pressure effects. Sorption and desorption: Gouy-Chapman equation, CD-MUSIC model (Charge Distribution MUltiSIte Complexation). Ion-exchange: Gaines-Thomas, Gapon or Vanselow convention. Kinetic reactions: Rate expressions defined using BASIC interpreter, multiple rate integration using Runge-Kutta explicit or the CVODE implicit (stiff) equation solver. Transport: Parameters such as diffusion coefficients, electrical double layer properties, dissolution, reversible and irreversible chemical reactions are used in advective-reactive transport simulations. Inverse modeling: Mole balance of elements, valance state and charge with specified uncertainty limits.
Input data requirements	Data input is free-format using chemical symbolism. Input, in the form of database files, can be built using provided graphical interface or Notepad++, arranged using keyword data blocks format. Input parameters may include, but is not limited to, elements, chemical formulas, chemical equations, charges, valance states, pH, pe, temperature, concentrations, physical states, equilibrium constants, volumes, partition coefficients, isotopes, uncertainty ranges, reaction rates, enthalpy and process time. Nine database files are inherently provided in the program: phreeqc.dat, amm.dat (decoupled ammonia data), wateq4f.dat, llnl.dat (thermodynamic data), minteq.dat, minteq.v4.dat, pitzer.dat (specific-ion interaction data), sit.dat (specific-ion interaction data), iso.dat (isotope data). Most input parameters can be obtained through chemical and kinetic databases, or from peer-reviewed literature.
Outputs	Output of graphs and table with user selected results such as concentration, activities, saturation indices, densities and transport distances.
Pre-processing and post-processing tools	Model application file, Notepad++ interface
Representation of uncertainty	Uncertainty accounted for in inverse modeling through user defined limits for all analytical data.
Prevalence	Very common: extensively used by governmental, educational, private and public entities. Used in the Delta to simulate subsurface and drain-water geochemical reactions (Deverel et al. 2007)
Ease of use for public entities	Easy to moderate, official user guide available in addition to many tutorials available through various sources.
Ease of obtaining information and availability of technical support	No commercial help desk available, many user groups and forums available online. User guide available at https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/.
Model and Source code availability	Model available for download from: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/. Source codes available upon request.
Status of model development	Model developed and available for use. Model updates are ongoing, status of model updates can be found at https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/status.htm.
Challenges for integration	Specificity of biogeochemical outputs may be difficult to scale to larger spatial resolutions often used in other models.

References

Deverel, S.J., Leighton, D.A., and Finlay, M.R., 2007. Processes Affecting Agricultural Drainwater Quality and Organic Carbon Loads in California's Sacramento–San Joaquin Delta. San Francisco Estuary and Watershed Science, 5(2). jmie_sfews_10988. Retrieved from: http://escholarship.org/uc/item/8db266mg.

Parkhurst, D.L. and Appelo, C.A.J., 2013. Description of input and examples for PHREEQC version 3: a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations (No. 6-A43). US Geological Survey.

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