PHREEQCI—A Graphical User Interface to the

Geochemical Model PHREEQC

U.S. Department of the Interior USGS Fact Sheet FS-031-02U.S. Geological Survey April 2002

Figure 1. Main screen of PHREEQCI, with the SOLUTION keyword dialog box open.

PHREEQCI Version 2 is a completeWindows-based graphical user interface tothe geochemical computer programPHREEQC (Version 2). PHREEQCI can beused interactively to perform all themodeling capabilities of PHREEQC—speciation, batch-reaction, one-dimensional (1D) reactive-transport, andinverse modeling.

Applications:

• Mine drainage• Radioactive-waste isolation• Contaminant migration• Natural and engineered aquiferremediation• Aquifer storage and recovery• Water treatment• Natural systems• Laboratory experiments

Features of the interface:

• Compatible with Windows 95 and 98,Windows NT 4.0, Windows ME, Windows2000, and Windows XP• Automated installation usingInstallShield• Interactive access to all of thecapabilities of PHREEQC—create andmodify input data files, run simulations,and display results

Modeling capabilities:

• Aqueous, mineral, gas, surface, ion-exchange, and solid-solution equilibria• Kinetic reactions• 1D diffusion or advection anddispersion with dual-porosity medium• A powerful inverse modeling capabilityallows identification of reactions thataccount for the chemical evolution inobserved water compositions• Extensive geochemical databases

Any use of trade, product, or firm names in thispublication is for descriptive purposes only anddoes not imply endorsement by the U.S.Government.

PHREEQCI Version 2

PHREEQCI Version 2 is a computerprogram for simulating chemicalreactions and transport processes innatural or contaminated water. PHREEQCIprovides all of the capabilities of thegeochemical model PHREEQC(Parkhurst and Appelo, 1999), includingspeciation, batch-reaction, 1D reactive-transport, and inverse modeling.

Data for PHREEQC are enteredthrough a series of keyword data blocks,each of which provides a specific type ofinformation. For example, theSOLUTION keyword data block definesthe chemical composition of a solution.PHREEQCI provides tabbed dialog boxesfor each PHREEQC keyword. Figure 1shows the main screen of PHREEQCI andthe SOLUTION keyword dialog box.

Speciation Modeling

Applications—Speciation modelingis useful in situations where thepossibility of mineral dissolution orprecipitation needs to be known, as inwater treatment, aquifer storage andrecovery, artificial recharge, and wellinjection.

Description—Speciation modelinguses a chemical analysis of a water tocalculate the distribution of aqueousspecies by using an ion-associationaqueous model. The most importantresults of speciation calculations aresaturation indices for minerals, whichindicate whether a mineral should dissolveor precipitate.

Keyword data blocks used inspeciation modeling include:

SOLUTION—define thechemical composition of asolution.

SOLUTION_SPREAD—definethe chemical composition ofmultiple solutions in aspreadsheet format.

Batch-Reaction Modeling

Applications—Batch-reactionmodeling can be applied to problems inlaboratory, natural, or contaminatedsystems. The reaction capabilities ofPHREEQC have been used frequently inthe study of mine drainage, radioactive-

waste disposal, degradation of organicmatter, and microbially mediatedreactions.

Description—Batch reactions can bedivided into equilibrium andnonequilibrium reactions. Equilibriumreactions include equilibration of asolution with an assemblage of mineralsand specified-pressure gases, ion-exchange sites, surface-complexationsites, a finite gas phase, and (or) solidsolutions. Nonequilibrium reactionsinclude kinetic reactions, addition orremoval of elements from solution,mixing, and changing temperature.

Keyword data blocks used in batch-reaction modeling include:

EQUILIBRIUM_PHASES—anassemblage of minerals and gasesthat react to equilibrium (or untilexhausted).

EXCHANGE—one or more setsof reaction sites for exchangeableions, an important reaction formajor cations.

SURFACE—one or more sets ofsites that react by surfacecomplexation, a major reaction forphosphorus, arsenic, and othertrace elements.

GAS_PHASE—a finite reservoirof gas that reacts with a solution.

SOLID_SOLUTIONS—solidsthat precipitate as mixtures ofminerals, an important reactionfor radionuclides and other tracemetals.

KINETICS—any nonequilibriumreaction, for which a rateexpression can be formulated.

RATES—BASIC languagestatements define rate expressionsfor kinetic reactions.

REACTION—addition andremoval of specified elementsfrom solution.

MIX—mixing together specifiedfractions of solutions.

REACTION_TEMPERATURE—changing the temperature of thereaction system.

USE—use the composition ofpreviously defined or saved(SAVE keyword) SOLUTION,EQUILIBRIUM_PHASES,EXCHANGE, SURFACE,GAS_PHASE,SOLID_SOLUTION, or apreviously defined KINETICS,REACTION, MIX, orREACTION_TEMPERATUREdata block.

SAVE—save the compositionfollowing the batch reaction of theSOLUTION,EQUILIBRIUM_PHASES,EXCHANGE, SURFACE,GAS_PHASE, orSOLID_SOLUTION for use insubsequent reactions.

Transport Modeling

Applications—The reactive transportcapabilities can be used to studycontaminant migration of nutrients,metals, radionuclides, and organiccompounds; natural and engineeredaquifer remediation; diffusion in sedimentpore water; the chemical evolution ofnatural systems; and laboratory columnexperiments.

Description—Reactive-transportmodeling simulates advection, dispersion,and chemical reactions as water movesthrough a 1D column. The column isdivided into a number of cells, andreactant compositions and nonequilibriumreactions can be defined for each cell. Allof the reactants and reactions describedfor batch reactions can be applied to thecells for transport modeling. TheTRANSPORT keyword data block is usedto simulate advection and dispersion, orpure diffusion, in the column. TheTRANSPORT data block also can be usedto simulate a dual-porosity medium,where water flows through the column butsimultaneously allows for diffusion intostagnant side pores. The ADVECTIONkeyword data block is used to simulatepurely advective transport.

Keyword data blocks for transportcalculations:

TRANSPORT—1D advection,dispersion, and reaction modeling.

ADVECTION—1D advection andreaction modeling.

Inverse Modeling

Applications—Inverse modeling canbe used to deduce geochemical reactionsand mixing in local and regional aquifersystems, and in aquifer storage andrecovery studies.

Description—Inverse modelingcalculates geochemical reactions thataccount for the change in chemicalcomposition of water along a flow path.For inverse modeling, at least twochemical analyses of water are needed atdifferent points along the flow path, aswell as a set of minerals and gases that arepotentially reactive. Mole transfers ofphases are calculated that account for thechange in water composition along theflow path. The numerical method accountsfor uncertainties in analytical data.

Keyword data block for inversemodeling:

INVERSE_MODELING—deducemixing, mineral, and gas reactionsthat account for the chemicalevolution of waters.

How to Obtain PHREEQCI, PHREEQC,and Additional Information

Versions of PHREEQCI and PHREEQCmay be obtained at no cost from theUSGS at World Wide Web addresshttp://water.usgs.gov/software. Manyother hydrologic computer models arealso available from this site.

Reports documenting USGS modelprograms can be purchased from thefollowing address:

U.S. Geological SurveyBranch of Information ServicesBox 25286Denver, CO 80225-0286

S.R. Charlton and D.L. Parkhurst

References

Parkhurst, D.L. and Appelo, C.A.J., 1999,User’s guide to PHREEQC (Version2)—A computer program forspeciation, batch-reaction, one-dimensional transport, and inversegeochemical calculations: U.S.Geological Survey Water-ResourcesInvestigations Report 99-4259, 310 p.