|waste disposal, degradation of organic matter, and
microbially mediated reactions.
Description—Batch reactions can be divided into equilibrium
and nonequilibrium reactions. Equilibrium reactions include equilibration
of a solution with an assemblage of minerals and specified-pressure gases,
ion-exchange sites, surface-complexation sites, a finite gas phase, and
(or) solid solutions. Nonequilibrium reactions include kinetic reactions,
addition or removal of elements from solution, mixing, and changing
Keyword data blocks used in batch-reaction modeling include:
EQUILIBRIUM_PHASES—an assemblage of minerals and
gases that react to equilibrium (or until exhausted).
||EXCHANGE—one or more sets of reaction sites for
exchangeable ions, an important reaction for major cations.
||SURFACE—one or more sets of sites that react by
surface complexation, a major reaction for phosphorus, arsenic, and
other trace elements
||GAS_PHASE—a finite reservoir of gas that reacts with
||SOLID_SOLUTIONS—solids that precipitate as mixtures
of minerals, an important reaction for radionuclides and other trace
||KINETICS—any nonequilibrium reaction, for which a
rate expression can be formulated.
||RATES—BASIC language statements define rate
expressions for kinetic reactions.
||REACTION—addition and removal of specified elements
||MIX—mixing together specified fractions of
||REACTION_TEMPERATURE—changing the temperature of the
||USE—use the composition of previously defined or
saved (SAVE keyword) SOLUTION, EQUILIBRIUM_PHASES, EXCHANGE,
SURFACE, GAS_PHASE, SOLID_SOLUTION, or a previously defined
KINETICS, REACTION, MIX, or REACTION_TEMPERATURE data block.
||SAVE—save the composition following the batch
reaction of the SOLUTION, EQUILIBRIUM_PHASES, EXCHANGE, SURFACE,
GAS_PHASE, or SOLID_SOLUTION for use in subsequent reactions.
Applications—The reactive transport capabilities can be used
to study contaminant migration of nutrients, metals, radionuclides, and
organic compounds; natural and engineered aquifer remediation; diffusion
in sediment pore water; the chemical evolution of natural systems; and
laboratory column experiments.
Description—Reactive-transport modeling simulates advection,
dispersion, and chemical reactions as water moves through a 1D column. The
column is divided into a number of cells, and reactant compositions and
nonequilibrium reactions can be defined for each cell. All of the
reactants and reactions described for batch reactions can be applied to
the cells for transport modeling. The TRANSPORT keyword data block is used
to simulate advection and dispersion, or pure diffusion, in the column.
The TRANSPORT data block also can be used to simulate a dual-porosity
medium, where water flows through the column but simultaneously allows for
diffusion into stagnant side pores. The ADVECTION keyword data block is
used to simulate purely advective transport.
Keyword data blocks for transport calculations:
||TRANSPORT—1D advection, dispersion, and reaction
||ADVECTION—1D advection and reaction modeling.
Applications—Inverse modeling can be used to deduce
geochemical reactions and mixing in local and regional aquifer systems,
and in aquifer storage and recovery studies.
Description—Inverse modeling calculates geochemical reactions
that account for the change in chemical composition of water along a flow
path. For inverse modeling, at least two chemical analyses of water are
needed at different points along the flow path, as well as a set of
minerals and gases that are potentially reactive. Mole transfers of phases
are calculated that account for the change in water composition along the
flow path. The numerical method accounts for uncertainties in analytical
Keyword data block for inverse modeling:
||INVERSE_MODELING—deduce mixing, mineral, and gas
reactions that account for the chemical evolution of waters.
How to Obtain PhreeqcI, PHREEQC, and Additional Information
Versions of PhreeqcI
and PHREEQC may be obtained at no cost from
the USGS at World Wide Web address http://water.usgs.gov/software. Many other hydrologic computer models are also available from this site.
Reports documenting USGS model programs can be purchased from the
U.S. Geological Survey
Branch of Information Services
Denver, CO 80225-0286
S.R. Charlton and D.L. Parkhurst
Parkhurst, D.L. and Appelo, C.A.J., 1999, User's guide to PHREEQC
(Version 2)—A computer program for speciation, batch-reaction,
one-dimensional transport, and inverse geochemical calculations: U.S.
Geological Survey Water-Resources Investigations Report 99-4259, 310 p.