The RCT: Chemical Reaction Package pane is is on the MODFLOW Packages and Programs dialog box under MT3DMS or MT3D-USGS. It is used to specify data for the RCT package in MT3DMS or MT3D-USGS.
ISOTHM indicates which type of sorption (or dual-domain mass transfer) is simulated:
ISOTHM = 0, no sorption is simulated;
=1, Linear isotherm (equilibrium-controlled);
=2, Freundlich isotherm (equilibrium-controlled);
=3, Langmuir isotherm (equilibrium-controlled);
=4, First-order kinetic sorption (nonequilibrium);
=5, Dual-domain mass transfer (without sorption);
=6, Dual-domain mass transfer (with sorption);
=-6 Dual-domain mass transfer (with different sorption coefficients in mobile and immobile domains). (only available in MT3D-USGS)
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IREACT indicates which type of kinetic rate reaction is simulated:
IREACT = 0, no kinetic rate reaction is simulated;
IREACT = 1, first-order irreversible reaction. Note that this reaction package is not intended for modeling chemical reactions between species. An add-on reaction package developed specifically for that purpose may be used.
IREACT = 2, MONOD kinetic reaction is simulated; (only available in MT3D-USGS)
IREACT = 3, first-order chain reaction is simulated. (only available in MT3D-USGS)
IREACT =100, zeroth-order reaction (decay or production).
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IGETSC indicates whether the initial concentration for the nonequilibrium sorbed or immobile phase of all species should be read when nonequilibrium sorption (ISOTHM = 4) or dual-domain mass transfer (ISOTHM = 5, 6 or -6) is simulated:
If the checkbox is not checked, the initial concentration for the sorbed or immobile phase is not read. By default, the sorbed phase is assumed to be in equilibrium with the dissolved phase (ISOTHM = 4), and the immobile domain is assumed to have zero concentration (ISOTHM = 5 or 6).
If the checkbox is checked, the initial concentration for the sorbed phase or immobile liquid phase of all species will be read.
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IREACTION is used to select a reaction module. At least 2 species must be simulated when this option is used. (EA = Electron Acceptor; ED = Electron Donor) (only available in MT3D-USGS)
If IREACTION=0, no reaction is simulated.
If IREACTION=1, instantaneous EA/ED reaction is simulated between an ED and an EA.
If IREACTION=2, kinetic reaction is simulated between multiple EAs and EDs.
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IED is the electron donor participating in the EA/ED reaction.
IEA is the electron acceptor participating in the EA/ED reaction.
F is the stoiciometric ratio in the simulated equation ED + F*EA -> Product
If IREACTION = 2, additional tabs will appear.
The Kinetic tab is used to specify the electron donors, acceptors, special cases, and whether to simulate solid phase Fe. When listing the species in the MT3D Basic package, the electron donors must be listed first followed by the electron acceptors. When you specify the number of electron donors and acceptors, the species will be listed below the edit boxes for the counts.
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For each special case identified on the Kinetic tab, the modeler must specify the treatment and the maximum express field capacity.
Treatment (SPECIAL) Three possible treatments are as follows:
•SOLID – The solid phase concentration is used; it is for the iron reduction process. For this case, Fe3+ solid phase will be tracked.
•MAXEC – The method of Lu et al. (1999) to deal with the iron reduction and methanogenesis simulation is used.
•STORE – This keyword is for the methanogenesis simulation only. If the methane concentration is over the maximum express field capacity (EFC), the additional mass of methane will be stored, and the result will be output as an unformatted file with a name of “MT3D_Ad_methane.UCN”. This option uses the formula developed by Neville and Vlassopoulos (2008).
EFCMAX is the maximum express field capacity (EFC). If keyword SOLID is used, then this variable is read but not used.
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The Elec. Accept tab is where the modeler specifies properties for the electron acceptors.
HSC is the half saturation constant.
IC is the inhibition constants.
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DECAYRATE is the decay rate of each electron acceptor corresponding to each electron donor.
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YIELDC is the yield coefficient of each component corresponding to each electron donor
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