﻿ Numerical Modeling Workflow - Finite Difference Grids > Define Objectives > Model Parameters

# Model Parameters

Model parameters consist of material properties relevant to fate and transport modeling associated with the geologic units that are independent of the constituent species to be modeled.  The specific model parameters required as part of each simulation are defined by the retardation model and reaction model.

### General Model Parameters:

IRCTOP

IRCTOP is an integer flag indicating how reaction variables are entered:

By Cell (IRCTOP=2) [Default] - all reaction variables are specified as 3-D arrays on a cell-by-cell basis.

By Layer (IRCTOP=0) - all reaction variables are specified as a 1-D array with each value in the array corresponding to a single layer. This option is mainly for retaining compatibility with the previous versions of MT3D.

IGETSC

IGETSC is an integer flag indicating whether the initial concentration for the non-equilibrium sorbed or immobile phase of all species should be read when non-equilibrium sorption (ISOTHM = 4) or dual-domain mass transfer (ISOTHM = 5 or 6) is simulated.  The flag is set by answering the question:

Should sorbed or immobile-phase concentrations be read?

No - then IGETSC = 0 and 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).

Yes - then IGETSC =1 and the initial concentration for the sorbed phase or immobile liquid phase of all species will be read, as applicable.

### Retardation Model - Model Parameters

Bulk Density

Soil Bulk Density is used to calculate the retardation coefficient for each chemical species according to the following formula:

where:

Ri = Retardation Coefficient of Species i [-]

ρb = Soil Bulk Density [M/L3]

n = Effective Soil Porosity [L3/L3] à [-]

Kd(i) = Distribution Coefficient of Species i [L3/M]

The retardation coefficient is used to calculate the ‘retarded’ flow velocity (VR(i)) of each chemical species according to the following formula:

where:

VR(i) = Retarded Flow Velocity of Species i [L/T]

V = Average Linear Groundwater Flow Velocity [L/T]

Ri = Retardation Coefficient of Species i [-]

The retarded flow velocity is used to calculate the advective transport of each species.

Unless otherwise specified during the setup of the transport model, the default soil Bulk Density value for any new model created is 1700 kg/m3.

If no sorption method is selected in the current transport variant, then no Bulk Density values are required for the simulation, and all of the options in the left-hand toolbar will be disabled.

Reaction Model/Modules - Model Parameters

The reaction modules associated with RT3D may require model parameter values and settings to be selected at the Define Modeling Objectives workflow step, these include:

Instantaneous aerobic degradation of BTEX [IREACT=1];

Rate-Limited Sorption [IREACT=4];

Double-Monod Model [IREACT=5];

Sequential First-Order Decay [IREACT=6]; and

Aerobic/Anaerobic PCE/TCE Dechlorination [IREACT=7]

where IREACT is a flag in the RT3D RCT package file whose value determines which reaction module is active in the transport model run.

Note: yield values associated with several reaction modules in RT3D are defined on a mg/L basis. To be consistent the user must use mg/L units for all concentrations when using these reaction modules. See additional notes for each reaction module.

Parameter

Caption

Description

Default

Units

Constant /

Spatially Variable

Node in Model Explorer

RC1

Y_O2/BTEX

Stoichiometric ratio of oxygen (consumed) to BTEX

3.14

[ - ]

Const

n/a

Stoichiometry

 * Note: By changing the value of RC1 instantaneous reactions between any other chemicals can be simulated using this module

Six-Species, First-Order, Rate-Limited, BTEX Degradation using Sequential Electron Acceptors

Parameter

Caption

Description

Default

Units

Constant /

Spatially Variable

Node in Model Explorer

RC1

Cmax_Fe2+

Maximum concentration of Fe2+

0

[M/L3]

Const/Var

R3-3(Reduction Capacities)

RC2

Cmax_CH4

Maximum concentration of CH4

0

[M/L3]

Const/Var

RC3

K_O2

Hydrocarbon decay rate via aerobic process

0

[1/T]

Const/Var

R3-3(Rates)

RC4

K_NO3

Hydrocarbon decay rate via denitrification

0

[1/T]

Const/Var

RC5

K_Fe3+

Hydrocarbon decay rate via iron reduction

0

[1/T]

Const/Var

RC6

K_SO4

Hydrocarbon decay rate via sulfate reduction

0

[1/T]

Const/Var

RC7

K_CH4

Hydrocarbon decay rate via methanogenesis

0

[1/T]

Const/Var

RC8

Ks_O2

Half-saturation constant for oxygen

0.5

[M/L3]

Const/Var

R3-3(Saturations)

RC9

Ks_NO3

Half-saturation constant for nitrate

0.5

[M/L3]

Const/Var

RC10

Ks_Fe3+

Half-saturation constant for Fe3+

0.5

[M/L3]

Const/Var

RC11

Ks_SO4

Half-saturation constant for sulfate

0.5

[M/L3]

Const/Var

RC12

Ks_CH4

Half-saturation constant for methane

0.5

[M/L3]

Const/Var

RC13

Ki_O2

Inhibition coefficient for the oxygen reaction

0.01

[M/L3]

Const/Var

R3-3(Inhibitions)

RC14

Ki_NO3

Inhibition coefficient for the nitrate reaction

0.01

[M/L3]

Const/Var

RC15

Ki_Fe3+

Inhibition coefficient for the Fe3+ reaction

10

[M/L3]

Const/Var

RC16

Ki_SO4

Inhibition coefficient for the sulfate reaction

0.01

[M/L3]

Const/Var

RC17

Y_O2/BTEX

Stoichiometric ratio of oxygen (consumed) to BTEX

3.14

[ - ]*

Const

n/a

Stoichiometry

RC18

Y_NO3/BTEX

Stoichiometric ratio of nitrate (consumed) to BTEX

4.9

[ - ]*

Const

RC19

Y_Fe/BTEX

Stoichiometric ratio of Fe2+ (produced) to BTEX

21.8

[ - ]*

Const

RC20

Y_SO4/BTEX

Stoichiometric ratio of sulfate (consumed) to BTEX

4.7

[ - ]*

Const

RC21

Y_CH4/BTEX

Stoichiometric ratio of methane (produced) to BTEX

0.78

[ - ]*

Const

 * Note: Yield values are on a mg/L basis; to be consistent, the user must use mg/L units for all concentrations when using this reaction module

Rate-Limited Sorption

Parameter

Caption

Description

Default

Units

Constant /

Spatially Variable

Node in Model Explorer

RC1

K_mt

Mass-transfer rate coefficient

0

[1/T]

Const/Var

R3-4(Sorption)

RC2

Kd

Linear partitioning coefficient (Kd)

1

[L3/M]

Const/Var

Double-Monod Model

Parameter

Caption

Description

Default

Units

Constant /

Spatially Variable

Node in Model Explorer

RC1

μ_m(w)

Specific utilization rate

0

[1/T]

Const/Var

R3-5(Rates)

RC2

Ks_d

Monod half-saturation constant for electron donor

0.5

[M/L3]

Const/Var

R3-5(Saturations)

RC3

Ks_a

Monod half-saturation constant for electron acceptor

0.5

[M/L3]

Const/Var

RC4

Y_x/d

Biomass produced per unit of electron donor utilized

0

[ - ]

Const

n/a

Stoichiometry

RC5

Y_a/d

Electron acceptor used per unit of electron donor utilized

0

[ - ]

Const

RC6

K_decay

First-order bacterial death or decay rate

0

[1/T]

Const/Var

R3-5(Rates)

RC7

K_attach

First-order bacterial attachment rate

0

[1/T]

Const/Var

RC8

K_detach

First-order bacterial detachment rate

0

[1/T]

Const/Var

 * Note: This reaction module describes a general double Monod model. By setting appropriate yield and kinetic rate constants, users can model many types of biological systems. Kinetic constants for an aerobic system are given in Clement et al. (1998), and for an anaerobic denitrifying system are given in Clement et al. (1997). Also see Taylor and Jaffe (1990); Hornberger et al. (1992); Zysset et al. (1994); and Reddy and Ford (1996).

Sequential First-Order Decay

Parameter

Caption

Description

Default

Units

Constant /

Spatially Variable

Node in Model Explorer

RC1

K_A

0

[1/T]

Const/Var

R3-6(Rates)

RC2

K_B

0

[1/T]

Const/Var

RC3

K_C

0

[1/T]

Const/Var

RC4

K_D

0

[1/T]

Const/Var

RC5

Y_tce/pce

Yield coefficient, TCE/PCE

0.792

[ - ]*

Const

n/a

Stoichiometry

RC6

Y_dce/tce

Yield coefficient, DCE/TCE

0.738

[ - ]*

Const

RC7

Y_vc/dce

Yield coefficient, VC/DCE

0.644

[ - ]*

Const

 * Note: Yield values are on a mg/L basis; to be consistent, the user must use mg/L units for all concentrations when using this reaction module

Aerobic/Anaerobic PCE/TCE Dechlorination

Parameter

Caption

Description

Default

Units

Constant /

Spatially Variable

Node in Model Explorer

RC1

Kp

Sequential (anaerobic) reaction rate for PCE

0

[1/T]

Const/Var

R3-7(Rates)

RC2

Kt1

Sequential (anaerobic) reaction rate for TCE

0

[1/T]

Const/Var

RC3

Kt2

Aerobic decay rate for TCE

0

[1/T]

Const/Var

RC4

Kd1

Sequential (anaerobic) reaction rate for DCE

0

[1/T]

Const/Var

RC5

Kd2

Aerobic decay rate for DCE

0

[1/T]

Const/Var

RC6

Kv1

Sequential (anaerobic) reaction rate for VC

0

[1/T]

Const/Var

RC7

Kv2

Aerobic decay rate for VC

0

[1/T]

Const/Var

RC8

Ke1

Sequential (anaerobic) reaction rate for ETH

0

[1/T]

Const/Var

RC9

Ke2

Aerobic decay rate for ETH

0

[1/T]

Const/Var

 * Note: All the yield values are fixed internally; to be consistent, use mg/L units for all species concentrations.