The Species Parameters include the Sorption and Reaction parameters used by the selected transport settings. The available parameters will depend on what sorption and reaction settings you selected in the Modeling Objectives workflow step. The parameters presented in the Species Parameters Database window are from the parameters listed in the Species Parameters Tab in the Modeling Objectives workflow step.

If no sorption or reactions are selected in the current transport variant, then no sorption or reaction parameters are required for the simulation, and there will not be an option for "Species Parameters" at the Define Properties workflow step.

The parameters are described by retardation model and reaction model below and for the special case of variable density flow (SEAWAT) models.

The following species parameters will be available for any model where transport is a defined modeling objective:

•Name: the short name of each species to be modeled (e.g. TCE for trichloroethylene)

•Description: a more descriptive name for each species to be modeled (e.g. trichloroethylene when modeling TCE)

•Initial Concentration: the concentration of each species throughout the model domain at the start of the simulation [default=0]. See Theory.

The list of parameters required for each species based on each retardation model is provided below, associated base units are provided in square brackets [ ], where L signifies length (e.g. meters, feet), M signifies mass (e.g. kilograms, pounds), and T signifies time (e.g. seconds, days, years).

A tabulation of which retardation model is supported by each of the transport engines is provided in the Define Modeling Objectives workflow step. For more detailed documentation on each of the retardation model parameters, please refer to the documentation of the relevant transport engine (i.e. MT3DMS, RT3D, SEAWAT, or MODFLOW-SURFACT).

Linear Isotherm (equilibrium controlled)

•Kd is the distribution coefficient [1/(M/L3)]

Freundlich (equilibrium-controlled)

•Kf is the Freundlich constant [1/(M/L3)a]

•a is the Freundlich exponent [-]

Langmuir (equilibrium-controlled)

•Kl is the Langmuir is constant [1/(M/L3)]

•S is the total concentration of sorption sites available [-]

First order kinetic sorption (non-equilibrium)

•Kd is the distribution coefficient [1/(M/L3)]

•K_mass is the first-order kinetic mass transfer coefficient [1/T]

Dual-domain mass-transfer without sorption

•SCONCIM is the initial concentration in the distribution coefficient [1/(M/L3)]

•K_mass is the first-order kinetic mass transfer coefficient [1/T]

Dual-domain mass-transfer with linear sorption in mobile domain

•SCONCIM is the initial concentration in the immobile domain [M/L3]

•Kd is the distribution coefficient for the mobile domain [1/(M/L3)]

•K_mass is the first-order kinetic mass transfer coefficient [1/T]

Dual-domain mass-transfer (with the same linear sorption in mobile and immobile domains)

•SCONCIM is the initial concentration in the immobile domain [M/L3]

•Kd is the distribution coefficient for both the mobile and immobile domain [1/(M/L3)]

•K_mass is the first-order kinetic mass transfer coefficient [1/T]

Dual-domain mass-transfer (with different linear sorption in mobile and immobile domains)

•SCONCIM is the initial concentration in the immobile domain [M/L3]

•Kd is the distribution coefficient for the mobile domain [1/(M/L3)]

•K_mass is the first-order kinetic mass transfer coefficient [1/T]

•KdIm is the distribution coefficient for the immobile domain [1/(M/L3)]

Dual-domain mass-transfer with nonlinear sorption in mobile domain

•SCONCIM is the initial concentration in the immobile domain [M/L3]

•Kf is the Freundlich constant for the mobile domain [1/(M/L3)a]

•a is the Freundlich exponent for the mobile domain [-]

•K_mass is the first-order kinetic mass transfer coefficient [1/T]

Dual-domain mass-transfer (with the same nonlinear sorption in mobile and immobile domains)

•SCONCIM is the initial concentration in the immobile domain [M/L3]

•Kf is the Freundlich constant for both the mobile and immobile domain [1/(M/L3)a]

•a is the Freundlich exponent for both the mobile and immobile domain [-]

•K_mass is the first-order kinetic mass transfer coefficient [1/T]

Dual-domain mass-transfer (with different linear sorption in mobile and immobile domains)

•SCONCIM is the initial concentration in the immobile domain [M/L3]

•Kf is the Freundlich constant for the mobile domain [1/(M/L3)a]

•a is the Freundlich exponent for the mobile domain [-]

•K_mass is the first-order kinetic mass transfer coefficient [1/T]

•KfIm is the Freundlich constant for the immobile domain [1/(M/L3)aim]

•aim is the Freundlich exponent for the immobile domain [-]

The list of parameters required for each species based on each reaction model is provided below, associated base units are provided in square brackets [ ], where L signifies length (e.g. meters, feet), M signifies mass (e.g. kilograms, pounds), and T signifies time (e.g. seconds, days, years).

A tabulation of which reaction model is supported by each of the transport engines is provided in the Define Modeling Objectives workflow step. For more detailed documentation on each of the retardation model parameters, please refer to the documentation of the relevant transport engine (i.e. MT3DMS, RT3D, SEAWAT, or MODFLOW-SURFACT).

NOTE: The RT3D reactions have specific default species for each of its associated reaction modules. Selecting a reaction module specific to RT3D will reset the species list and all associated species and model parameters.

First-Order Irreversible Decay

•K_mobile is the first-order decay rate coefficient for dissolved constituents in both the mobile and immobile domain [1/T]

•K_sorbed is the first-order decay rate coefficient for sorbed constituents in both the mobile and immobile domain [1/T]

Note: First-order decay rate coefficients can be derived from half-life values, which are more commonly available. Concentrations that follow first-order decay can be shown to change over time by the equation:

where:

Ct is the concentration of the constituent at time t [M/L3]

C0 is the initial concentration of the constituent [M/L3]

k is the first order decay rate [1/T]

t is elapsed time [T]

When the constituent concentration reaches half of its initial concentration (i.e. Ct = 0.5 x C0), the equation above can be rewritten as:

k = ln 2 / t1/2

where:

k is the first order decay rate [1/T]

t1/2 is the half-life of the constituent C [T]

Zeroth-Order Irreversible Decay

•K_mobile is the zeroth-order decay rate coefficient for dissolved constituents in both the mobile and immobile domain [1/T]

•K_sorbed is the zeroth-order decay rate coefficient for sorbed constituents in both the mobile and immobile domain [1/T]

RT3D Reaction Modules

When selecting one of the reaction modules associated with RT3D:

•Instantaneous aerobic degradation of of BTEX;

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

•Rate-Limited Sorption;

•Double-Monod Model;

•Sequential First-Order Decay; or

•Aerobic/Anaerobic PCE/TCE Dechlorination

The Absolute Tolerance (ATOL) and Relative Tolerance (RTOL) species parameters will be added for each species. These tolerance parameters are used by the differential equations solver in RT3D to control convergence errors while solving the applicable reaction model. Setting ATOL(i) = 0.0 results in a pure relative error test on Species i, while setting RTOL(i) = 0.0 results in a pure absolute error test on Species i. For practical problems, the following rules of thumb may be used to set ATOL and RTOL values:

•set RTOL(i) = 10-(m+1), where m is the desired number of significant digits for concentration output for species i (Ci )

•set ATOL(i) to a small value at which the absolute value of Ci is essentially insignificant (typically between 10-6 to 10-9)

Note: that RTOL + ATOL > 0

Caution: Actual (global) errors may exceed the local tolerances, so choose ATOL(i) and RTOL(i) conservatively.

The list of parameters required for a variable density flow model are presented below, associated base units are provided in square brackets [ ], where L signifies length (e.g. meters, feet), M signifies mass (e.g. kilograms, pounds), T signifies time (e.g. seconds, days, years), and Θ signifies temperature (currently only degrees Celsius [oC] are supported, as these units are hard-coded into the governing equations implemented in SEAWAT). For more detailed information, see discussion on the variable density and viscosity packages.

•Temperature: this is a logical flag to select the species representing temperature. At most one species may be selected. This will link the selected temperature species to the viscosity package and change the concentration units to temperature. [-]

•Initial Concentration: same as above [see units item below]

•CREF: represents the reference concentration for the density and viscosity relationships for each species. [see units item below]

•Units: shows the project concentration units for all species [M/L3, except for the selected temperature species (if any) which will be shown in oC].

•DRHODC (ẟρ/ẟCi): the density versus concentration slope [-], for the temperature species, it represents ẟρ/ẟT

•DMUDC (ẟμ/ẟCi): the viscosity versus concentration slope [-], for the temperature species, it represents ẟμ/ẟT, if the linear-relationship option is selected.

•MDCOEFF: species-specific diffusion coefficient [L2/T]. Note: this is only used if the multi-diffusion option is set to true in the DSP package advanced settings.

Note: If you are developing a multi-species variable-density model, the major density-controlling species (typically salt) should be Species 1.

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