Chemical Species

By default, mass-transport simulations in FEFLOW are done for a single chemical species. If multiple species (dissolved constituents) are to be simulated, the number of species, the phase a species is associated with and also the reaction type are to be defined on this page of the Problem Settings dialog.

The small toolbar above the species list can be used to add and remove species and to change their order:

 

Symbol	Description
	Add new species below the selected one
	Delete selected species
	Move selected species one step up
	Move selected species one step down
Import...	Import Species Definition File (*.fsd)
Export...	Export Species Definition File (*.fsd)

 

The order of chemical species is significant if reactions are defined as the solution of the mass-transport for the species is done sequentially.

 

The Species Definition File (*.fsd) is used to store the number of chemical species and their associations (chemical reactions). File format is based on standard XML. File can be used to conveniently import/export multi-species transport settings from one project to another.

 

For each species the following settings have to be defined:

 

Property	Description
Name	For each species, an arbitrary identifying name can be defined.
Phase	If multiple chemical species are simulated, each species has to be associated with a phase. Fluid: Species is dissolved in a mobile phase and subject to advection and dispersion. Solid: Species of an immobile solid phase are bound to the soil matrix, having no advection and no dispersion. This setting can be used to represent the sorbed part of a contaminant.
Reaction	Reaction type for each species. The available reaction types for simulations with a single species and with multiple chemical species are explained in the respective sections below.

Simulations with a single chemical species

For mass-transport simulations with a single chemical species, the species does not have to be associated with a phase. Instead, the user has to select in which phase the chemical reaction takes place. The available options are

• In both the fluid and the solid phase (default)
• In the fluid phase only
• In the solid phase only

Reaction types

The following reaction types are available of a single-specie mass transport problem:

• First-order decay
• Michaelis-Menten kinetics

Simulations with multiple chemical species

FEFLOW allows the mass transport with multiple chemical species. These species can be conservative or can react. For each species, one of the following reaction kinetics can be used:

• Degradation
• Arrhenius
• Monod
• User-defined

For the predefined Degradation, Arrhenius and Monod kinetics the user only has to set the coefficients for each species correctly in order to run a simulation. Each of the reaction types have its own template for setting up the coefficients. For more complex cases where the default kinetics are not sufficient, an editor to define user-specific equations is available.

All reaction kinetics can be edited in the Formula Editor which is invoked with a double click on the reaction and on Edit.

Positive terms in an equation represent a creation of the species, negative ones a decomposition of the contaminant. All the equations are written in a way so that they relate creation and decomposition of a species to concentrations of other species possibly using additional factors and reaction rates. Indices relate to the species number, dark red indices can be edited with a click.

 

There is a difference between reaction rates in single-species and multi-species transport in FEFLOW: Reaction (e.g., decay) in single-species transport only handles a species dissolved in the fluid phase, therefore all reactions are so-called homogeneous reactions, i.e., reactions within one phase. In multi-species transport both homogeneous and heterogeneous reactions, i.e., reactions between species in fluid and in solid phase, can occur. Thus only in single-species transport it is possible to deal with porosity internally, internally multiplying the reaction rate with the porosity. In multi-species transport, however, porosity has to be considered by the user modifying the reaction-rate constant or the coefficients of the reaction equation. For example, for the same first order decay reaction, in the multi-species case the reaction-rate constant would be the single-species decay rate multiplied by the porosity. In case of user-defined reactions, porosity is available as a variable in the equation.

 

White Papers, Vol. 1, chapter 10 and White Papers, Vol. IV for details and corresponding equations.