The Chem Species (MT3D and GWT) pane is in the MODFLOW Packages and Programs dialog box under Groundwater Transport. It is used to define the chemical species used for groundwater transpor for both MT3D and the Groundwater Transport Process of MODFLOW 6.
“Mobile species” are involved in both transport and reaction while “immobile” species are involved in reaction only. Immobile species are only used in MT3DMS and MT3D-USGS. Also, for each species, MT3DMS or MT3D-USGS automatically tracks a sorbed or immobile counterpart if a sorption isotherm or dual-domain mass transfer is specified through the Chemical Reaction Package. Thus, there is no need to define separate “immobile” species to simulate sorption or a dual-domain system. The ability to define separate immobile species is only intended for using MT3DMS or MT3D-USGS with add-on reaction packages.
The initial values for either the mobile or immobile species can be read from a concentration file created by MT3DMS or MT3D-USGS. The data in the file must have the same number of columns, rows, and layers as the model. To use such a file, check the check box in the table next to the chemical species name in the column labeled Use Initial Concentration File and then enter the file name in the column labeled File Name. You can click in the File Name column to display a Select button. Clicking the Select button will allow you to select the file. You will also need to specify the Stress period, Time step, and Transport step of the concentrations that you want to use. Use of external initial concentration files is not supported by ModelMuse for GWT.
If the Buoyancy or Viscosity packages are selected, there are options in those packages to specify the density or viscosity of water entering the groundwater through some boundary conditions (GHB, RIV, LAK, SFR, and MAW). if those options are selected "Density" or "Viscosity" will be automatically added to the list of chemical species but will not be used in separate transport models.
