User's guide of Proceessing Modflow 5.0

Processing Modflow 127
beginning of each transport step. ¯C j,i,k is in equilibrium with solute concentration C j,i,k in the
cell [j, i, k].
The rate constant 8 is usually given in terms of the half-life t (see eq. 3.32). Generally,
1/2
if the reaction is radioactive decay, 8 should be set equal to 8 . However, for certain types
2 1
of biodegradation, 8 may be different from 8 .
2 1
MT3D < Chemical Reaction < Cell by Cell (only MT3D96)
Chemical Reactions Package 2 supported by the MT3D96 transport model includes all functions
of the Chemical Reactions Package of MT3D (version 1.xx). Using the Data Editor, chemical
reaction coefficients may be entered on a three-dimensional cell-by-cell basis. This option
provides the ability to have different reaction coefficients for different areas in a single model
layer. See MT3D < Chemical Reaction < Layer by Layer for details about the parameters.
MT3D < Sink/Source Concentration
This menu is used for specifying the concentration associated with the fluid of point or
areally-distributed sources or sinks. The concentration of a particular source or sink is specified
by the Data Editor. Point sources include wells, general-head boundary cells, fixed-head cells,
rivers and streams. Recharge is the only areally-distributed source whereas evapotranspiration
is the only sink whose concentration can be specified. Note that MT3D does not allow the
concurrent use of the rivers and the streams. This does not cause problems in any case, because
the Streamflow-Routing package has all functions of the River package.
The concentration of a sink cannot be greater than that of the aquifer at the sink cell. If the
sink concentration is specified greater than that of the aquifer, it is automatically set equal to the
concentration of the aquifer.
Menu items of this menu are dimmed if the corresponding hydraulic features given in the
Models