User's guide of Proceessing Modflow 5.0

Processing Modflow 125
MT3D < Chemical Reaction < Layer by Layer
Chemical reactions supported by MT3D include equilibrium-controlled sorption and first-order
irreversible rate reactions, such as radioactive decay or biodegradation. It is generally assumed
that equilibrium conditions exist between the aqueous-phase and solid-phase concentrations and
that the sorption reaction is fast enough relative to groundwater velocity so that it can be treated
as instantaneous. Consider to use MT3DMS, if nonequilibrium (rate-limited) sorption need to
be simulated.
Use this menu item to open the Chemical Reaction Package (MTRCT1) dialog box (Fig.
3.44) to specify the required parameters on a layer-by-layer basis. The parameters are described
below.
Fig. 3.44 The Chemical Reaction Package (MTRCT1) dialog box
< Type of sorption: Sorption is implemented in MT3D through use of the retardation factor
R. MT3D provides three types of sorptions: the linear equilibrium isotherm, Freundlich
nonlinear equilibrium isotherm and Langmuir nonlinear equilibrium isotherm.
The linear sorption isotherm assumes that the sorbed concentration ¯C j,i,k is directly
proportional to the dissolved concentration C j, i, k (eq. 3.41) and the retardation factor is
independent of the concentration field. The retardation factor is calculated only once for each
cell at the beginning of the simulation by eq. 3.42.
¯C i,j,k ' K d @ C j,i,k
Rj,i,k ' 1 % Db @Kd nj,i,k (3.41)
(3.42)
3.6.3 MT3D