User's guide of Proceessing Modflow 5.0

134 Processing Modflow
described below. Using these two options, you have the choice of specifying the initial
concentration for the sorbed or immobile phase for each species. To do this, simple check Use
the initial concentration for the sorbed or immobile phase of all species and specify the
concentration value to Initial concentration for the sorbed phase or Initial concentration for
the immobile phase in the Chemical Reaction (MT3DMS) dialog box.
D b @ M ¯C
Mt
C & ¯ C
K d
' $ C & ¯C
K d
-1
where $ [T ] is the first-order mass transfer rate between the dissolved and sorbed phases,
-3
D [ML ] is the bulk density of the porous medium, ¯C is the sorbed concentration and K
' Db $ @ M ¯ C
Mt
¯C M ¯ If sufficient time is available for the system to reach equilibrium (for example, the flow
velocity of groundwater is very slow), then there is no further change in and C/Mt'0 ,
3.6.4 MT3DMS
Fig. 3.50 The Chemical Reaction (MT3DMS) dialog box
< First-order kinetic sorption: When the local equilibrium assumption is not valid, it is
assumed in MT3DMS that sorption can be represented by a first-order reversible kinetic
sorption described by
(3.48)
b d
3 -1
[L M ] is the distribution coefficient as defined previously in eq. 3.41.
Eq. 3.48 can be rearranged in
(3.49)
so that eq. 3.49 is reduced to linear sorption (eq. 3.41). If the first-order mass transfer rate