User's guide of Proceessing Modflow 5.0

32 Processing Modflow
2.2.1 Perform Transport Simulation with MT3D
MT3D requires a boundary condition code for each model cell which indicates whether (1)
solute concentration varies with time (active concentration cell), (2) the concentration is kept
fixed at a constant value (constant-concentration cell), or (3) the cell is an inactive concentration
cell. Use 1 for an active concentration cell, -1 for a constant-concentration cell, and 0 for an
inactive concentration cell. Active, variable-head cells can be treated as inactive concentration
cells to minimize the area needed for transport simulation, as long as the solute concentration is
insignificant near those cells.
Similar to the flow model, you must specify the initial concentration for each model cell.
The initial concentration at a constant-concentration cell will be kept constant during a transport
simulation. The other concentration are starting values in a transport simulation.
< To assign the boundary condition to MT3D
1. Choose Boundary Conditions < ICBUND (MT3D/MT3DMS) from the Grid menu.
For the current example, we accept the default value 1 for all cells.
2. Choose Leave Editor from the File menu or click the leave editor button .
< To set the initial concentration
1. Choose MT3D < Initial Concentration from the Models menu.
For the current example, we accept the default value 0 for all cells.
2. Choose Leave Editor from the File menu or click the leave editor button .
< To assign the input rate of contaminants
1. Choose MT3D < Sink/Source Concentration < Recharge from the Models menu.
3
2. Assign 12500 [µg/m ] to the cells within the contaminated area.
This value is the concentration associated with the recharge flux. Since the recharge rate is
-9 3 2 -4 2
8 × 10 [m /m /s] and the dissolution rate is 1 × 10 [µg/s/m ], the concentration associated
-4 -9 3
with the recharge flux is 1 × 10 / 8 × 10 = 12500 [µg/m ]
3. Choose Leave Editor from the File menu or click the leave editor button .
< To assign the transport parameters to the Advection Package
1. Choose MT3D < Advection... from the Models menu.
An Advection Package (MTADV1) dialog box appears. Enter the values as shown in
Fig. 2.22 into the dialog box, select Method of Characteritics (MOC) for the solution
scheme and First-order Euler for the particle tracking algorithm.
2.2.1 Perform Transport Simulation with MT3D