GMS:MODPATH Particle Tracking

Requirements

Before you can do particle tracking with MODPATH, you must have:

1. A MODFLOW simulation in memory.
2. A MODFLOW solution in memory computed using MODFLOW 2000 with a valid head and flow field.

If the Conceptual Model approach is used to build the MODFLOW model, the same conceptual model can be used to initialize some of the input data for MODPATH. Both the zone codes and porosities can be defined using polygons in a MODFLOW / MT3D / MODPATH Layer Attributes type coverage. The values are assigned to the cells when the Map → MODPATH command is selected.

Steps

Once you have a MODFLOW solution, all that is necessary to do particle tracking is to:

1. Create particle starting locations.

As soon as you create the particle starting locations GMS immediately does a number of things depending on the Run MODPATH automatically for steady state models option in the MODPATH General Options dialog. If the option is on and the MODFLOW model is steady state.

Automatic option on

1. GMS saves the MODPATH input files.
2. GMS launches MODPATH and passes it the input files.
3. GMS waits for MODPATH to compute the pathlines.
4. GMS then reads in and displays the pathlines.

All this is done automatically and behind the scenes and usually takes just a second or two. Thus, pathlines will appear almost immediately after you define the particle starting locations. If MODPATH encountered problems attempting to generate the path lines, the errors will be displayed in the MODPATH Errors dialog.

Automatic option off

If the Run MODPATH automatically for steady state models option is off, or the MODFLOW model is transient, you must save and run MODPATH manually after creating particle starting locations before pathlines will appear. This is to prevent having GMS spend lots of time updating pathlines because MODPATH can take a considerable amount of time with large, transient models. The MODPATH | Run MODPATH menu command or equivalent toolbar macro can be used to run MODPATH.

Generating Particles

There are two ways to generate particle starting locations in GMS, both available via the MODPATH menu:

Generate Particles at Wells – Accessed in the MODPATH menu. This command automatically generates particle starting locations in cells containing wells. The specified number of particle starting locations are distributed evenly in a horizontal circle around the center of the cells containing the wells. You specify the number of starting locations per cell, and whether you want starting locations created in extraction well cells, or injection well cells.

Injection Wells – If this option is selected, particles are created in cells with wells that have a positive flow rate. The tracking direction for the current particle set is changed to forward.

Extraction Wells – If this option is selected, particles are created in cells with wells that have a negative flow rate. The tracking direction for the current particle set is changed to backward.

Selected Wells – This option makes it possible to generate particles at either all wells or only at wells within selected cells.

Generate Particles at Selected Cells – Accessed in the MODPATH menu. This command opens the Generate Particles dialog, and when you select OK, particles are created in all the selected cells according to the options specified.

Number of Particles – If the More options toggle is not selected, you can use the slider to change the number of particles created per cell. The minimum is 1, and the maximum depends on the option being used to distribute the particles. The number of particles jumps as the slider is moved in order to create an even distribution of particles. If the More options toggle is selected, the slider is not available, and the user has more control over the total number of particles and how they are distributed.

Distribute Particles – There are three options for distributing the particles within the selected cells.

• Within Cell – With this option, the particles are distributed in the interior of the cells.

• On Cell Faces – With this option, the particles are distributed on the faces of the cells.

• Distribute Starting Points on Water Table Surface – With this option, the particles are restricted to the top face only. When using this option, you should be sure to select the cells corresponding to the location of the water table surface.

• More Options – The More options toggle expands the dialog, allowing you to further define how particles are distributed within the selected cells.

(These commands are only available if a MODFLOW CCF solution generated by MODFLOW 2000 exists, and you have 3D grid cells selected.)

Transient MODFLOW Solution

For each time step, particle paths are computed just as for the steady state case until the end of the time step is reached. A new velocity distribution is then calculated for the next time step and the computation of particle paths is resumed. The computation of paths forward or backward, boundary conditions, and the path line termination criteria are handled the same as for steady state flow.