What to do when your MODFLOW MT3D contaminant plume doesn’t migrate?


This is a problem that every hydrogeologist – numerical modeler has faced many times over his/her professional career independently from the theoretical background and skills with the code, graphical user interface or Python library. 

The solitude psychological problem

Facing a problem like this or others related to groundwater modeling could be diagnosed as a period of high stress that lasts over days or weeks until the problem it’s solved. The term “solitude” comes from the “einsamkeit” – loneliness in the work of numerical modelers. Most students, researchers or professionals have few options of direct and effective support from their teachers or colleagues; therefore the problem solution process involves a violent mix of trial and error, partial documentation review, and questions to our blog.

We know the ground of this problem, although we can come up with an effective solution or an alternative plan; we just can give a checklist of the main issues related to plume migration.

When your contaminant plume doesn’t migrate

We don’t know why groundwater “flows” and a contaminant plume “migrates” when both move on the aquifer system. After you construct you model geometry, setup the flow boundary condition, specify your transport package, define your contaminant sources, run your MODFLOW flow model and then run your MT3D model (or even your MT3DMS model) you will find that there is no plume, none, nada, nichts. 

The most rational thing could be to read the .list file for flow information or the .mls for transport information however, taking into consideration the stress involved, the chances of a careful and conscious review are scarce. To overcome this problem we have developed a list of the most common issues when your plume doest migrate.

Perfection requires a hand of time as well as contaminant plume migration

We have to change our schema of temporal discretization related to seconds, days, months to  lustrum, decades or even centuries when we deal with contaminant plumes since they take lots of time to migrate. 

Check the cell size

A contaminant plume could be migrating, but the whole plume extension can fit on the cell and you will have just one concentration value. 

Review your K

If you are working on granular aquifers, you might see some migration after some months, but if you are dealing with low K aquifers that migration could be minimal.

Are you applying enough contaminant mass flux?

Depending on the source type and the related boundary condition the resulting mass flux (flow x concentration) can be so small to produce a contaminant migration or the concentration values would be very small.

Have you specified enough iterations for your MT3D model?

A common error on modeling contaminant plumes where the iterations for the transport model are not enough to complete the simulation of the contaminant plume. The maximum transport steps per flow step (MXSTRN) has to be large to reach the end of the transport step.

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