What is fate and transport modeling?

It combines a groundwater flow solution with contaminant transport processes—advection, dispersion, sorption, and decay—to estimate concentrations through space and time under stated assumptions, including the source term.

When is an analytical model enough?

When the hydrogeologic setting and the decision are simple enough that a grid would add parameters you cannot support. Complexity should follow the question and the data, not the software on hand.

Can you review a model built by another firm?

Yes, when the report and input files are available. The review covers the conceptual model, boundaries, calibration targets, parameter plausibility, sensitivity, and whether the predictions match site observations.

Insights · October 15, 2026

What a fate and transport model report should show

A fate and transport model report should let a reviewer reproduce the logic: the conceptual site model, the grid and boundaries, the parameters and where they came from, the calibration targets, the sensitivity of the answer, and what each scenario is allowed to mean. Color contours without that file are an illustration, not an opinion.

Aerial view of an industrial facility, grass well pads, and a creek winding through the floodplain. — Facility, floodplain, and creek in one frame—the setting a transport model has to respect.

Start with the conceptual site model

Fate and transport is groundwater flow plus the processes that move or stall a constituent: advection, dispersion, sorption, decay, and the source term you are willing to defend. None of that is interpretable until the conceptual site model says which units exist, which ones connect, where water enters and leaves, and where the release is assumed to have happened.

If the report opens on a plot of simulated concentrations, ask for the cross-section and the water balance first. Fate and transport modeling that skips the framework usually hides the assumption inside a conductivity zone.

Document the flow model before the transport model

Reviewers should be able to see layering tied to stratigraphy, boundary conditions (and whether they create or destroy water), recharge, pumping, and the observations used as targets. A single error statistic is not a calibration. The report should say which wells were targets, how residuals are distributed, and which observations were left out on purpose.

Numerical tools in this practice include the MODFLOW family—MODFLOW, MODPATH, MT3DMS, RT3D—and PEST where parameter estimation earns its cost, plus analytical models when the question is simpler than a grid. The code name is not the opinion. The input files, the version, and a short log of material changes are.

Transport assumptions have to be visible

Retardation, decay, dispersivity, and source concentration or mass each move arrival time and peak concentration. A defensible report gives the value, the basis (site test, literature range, or assumption), and a sensitivity run on the lever that matters for the decision. If timing is the dispute, porosity and retardation belong in that run. If extent is the dispute, source mass and the connection between units usually matter more.

Scenarios are comparisons under stated assumptions. They are not proof of allocation. Say what was held constant. The note on groundwater model admissibility covers how that documentation is scrutinized once the model is in a dispute.

What peer review should be able to do

An independent reviewer should be able to open the files and find the conceptual model, the targets, the parameters, and the sensitivity cases without a phone call. If the only deliverable is a PDF of contours, the review will be about the picture, not the model.

We use that standard on original builds and on reviews of another firm’s model. Either way, the work sits next to hydrogeology consulting interpretations of heads, tests, and well construction—the model does not replace them.

Questions counsel ask

What is fate and transport modeling?: It combines a groundwater flow solution with contaminant transport processes—advection, dispersion, sorption, and decay—to estimate concentrations through space and time under stated assumptions, including the source term.
When is an analytical model enough?: When the hydrogeologic setting and the decision are simple enough that a grid would add parameters you cannot support. Complexity should follow the question and the data, not the software on hand.
Can you review a model built by another firm?: Yes, when the report and input files are available. The review covers the conceptual model, boundaries, calibration targets, parameter plausibility, sensitivity, and whether the predictions match site observations.

Daniel S. Fisher, P.G.

Owner and principal hydrogeologist, Wetstone Solutions LLC. Pennsylvania professional geologist since 1994. This note is for education. It is not legal advice and not an opinion on any matter.

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