Contaminants move by physics, not by argument. Whether a substance could have travelled from a given source to a given receptor in the time available is often the most testable question in the case.
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Fate and transport analysis asks what happened to a substance between release and receptor: which way it moved, how fast, how much it dispersed or degraded on the way, and what concentration remained when it arrived. It is the discipline that connects a source to a location, and it cuts in both directions — the same modeling that establishes a plume reached a neighbourhood can demonstrate that a different, more distant source could not have. Air, groundwater, and vapor each behave differently and are modeled with different tools, but the failure modes are shared: models run on defaults where site data existed, models never calibrated against measurements, and models pushed past the conditions they were built for.
Different media, different physics, different models — and different evidence required to constrain each one.
Stack and fugitive emissions carried downwind, governed by release parameters, terrain, and meteorology over the exposure period.
Dissolved plumes moving with the aquifer, retarded by sorption and shaped by hydraulic conductivity and gradient.
Volatiles partitioning from subsurface contamination into soil gas and entering buildings through the foundation.
How a substance distributes between soil, water, and air phases, which governs what is available to move at all.
Whether the substance breaks down on a timescale that matters, and what the degradation products are. Some, notably PFAS, essentially do not.
Uptake into fish, produce, or livestock creating an ingestion route that bypasses air and water entirely.
How transport analysis is done and tested.
It is the discipline most capable of eliminating a candidate source outright, which makes it valuable to whichever side the physics favours.
A plume that would need forty years to reach a well cannot explain contamination that appeared in fifteen. Timing arguments are quantitative, hard to rebut with narrative, and frequently decisive on source attribution.
It is well accepted where the hydrogeology is characterised and the model is calibrated, and vulnerable where it is not. Subsurface heterogeneity is the central difficulty: preferential pathways, fractured bedrock, and variable conductivity can produce transport behaviour that a homogeneous model will not reproduce. The strongest analyses are constrained by real monitoring data from multiple wells over time, state the sensitivity of conclusions to the parameters that are least known, and avoid claiming precision the subsurface does not permit.
Frequently, and it is one of the more powerful defensive uses of the discipline. If the groundwater gradient runs away from the receptor, if travel time exceeds the elapsed period, or if the contaminant profile is inconsistent with what the site handled, transport analysis makes that concrete rather than argumentative. Combined with source attribution work it can point affirmatively at a different origin, which is considerably more persuasive than simply denying responsibility.
The attenuation between subsurface contamination and indoor air is large, variable, and building-specific, which makes generic screening factors blunt instruments for litigation. Building construction, foundation condition, heating and ventilation behaviour, and even seasonal pressure differences change indoor concentrations substantially. Indoor air sampling is more direct but confounded by background sources — consumer products and building materials contain many of the same volatiles — so distinguishing subsurface contribution from ordinary indoor background is usually the contested question.
It becomes simpler in one respect and harder in another. Persistent compounds such as PFAS remove the degradation term, so the plume does not attenuate the way a solvent plume does and can extend much further from the source than intuition suggests. But sorption and transport behaviour for these compounds is still an active research area, and older models calibrated on degradable solvents can substantially misrepresent how far and how fast they travel. Analyses in this space should be explicit about which parameters are well established and which are not.
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