One of these variables is the sediments suspended in the water. Not only is the estuary not a simple pipeline, but moreover, the water of the Scheldt is not clear like tap water. It is troublesome, filled with micro particles of organic and mineral origins. “Certain pollutions have a propensity to attach themselves to these micro particles”, explains Eric Delhez, “which evidently influences their transport dynamic. This will be different from that of a substance which dissolves in water.” Taking in to account what the researchers call the partition coefficient (the ability of a pollutant to attach to sediments) it is possible to calculate the time that a specific pollutant would take to travel down the river. The Liege researcher’s model shows that a glass of pure water poured in to the Escaut in Gent will arrive 60 days later in Vlissingen, 160km further down the estuary, whereas, pollution which has a strong tendency to attach to sediments will arrive in Vlissingen after only 160 days. Why this difference? By attaching to particles, the pollution is going to be deposited on the river bed, before being moved again when the river is agitated, for example, by the spring tides or even storms, and then it will go and deposit itself again further down and so on and so forth…

The theory of age

"This modeling, explains Eric Delhez, uses the notion of “age” or “characteristic time”.  They are concepts and equations that help to quantify the speed at which many processes occur, in this case here a transportation process. It is an idea that can integrate biologists and chemists, a currency of exchange between the office engineers that we are and other scientists who are interested in the state of our watercourses". In fact, a chemist can estimate if such and such processes have the time to happen in 60 or 160 days. In the same way, a biologist could estimate the impact of pollution, heavy metals for example, on the flora and forna according to the time the pollutant stays in the estuary. The theory of age, which is part of the Liege numeric model, is a general concept which can be used outside hydrological models. It can also be used to calculate the spread of a radioactive cloud following a nuclear accident like in Chernobyl or Fukushima.

We can imagine checking the validity of a numeric simulation by conducting field work. “Certain aspects of the model are validated by comparing the concentrations calculated with measurements taken in situ by the field oceanographers. On the other hand, the notion of age is not an easily accessible experience, says Eric Delhez. If you experiment with pouring several liters of pollution in the river in Gent, no instrument currently available can measure anything 160 days later in Vlissingen. The Escaut estuary is just too large a natural laboratory. The only possibility would be to take measurements during a big accidental pollution incident, but hopefully that is not a step any researcher would take." We can perhaps simply wait for it to happen and prepare ourselves to react to the event.