About pollutants transport modelling in groundwater
Groundwater quality modelling has become a tool for water management. The accuracy of the simulations closely depends on the available knowledge concerning the transport processes and the parameters used in the model. The accuracy of the results depends on the choice ofa suitable model adapted to the observation scale, the measurement of the effective parameters linked to the discretization of the field and the spatialization of the local measurements. The mathematical model used to describe mass transport in porous media is the dispersion-convection equation. The velocity is calculated by solving the flow equation. For heterogeneous media, numerical schemes which simultaneously solve heads and velocities have to be preferred to classical finite element or finite differences techniques. The dispersion coefficient represents the velocity fluctuations around the average velocity. Therefore, it strongly depends on the dimension and the scale of the discretization. A predictive simulation of the Twin Lake Tracer Test experiment has been done. After a very fine calibration of the flow (differences between measured and calculated heads less than 1 cm), the transport simulation did not succeed. The headgradients were not calculated with enough accuracy and the simulated plume travelled in a wrong direction. Due to the nature of groundwater (invisible, expensive rneasurements), knowledge of the structure of the aquifer will always be too incomplete to perform very detailed simulations. Stochastic computations may be the way to take into account uncertainities in groundwater modening.