Centrifuge modeling of transport processes for pollutants in soils

The physico-chemical processes of advection, dispersion, adsorption, and degradation that control pollutant transport processes in groundwater are described. Dimensional analysis is presented, and the scaling requirements for centrifuge modeling of these processes is developed. The validity of these scaling laws is examined by conducting modeling of models tests using several types of soils in idealized models of one-dimensional flow situations. The importance of scaling gravity (by using a centrifuge) is highlighted. The centrifuge permits simulation of prototype stress levels and thus makes it possible to obtain identical soil properties such as permeability in a small-scale model as compared to a full-scale soil deposit. Simulation of capillary effects and a phreatic surface is also possible in a centrifuge model, but not in a 1-g scale model. It is concluded that the scaling laws are valid for adsorption and advection in the reported model tests, but in coarse-grained soils where the Peclet number is often greater than one, the dispersion process cannot be directly scaled from model to prototype. Even if direct scaling is not possible, the centrifuge is a useful tool for providing data to test or verify numerical models, since full-scale test data are not abundant, and the full-scale boundary and site conditions are poorly defined.