Pore-scale redistribution of water during wetting of air-dried soils as studied by low-field NMR relaxometry.

The kinetics of water uptake and redistribution in several soils and their components are studied using NMR relaxometry. Unlike the normal behavior observed in stable porous media, entry into micropores in the soil is a slow process as compared to entry into macro- and mesopores. This indicates that soils air-dried at ambient temperature include gel phases that have collapsed or reoriented, closing micropores, during drying. Wetting must then include the swelling processes that re-open micropores. This can even exhibit temperature dependence giving an "apparent activation energy" comparable to that of a chemical reaction, for example, ester hydrolysis. The processes of micropore opening may play a role in slow uptake of contaminants into soils.