How Differential Capillary Stresses Promote Particle Mobilization in Macroporous Soils: A Novel Conceptual Model

Soil particles of colloidal size have been known for more than two decades to facilitate the transport of adsorbed contaminants through the vadose zone. Understanding the mobilization mechanisms of these particles is thus essential for environment and water resource protection. It was recently shown that when the dry period before a rainfall event varies from 1 h to a few days, the mass of mobilized particles increases by more than an order of magnitude. This mobilization increase was indirectly linked to water content variations in preferential flow pathways. In this study, we developed a novel conceptual model of autochthonous particle mobilization in macroporous soils that explains this observation. We assumed that during a rain interruption, water loss from the macropore walls induces differential capillary stresses that weaken the structure of the walls. This weakening promotes mobilization during the passage of the infiltration front at the beginning of a subsequent rainfall event. The model computes the number of mobilized particles as a function of the rain interruption duration. We compared the computed mobilization with data obtained from a series of successive rainfall events performed at the column scale on a calcareous soil. Our simple model reproduced qualitatively well the observed variations of mobilization with rain interruption duration. This agreement strengthens the hypothesis of a mobilization process linked to capillary stresses occurring in the macropore walls. The model also provides insight into how the chronology of rainfall events undergone by the soil influences mobilization during successive events. Finally, it provides a novel link between colloid mobilization and pore structure evolution.