International audienceUnderstanding particle mobilization and transport in soils is a major concern for environmental protection and water resource management as they can act as vectors for sorbing pollutants. In natural soils, the existence of a finite size and renewable pool of dispersible particles has been hypothesized. Even though freeze-thaw and wetting-drying cycles have been identified as possible mechanisms of pool replenishment between rainfall events, to date the underlying phenomena ruling the renewal of particle pools are still largely unexplored. We carried out a series of infiltration-drainage experiments to study systematically the effects of periods without rain (pauses) on in situ particle mobilization in undisturbed soil columns. We found that, for a given column, pause duration between two rainfall events has a major influence on subsequent particle mobilization: the mass of leached particles increases with pause duration until it reaches a maximum (mass for a 200-hours pause is 15 time greater than for a 1-hour pause), and then it decreases for even longer pauses. This behaviour was correlated with soil water content, and can be explained by soil matrix weakening due to differential capillary stresses during drying. The consequences of this finding are important because the 15-fold increase in mass of leached particles, when pause duration is changed from 1 hour to 4 days, might overwhelm variations caused by changes in other parameters such as the ionic strength of the incoming solution or the rainfall intensity

International audienceoil 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

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16The Upper Rhine alluvial aquifer is an important transboundary water resource. However, as 17 in many alluvial systems, the aquifer inflows and outflows are not precisely known, due to the 18 difficulty in estimating the river infiltration flux and the boundary subsurface flow. To 19 provide a thorough representation of the aquifer system, a coupled surface-subsurface model 20 was applied on the whole aquifer basin, and several parameter sets were tested to investigate 21 the uncertainty due to poorly known parameters (e.g. aquifer transmissivity computed by an 22 inverse model, river bed characteristics). Twelve simulations were run and analysed using 23 standard statistical criteria, as well as a more advanced statistical method, the Karhunen 24 This quantity is larger than estimated in previous studies, but also in agreement with some 32 results obtained during low water periods. This important conclusion highlights the 33 vulnerability of the Upper Rhine Graben aquifer to pollution from the rivers and to climate 34 change since it is highly probable that the rivers' regime from the neighbouring mountain 35 ranges will be affected by a reduced snow cover. 36 37