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

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

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[1] Understanding particle movement in soils is a major concern for both geotechnics and soil physics with regard to environmental protection and water resources management. This paper describes a model for mobilization and preferential transport of soil particles through structured soils. The approach combines a kinematic-dispersive wave model for preferential water flow with a convective-dispersive equation subject to a source/sink term for particle transport and mobilization. Particle detachment from macropore walls is considered during both the steady and transient water flow regimes. It is assumed to follow first-order kinetics with a varying detachment efficiency, which depends on the history of the detachment process. Estimates of model parameters are obtained by comparing simulations with experimental particle breakthrough curves obtained during infiltrations through undisturbed soil columns. Both water flux and particle concentrations are satisfactorily simulated by the model. Particle mobilization parameters favoring both attachment and detachment of particles are related to the incoming solution ionic strength by a Fermi-type function.Citation: Majdalani, S., E. Michel, L. Di Pietro, R. Angulo-Jaramillo, and M. Rousseau (2007), Mobilization and preferential transport of soil particles during infiltration: A core-scale modeling approach, Water Resour. Res., 43, W05401,

The objective of this work was to evaluate the transport of Escherichia coli cells in undisturbed cores of a brown leached soil collected at La Côte St André (France). Two undisturbed soil cores subjected to repeated injections of bacterial cells and/or bromide tracer were used to investigate the effect of soil hydrodynamics and ionic strength on cell mobility. Under the tested experimental conditions, E. coli cells were shown to be transported at the water velocity (retardation factor close to 1) and their retention appeared almost insensitive to water flow and ionic strength variations, both factors being known to control bacterial transport in model saturated porous media. In contrast, E. coli breakthrough curves evolved significantly along with the repetition of the cell injections in each soil core, with a progressive acceleration of their transport. The evolution of E. coli cells BTCs was shown to be due to the evolution of the structure of soil hydraulic pathways caused by the repeated water infiltrations and drainage as may occur in the field. This evolution was demonstrated through mercury intrusion porosimetry (MIP) performed on soil aggregates before and after the repeated infiltrations of bacteria. MIP revealed a progressive and important reduction of the soil aggregate porosity, n, that decreased from approximately 0.5 to 0.3, along with a decrease of the soil percolating step from 27 to 2 μm. From this result a clear compaction of soil aggregates was evidenced that concerned preferentially the pores larger than 2 μm equivalent diameter, i.e. those allowing bacterial cell passage. Since no significant reduction of the global soil volume was observed at the core scale, this aggregate compaction was accompanied by macropore formation that became progressively the preferential hydraulic pathway in the soil cores, leading to transiently bi-modal bacterial BTCs. The evolution of the soil pore structure induced a modification of the main hydrodynamic processes, evolving from a matrix-dominant transfer of water and bacteria to a macropore-dominant transfer. This work points out the importance of using undisturbed natural soils to evaluate the mobility of bacteria in the field, since the evolving hydrodynamic properties of soils appeared to dominate most physicochemical factors.

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The identification of groundwater parameters in heterogeneous systems is a major challenge in groundwater modeling. Flexible parameterization methods are needed to assess the complexity of the spatial distributions of these parameters in real aquifers. In this article, we introduce an adaptative parameterization to identify the distribution of hydraulic conductivity within the large-scale (4400 km(2) ) Upper Rhine aquifer. The method is based on adaptative multiscale triangulation (AMT) coupled with an inverse problem procedure that identifies the parameters' distributions by reducing the error between measured and simulated heads. The AMT method has the advantage of combining both zonation and interpolation approaches. The AMT method uses area-based interpolation rather than an interpolation based on stochastic features. The method is applied to a standard 2D groundwater model that takes into account the interactions between the aquifer and surface water bodies, groundwater recharge, and pumping wells. The simulation period covers 204 months, from January 1986 to December 2002. Recordings at 109 piezometers are used for model calibration. The simulated heads are globally quite accurate and reproduce the main dynamics of the system. The local hydraulic conductivities resulting from the AMT method agree qualitatively with existing local experimental observations across the Rhine aquifer.