Eric Michel

Mobilization and preferential transport of soil particles during infiltration: A core‐scale modeling approach

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.

Combined time-lapse magnetic resonance imaging and modeling to investigate colloid deposition and transport in porous media

Colloidal particles can act as vectors of adsorbed pollutants in the subsurface, or be themselves pollutants. They can reach the aquifer and impair groundwater quality. The mechanisms of colloid transport and deposition are often studied in columns filled with saturated porous media. Time-lapse profiles of colloid concentration inside the columns have occasionally been derived from magnetic resonance imaging (MRI) data recorded in transport experiments. These profiles are valuable, in addition to particle breakthrough curves (BTCs), for testing and improving colloid transport models. We show that concentrations could not be simply computed from MRI data when both deposited and suspended colloids contributed to the signal. We propose a generic method whereby these data can still be used to quantitatively appraise colloid transport models. It uses the modeled suspended and deposited particle concentrations to compute modeled MRI data that are compared to the experimental data. We tested this method by performing transport experiments with sorbing colloids in sand, and assessed for the first time the capacity of the model calibrated from BTCs to reproduce the MRI data. Interestingly, the dispersion coefficient and deposition rate calibrated from the BTC were respectively overestimated and underestimated compared with those calibrated from the MRI data, suggesting that these quantities, when determined from BTCs, need to be interpreted with care. In a broader perspective, we consider that combining MRI and modeling offers great potential for the quantitative analysis of complex MRI data recorded during transport experiments in complex environmentally relevant porous media, and can help improve our understanding of the fate of colloids and solutes, first in these media, and later in soils.

Transport and Adsorption of Nano-Colloids in Porous Media Observed by Magnetic Resonance Imaging

We use magnetic resonance imaging to follow the adsorption of colloids during their transport through a porous medium (grain packing). We injected successive pulses of a suspension of nanoparticles able to adsorb onto the grains. To get quantitative information we carry out 2D imaging and 1D measurements of the evolution in time of the distribution profile of all particles (suspended or adsorbed) in cross-sectional layers along the sample axis during the flow. For the first injections we observe the 1D profile amplitude progressively damping as particles advance through the sample, due to their adsorption. 2D imaging shows that successive injections finally result in a coverage of grains by adsorbed particles regularly progressing along the sample. The analysis of the results makes it possible to get a clear description of the adsorption process. In our specific case (particle charged oppositely to the adsorption sites) it appears that the particles rapidly explore the pores and adsorb as soon as they encounter available sites on grains, and the surplus of particles goes on advancing in the sample. A further analysis of the profiles makes it possible to distinguish the respective concentration distribution of suspended and adsorbed particles over time at each step of the process.

Magnetic Resonance Imaging and Relaxometry as Tools to Investigate Water Distribution in Soils

Relaxation times and two imaging sequences (spin echo and single point imaging) were performed onto repacked soil samples to study respectively water distribution within the porosity and to measure water content profiles, distinguishing water contained in large pores from water contained in the whole porosity. These methods were applied to 25 samples of the same soil that was prepared to obtain aggregates of three different size, then repacked to five bulk densities. Samples were then equilibrated with water at five matric potentials. We found that T1 and T2 measurements present similar time distributions with essentially four peaks. We attributed the two shortest times to textural pores, and the two longest times to structural pores. The water profile measured with spin echo sequence was attributed to water contained in structural pores.