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Preferential flow is quite usual in natural environments. Non-uniform and preferential flows co-exist or alternate, impacting water transport and contaminant transfer through the vadose zone. In this study, we investigated how macropore-induced flow affects manufactured nanoparticles, as emerging contaminants reactive transfer. Previous studies showed that the presence of a macropore into water-saturated soil columns can foster preferential water flow within the macropore. One could expect that this preferential flow may increase contaminant transfer and reduce retention by the matrix in the case of contaminant, as previously reported. In this study, we injected pulses of silver nanoparticles to assess their transfer through sand columns with and without a macropore. Both systems (with and without macropore) were studied under similar conditions. An unexpected result was obtained: more nanoparticles were retained in the system with a macropore, i.e., with a preferential flow. This result is quite counter-intuitive. It appears that the relation between flow homogeneity and contaminant retention is not straightforward. Some possible explanations, related to chemical and physical kinetics, are put forward to explain the experimental results.

The effect of macropore flow on solute transport has spurred much research over the last forty years. In this study, non-reactive solute transport in columns filled by macropored porous media was experimentally and numerically investigated, and the emphasis was put on the study of exit effects, whose very existence is inherent to the finite size of any experimental column. We specifically investigated the impact of the presence of a filter at the column outlet on water flow and solute transport in macropored systems. Experiments involving breakthrough measurements and magnetic resonance imaging (MRI) showed that solute transport displayed some significant non-unidirectional features, with a strong mass exchange at the interface between the macropore and the matrix. Fluid dynamics and transport simulations indicated that this was due to the non-unidirectional nature of the flow field close to the outlet filter. The flow near the exit of the column was shown to be strongly impacted by the presence of the outlet filter, which acts as a barrier and redistributes water from the macropore to the matrix. This impact was apparent on the breakthrough curves and the MRI images. It was also confirmed by computer simulations and could, if not properly taken into account, impede the accurate inference of the transport properties of macropored porous media from breakthrough experiments.

In natural systems, preferential flow is the rule rather than the exception. Non-uniform and preferential flows significantly impact mass transport, and by this way most of geochemical processes and pollutant dispersion in the environment. Laboratory columns are experimental devices used for the monitoring of solute transfer through porous. In particular, several studies used such experimental devices for characterizing mass transfer through heterogeneous systems and macropored systems. However, the design of these devices and its impacts on the experimental results has never been investigated in depth so far. In particular, the edge effect is rarely questioned and the transfer is always hypothesized to correspond to a fully developed flow (i.e., flow in an equivalent infinite system). In this study, we question this hypothesis both experimentally and numerically for the case of a macropored system. Tracer elutions, magnetic resonance imaging (MRI), and modeling using multiphysics approaches (Comsol) are conducted to demonstrate that flow is affected by edge effects close to the inlet and the outlet of the column, and that the presence of filters (used to prevent particles from exiting the system and clogging the outlet) do impact the flow and transfer breakthrough. Consequently, these edge effects should be considered when analyzing the results and concluding on the involved processes, in particular for the case of soils and systems with macropores.