Characterization of the Heterogeneous Flow and Pollutant Transfer in the Unsaturated Zone in the Fluvio-glacial Deposit

Abstract

Infiltration basins are part of the best management practices. They are aimed at infiltrating stormwater to prevent additional collection and treatment through rainwater systems. In the suburbs of Lyon (France), many of these infiltration basins were built over fluvio-glacial deposit. These basins have been the subject of research programs on vadose zone flow and fate of pollutants. This study focuses on the impact of the heterogeneity of the fluvio-glacial deposit on both flow pattern and solute transfer. A proper geological and sedimentological description is first proposed to characterize the efficient water transfer properties of the fluvio-glacial deposit at the work scale (1ha). The local geological and sedimentological architecture of the deposit and its lithofacies were investigated locally through trenches using both particle size analysis and sedimentological approach. This information was extended to the whole work by combining several geophysical techniques, i.e. GPR, electric resistivity and seismic refraction tomography (data not shown). Then water infiltration experiments were performed on each lithofacies to derive the hydrodynamic properties through BEST algorithm (Beerkan estimation of Soil Transfer properties), leading to the corresponding hydrofacies. In addition, soil-column and batch experiments were performed to estimate hydrodispersive parameters by tracer experiments and the geochemical properties of lithofacies for a model pollutant, i.e. copper (Cu). All these data were implemented into Hydrus to model flow and solute transfer through a 2D soil profile with a precise description of the hydrofacies at the basin scale (flow domain 14x2 m2). The results are highly relevant because they emphasize different types of preferential flow due to either the presence of capillary barriers, drainage layers or pipe flow, which may be responsible for the enhancement of pollutant transfer. In particular, they show that sand lenses may play an important role whereas unconnected gravels may have insignificant effect on flow. This methodology may help in understanding the mechanisms which that are responsible for preferential solute transfer. A sensitivity analysis combining the distribution of lithofacies, the soil initial water content and unsaturated hydraulic properties allows bettering understanding the development of preferential flows across the vadose zone underneath the basin. This data is of great interest in terms of the optimization of basin infiltration monitoring.