Three-dimensional modeling of the scale- and flow rate-dependency of dispersion in a heterogeneous unsaturated sandy monolith

Spatial heterogeneity in soil hydraulic properties strongly affects solute transport. However, the level of precision needed in the description of spatial variability to properly reproduce the solute mixing regime is still an open question. We investigated this problem by analyzing observed inert solute transport using three-dimensional simulations with different levels of complexity in the material description. The scale- and flow-dependency of the dispersivity was first characterized from a series of leaching experiments during unsaturated steady-state flow in a heterogeneous sandy monolith. The structure and hydraulic property variability within the monolith was investigated as well by means of an exhaustive survey of the monolith, and by intensive soil core sampling allowing for hydraulic characterization. In this study, three three-dimensional models are constructed, involving several levels of complexity. In Case I, only the macrostructure variability is represented. In Case II, scaling factors encoding the spatial variability in the hydraulic properties of the sandy matrix are implemented. In Case III, an anisotropy factor for hydraulic conductivity is added to the macrostructure and the microheterogeneity of the sand matrix. Results show that microheterogeneity is needed to reproduce qualitatively the scale- and flow rate-dependency of the transport parameters. Despite the elaborate effort devoted to the structure characterization, no model was fully capable of reproducing observed solute transport in the monolith and at the outlet.