Solute transport in a heterogeneous unsaturated subsoil : experiments and modeling
The impact of the soil structure on flow and transport in partially water saturated soils is currently still a matter of scientific debate. The major aim of this thesis was to investigate the relation between heterogeneity and transport for a natural unsaturated heterogeneous Tertiary sand deposit. In the first part, we analyzed the flow and transport at the scale of an undisturbed monolith. Chloride breakthrough curve experiments were used to derive an apparent dispersion coefficient at the TDR sampling and monolith scale. Application of a Brilliant Blue pulse allowed further the visualization of flow distribution within the monolith. Small undisturbed soil cores were sampled throughout the monolith and the hydraulic characteristic curves at the scale of the cores were determined. Textural variability and structure as inferred from the inspection of the Brilliant Blue pattern and analysis of the small core sampling were subsequently implemented in a 3-D model and transport was simulated. The simulations clearly revealed the importance of the macro-structure on the transport behavior of the soil. We also showed that the micro-variability heterogeneity component was needed to assess the scaling of the effective and local scale dispersivity.
In the second part, we studied in-situ chloride transport in the vadose formation separating the bottom of a lake and an unconfined aquifer. First the uncertainty generated by the undersampling of the lake chloride concentration time series were investigated. Subsequently, velocity and dispersivity profiles were assessed by inverse modeling of the soil chloride concentration time series. We observed that the clay layers induced an increase of the dispersivity below them. We hypothesize that fingering flow or convergence phenomena, occurring below sand-clay interfaces, lead to non-representative artificially high dispersivity values. Velocity and dispersivity values just above the clay layers however seem more reliable due to convergence phenomena and better lateral mixing induced by a larger water content.
In this formation, the transport behavior could be characterized considering a hierarchical structure of the subsoil heterogeneity. In this model, the flow field micro-variability is influenced by pore structure (possibly characterized by scaling factors). The next complexity level is induced by the slight layering resulting from the sedimentation process (not investigated in this work). Then, the third hierarchical level is assessed by the macro-structure and the sequence of clay layers in the sand. Each of these levels is assumed to have an effect on the solute mixing process and effective macro-dispersivity.
School:Université catholique de Louvain
Source Type:Master's Thesis
Keywords:upscaling solute transport heterogeneity vadose zone structure
Date of Publication:05/28/2004