Bechtold, Michel: Experimental and numerical studies on solute transport in unsaturated heterogeneous porous media under evaporation conditions. - Bonn, 2012. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc:
author = {{Michel Bechtold}},
title = {Experimental and numerical studies on solute transport in unsaturated heterogeneous porous media under evaporation conditions},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2012,
month = aug,

volume = 143,
note = {Groundwater level rise, root water uptake, or evaporation induces local upward water and solute fluxes in soils, causing soil salinization and rise of contaminants to the soil surface, and influencing the migration of solutes to the groundwater. It is known that soil heterogeneity strongly controls transport under infiltration conditions, but its effect on transport under upward flow conditions has barely been investigated. In this thesis, laboratory tracer experiments were conducted in artificial porous media with known heterogeneity under evaporation conditions and observations were compared with numerical simulations in order to improve the understanding of upward flow and transport processes.
High concentration gradients due to solute accumulation at the soil surface caused by evaporation are posing very high demands on Eulerian schemes for solving the advection-dispersion equation (ADE), while they have no negative effect on the stability of random walk particle tracking (RWPT) schemes. However, RWPT loses accuracy when the dispersion tensor or the water content is spatially discontinuous, a topic that is frequently-debated in RWPT literature. In this thesis, a new RWPT algorithm is presented that builds on the former concept of representing the discontinuities by partially reflecting barriers. Three improvements were developed that enhance the accuracy and efficiency of this concept by orders of magnitude.
In a composite porous medium, consisting of a cylindrical inner core with coarse sand that was surrounded by fine sand, dye and salt tracer experiments were conducted under constant evaporation conditions, and a Gd-DTPA2- tracer experiment was monitored with magnetic resonance imaging (MRI) during a cycle of infiltration and evaporation. The key finding of these experiments was the formation of high solute concentration spots at the surface of the coarse material, which is contrary to the general expectation that solutes accumulate and precipitate in regions with finer texture and higher evaporation fluxes. Flow and transport simulations showed that molecular diffusion, which moves solutes away from the evaporating surface back into the porous medium, in combination with lateral water flow redistributes solutes towards locations with the lowest hydraulic head. The formation of high solute concentration spots at the surface of coarser regions, which usually represent preferential flow pathways during strong precipitation, may have an accelerating effect on the leaching of solutes.
In a three-dimensional spatially correlated heterogeneous laboratory soil composed of three different materials a salt tracer experiment was conducted under constant evaporation conditions and monitored with electrical resistivity tomography (ERT). The detailed comparison of monitored and modeled solute transport demonstrated that (1) the accuracy of the ERT observations was high enough to analyze errors of the flow and transport model, (2) a weak point of commonly-applied flow and transport models is the simplified representation of the evaporation boundary condition, in which mechanisms of lateral compensation of low evaporation zones are neglected, and (3) despite the deviations between monitored and modeled solute transport, there was a consistent and systematic transition of preferential upward transport pathways over the height of the laboratory soil.},

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