Présentations Colloques

    Oral Presentation
    Session 8.04: Isotopic and residence time tracers
    Stumpp Christine
    Importance of immobile water for transport in porous aquifers
    Targeted provisional session N°8.04**Porous aquifers can be heterogeneous containing immobile water regions characterized by low hydraulic conductivities. The diffusive mass exchange into such regions influences the fate of solutes in the subsurface. Consequently, transit times of solutes are larger compared to estimates from water fluxes. However, the spatial distribution and properties of immobile water regions are often unknown. Therefore, new tracer tools and modelling approaches are needed to quantify properties of mobile and immobile water regions. This information is important for evaluating the influence of immobile water regions on contaminant transport. In addition, contaminant plumes can have different densities than the ambient groundwater resulting in density-driven flow. Here, little is known about the influence of immobile water regions on transport. An analytical model was directly and indirectly validated for porous media by conducting multi-tracer experiments in a well-defined dual-porosity system at different pore-velocities. We showed that it can be used to estimate properties of both regions even if immobile water regions are randomly distributed. An extended version of this model, accounting for sorption and degradation, was used to quantify the transport of contaminants like nitroaromatic compounds or radioactive cesium (134Cs) in dual-porosity column experiments. Fitted first-order decay rates and retardation factors were found to be independent of pore-velocity. Sorption within immobile water regions enhances the mass exchange into such regions+ for 134Cs, sorption is more important compared to diffusion. Degradation of nitroaromatic compounds results in cuts of tailings caused by back diffusion which reduces the risk of a contaminant rebound after a site remediation. Experimental and numerical results on density-driven flow indicated that instabilities tend to move along the boundary surfaces and that the diffusive mass exchange into immobile water regions inhibits the growth of instabilities. These findings advance our understanding about the influence of immobile water regions on reactive transport which is important for prediction of contaminant rebounds and of transit times of contaminants in heterogeneous aquifers.


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