Immobile water regions – influence on contaminant transport and density-driven flow
O 13.6 in Reactive transport modeling
14.04.2016, 16:00-16:15, Audimax A, Geb. 30.95
Porous aquifers can be highly 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. 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 is important information to further evaluate 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 this transport process. An analytical model, originally developed for fissured systems, was directly and indirectly validated for porous media by conducting multi-tracer experiments in a well-defined dual-porosity system at different pore-velocities. This approach can be used to estimate properties of both regions even if immobile water regions are randomly distributed (Knorr et al. in press). An extended version of this model, accounting for sorption and degradation, was used to quantify the transport of nitroaromatic compounds 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. On the other hand degradation 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. The findings of this study advance our understanding about the influence of immobile water regions on reactive solute transport which is important for prediction of contaminant rebounds.
KNORR, B., MALOSZEWSKI, P., KRÄMER, F. & STUMPP, C. (in press): Diffusive mass exchange of non-reactive substances in dual-porosity porous systems – a column experiment under saturated conditions. Hydrological Processes, doi: 10.1002/hyp.10620
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