Water Flow Monitored by Tracer Transport in Natural Porous Media Using Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) was applied to the study of flow processes in model and natural soil cores. Flow velocities in soils are mostly too slow to be monitored directly by MRI flow velocity imaging. Therefore, we used for the first time diethylenetriaminepentaacetate in the form Gd-DTPA(2-) as a tracer in spin echo multislice imaging protocols with strong weighting of longitudinal relaxation time T-1 for probing slow flow velocities in soils. Apart from its chemical stability, the main advantage of Gd-DTPA(2-) is the anionic net charge in neutral aqueous solution. We showed that this property hinders adsorption at soil mineral surfaces and therefore retardation. We found that Gd-DTPA(2-) is a very convenient conservative tracer for the investigation of flow processes in model and natural soil cores. With respect to the flow processes in the coaxial model soil column and the natural soil column, we observed totally different flow patterns. In the first case, the tracer plume moved quite homogeneously in the inner highly conductive core only and the migration into the outer fine material was very limited. A numerical forward simulation based on independently obtained parameters showed good agreement between experiment and simulation and thus proves the convenience of Gd-DTPA as a tracer in MRI for soil physical investigations. The natural soil core, in contrast, showed a flow pattern characterized by preferential paths, avoiding dense regions and preferring loose structures. In the case of the simpler model column, the local flow velocities were also calculated by applying a peak tracking algorithm.