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

Magne c resonance imaging (MRI) was applied to the study of fl ow processes in model and natural soil cores. Flow veloci es in soils are mostly too slow to be monitored directly by MRI fl ow velocity imaging. Therefore, we used for the fi rst me diethylenetriaminepentaacetate in the form Gd-DTPA2− as a tracer in spin echo mul slice imaging protocols with strong weigh ng of longitudinal relaxa on me T1 for probing slow fl ow veloci es in soils. Apart from its chemical stability, the main advantage of Gd-DTPA2− is the anionic net charge in neutral aqueous solu on. We showed that this property hinders adsorp on at soil mineral surfaces and therefore retarda on. We found that Gd-DTPA2− is a very convenient conserva ve tracer for the inves ga on of fl ow processes in model and natural soil cores. With respect to the fl ow processes in the coaxial model soil column and the natural soil column, we observed totally diff erent fl ow pa erns. In the fi rst case, the tracer plume moved quite homogeneously in the inner highly conduc ve core only and the migra on into the outer fi ne material was very limited. A numerical forward simula on based on independently obtained parameters showed good agreement between experiment and simula on and thus proves the convenience of Gd-DTPA as a tracer in MRI for soil physical inves ga ons. The natural soil core, in contrast, showed a fl ow pa ern characterized by preferen al paths, avoiding dense regions and preferring loose structures. In the case of the simpler model column, the local fl ow veloci es were also calculated by applying a peak tracking algorithm.