Magnetic resonance imaging of paramagnetic tracers in porous media: Quantification of flow and transport parameters

Experiments in model soil columns made of 0.5-mm glass beads demonstrated that the convective and dispersive transport in porous media can be visualized and transport parameters can be quantified by using magnetic resonance imaging (MRI). Three-dimensional information of water or tracer distribution in columns was obtained at different times in slices of 10 mm thickness and with an image plane resolution of 2.34 mm. The total mass of tracer in the columns and the pore water velocities determined by MRI were found to be consistent with the input values and with those derived from the outflow breakthrough curves (OBC). The dispersivity for a homogeneous medium was found to be on the order of half the grain diameter increasing by a factor of 4 in a heterogeneous medium, which is consistent with literature values. The dispersivities derived from the OBC were 3 times higher owing to the flow pattern inside the columns which we identified and took into account in our MRI analysis but not in the standard one-dimensional analysis of the OBC. This apparatus-induced dispersion could be demonstrated explicitly and visually.