Imaging water fluxes in porous media by magnetic resonance imaging using D2O as a tracer

In this study, we investigate the usefulness of D2O as a conservative tracer for monitoring water flux by MRI in a heterogeneous sand column. The column consisted of a cylindrical 3×9-cm packing of fine sand in which an 8-mm diameter cylindrical obstacle was placed. Constant steady-state flux densities between Jw=0.07 and 0.28 cm min−1 corresponding to mean pore flow velocities between 0.20 and 0.79 cm min−1 were imposed at the top of the sand column, and a constant hydraulic head of −39 cm was maintained at the lower boundary. We injected pulses of 0.01 M NiCl2 and 55% D2O and monitored the motion of the tracer plumes by MRI using a fast spin echo sequence over a period of 20 min. We observed that the center of gravity of all plumes moved with the mean pore flow velocity, which showed that D2O behaves as a conservative tracer. The motion of the tracer plume at Jw=0.14 cm min−1 was validated by a numerical simulation using HYDRUS2D, which reproduced the experimentally observed behavior very satisfactorily.