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Miscible displacement of salinity fronts: Implications for colloid mobilization

Colloids can be mobilized in the subsurface when low ionic strength pore water displaces high ionic strength pore water. Various experiments in saturated porous media, where such a change in ionic strength was used to mobilize colloids, have been reported in the literature. Here we experimentally test the effect of the flow direction on the hydrodynamic stability of the salinity displacement front. A series of experiments was conducted in packed columns where a solution of 1000 mM NaCl was displaced by 1 mM NaNO3 in a saturated porous medium. The flow direction was vertically downward, horizontal, or vertically upward. Nitrate breakthrough curves were determined at the column outflow. Results show that for downward flow of a salinity front the displacement fronts were hydrodynamically stable, but breakthrough curves showed less dispersion than in the case of equimolar miscible displacement. For upward flow of the salinity front the displacement was unstable. Breakthrough curves were not reproducible, and a family of breakthrough curves was observed for repeated experiments. This was caused by the development of distinct fingers at the displacement front. For horizontal flow of the salinity front the displacement was unstable. Repeated breakthrough curves were reproducible, but there was more dispersion than under equimolar displacement, which we attribute to some instability in the displacement front. Front instability inferred from breakthrough curves was consistent with theoretical expectations for miscible displacement.