In situ measurements of colloid transport and retention using synchrotron X-ray fluorescence

[ 1] The physics regarding the retention and mobilization of colloids in saturated and unsaturated conditions remains poorly understood, partially because of the inability to measure colloid concentrations in situ. In this study, we attached Cd(+2) ions to clay colloids and used synchrotron X rays to cause the Cd to fluoresce. By measuring the fluorescence and attenuation of the X rays we obtained simultaneous in situ water saturations and colloid concentrations on timescales of tens of seconds. We used this technique to study the transport of colloids consisting of Na and Ca Montmorillonite clays through a preferential flow path in uniform well-sorted sand. This flow path had both saturated and unsaturated zones that travel downward with time. We found that the Na colloids showed little retention in the sand, while the Ca colloids were retarded with respect to the wetting front. By comparing the results to those obtained by infiltrations with a Cd solute we find that the retention of the colloids seen in the unsaturated portion of the column was no greater than that seen in the saturated portion. We discussed the advantages and limitations of this X-ray fluorescence technique and the implications for colloid transport.