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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #201660

Title: In-situ Measurements of Colloid Transport and Retention Using Synchroton X-ray Fluorescence

item Dicarlo, David
item GAO, BIN

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/8/2006
Publication Date: N/A
Citation: N/A

Interpretive Summary: Many contaminants (such as heavy metals and pesticides) are only transported through soils by attaching to small solid particles (colloids) which can become mobile. This study uses x-rays to measure how colloids are mobilized and retained in a soil column. The key advance is that by using the high powered x-rays, we can effectively see inside the column rather than just measuring what comes out of the column. This has the potential to revolutionize how colloid transport is studied and modeled.

Technical Abstract: The physics regarding the retention and mobilization of colloids in saturated and unsaturated conditions remains poorly understood, partially due to 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 time scales 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.