Mobilization of Colloids and Metals From Intact Cores of a Fractured Soil: Role of Pore Water Exchange Between Soil Matrix and Macropores.

Understanding the mechanisms of colloid mobilization is essential to predicting the importance of colloid-facilitated transport of contaminants in subsurface environment.  Under natural conditions, the effect of pore water exchange between the soil matrix and macropores on the mobilization of colloids and metals is not understood.  We conducted rainfall experiments in the laboratory to evaluate the effectiveness of pore water exchange in mobilizing cesium, strontium, and colloids from intact soil cores sampled from Oak Ridge National Laboratory in Tennessee.  The ionic strength of the rainfall was varied 0.01 to 10 mM.  Samples were collected from 19 ports in a hexagonal grid at the base of the soil core to identify regions of fast and slow flow attributed to macropore and matrix pathways. 

The mobilization of cesium and strontium increased as the ionic strength of the infiltrating water increased, whereas the mobilization of colloids decreased as the ionic strength of the infiltrating water increased.  Mobilization of both metals and colloids were hysteretic in response to changes in ionic strength – the amount of metals and colloids mobilized at a given ionic strength was not matched when the experiment was repeated following exposure to pore water of higher or lower ionic strengths.  The amount of colloid mobilization hysteresis was positively correlated to dispersion of bromide, which was used as an indicator of the rate of exchange of pore water between the soil macropores and matrix.  An exchange of pore water between the soil matrix and macropores was attributed as the primary reason for the hysteresis of metals and colloid mobilization during changes in ionic strength.