Contamination by slaked fragments with sorbed compounds in a structured soil

An adequate understanding of the mechanisms involved in solute transport through porous media is needed to help design management strategies aimed at controlling ground water contamination. The objectives of this study were, first, to evaluate the effect of soil structure changes on the processes of water and solute transport, and second, to assess the contribution of the fine particles detached during rapid wetting (slaked fragments) to sorbed solute transport. A laboratory study was conducted on a silty loam soil wetted at three rates (slow, medium, and rapid) after adding a soluble compound (Br) and a highly adsorbed compound (radioactive ¹³⁷Cs) to sieved aggregates that were then deposited on the untreated soil surface. Soil particle migration (obtained from ¹³⁷Cs measurements), Br transport, mean weight diameter (MWD), and wetting rate were measured following the wetting events. The results showed that, from the soil surface down to a depth of 100 mm, sorbed soil particle transport occurred in significantly greater amounts under the rapid wetting treatment than under the slow and medium wetting treatments. For one single rainfall event, the sorbed fraction that left the surface represented about 0.3% of the surface applied amount. This occurred despite the fact that soil disintegration following the rapid wetting process increased the residual mass of Br in the soil from 77% (slow wetting) to 88% (fast wetting) and decreased the speed of water and Br transport through the soil profile. These results imply that contaminants sorbed onto external aggregate surfaces could be transported through the soil profile more quickly and in greater quantities than predicted by conventional contaminant transport models.