Particle and plutonium mobilization in macroporous soils during rainfall simulations

To test the effects of infiltration velocity, soil type, and soil structure on the mobilization of particles in the unsaturated zone, we monitored particle concentrations and plutonium activities in water moving through macroporous soils during rainfall simulations at the Rocky Flats Plant in Colorado. Rainfall simulations were conducted in three soil pits at the same intensity and in one soil pit at three intensities. The rapid arrival of water at zero-tension samplers located at depths from 15 to 70 cm indicated that macropore flow dominated infiltration. Most of the particle mobilization occurred during the initially slow infiltration of the first small volumes of rainfall to pass through the soil, resulting in a lack of correlation between particle concentration and the discharge rate (Darcy velocity) of the infiltrating water. Particle size distributions (1-50 mu m) were steady during the simulation sand displayed greater mass in the larger size ranges. The amount of particle mobilization was not related to soil composition. Three successive applications of rainfall over 5-10-day intervals to the same soil pit depleted the supply of particles that could be mobilized. The plutonium content of the particles decreased with depth, and plutonium content of the particles decreased with depth, and plutonium transport was largely attenuated in the upper 15-20 cm of soil.