PARTICLE AND PLUTONIUM MOBILIZATION IN MACROPOROUS SOILS DURING RAINFALL SIMULATIONS

To test the effects of infiltration velocity, soil type, and soil stru cture on the mobilization of particles in the unsaturated zone, we mon itored particle concentrations and plutonium activities in water movin g through macroporous soils during rainfall simulations at the Rocky F lats Plant in Colorado. Rainfall simulations were conducted in three s oil pits at the same intensity and in one soil pit at three intensitie s. The rapid arrival of water at zero-tension samplers located at dept hs from 15 to 70 cm indicated that macropore flow dominated infiltrati on. Most of the particle mobilization occurred during the initially sl ow infiltration of the first small volumes of rainfall to pass through the soil, resulting in a lack of correlation between particle concent ration and the discharge rate (Darcy velocity) of the infiltrating wat er. Particle size distributions (1-50 mu m) were steady during the sim ulation sand displayed greater mass in the larger size ranges. The amo unt of particle mobilization was not related to soil composition. Thre e successive applications of rainfall over 5-10-day intervals to the s ame soil pit depleted the supply of particles that could be mobilized. The plutonium content of the particles decreased with depth, and plut onium content of the particles decreased with depth, and plutonium tra nsport was largely attenuated in the upper 15-20 cm of soil.