| To examine particle mobilization and particle-facilitated contaminant transport in the vadose zone two experimental approaches were employed. The first was a model soil system with known particle and sand geometry. Silica-coated particles (0.12 and 1.22 mum diameter) were deposited on quartz sand (0.45 mm mean diameter). To measure the spatial variability of infiltration and particle mobilization, samples were collected in 68 ports distributed over the base of the column. The first series of experiments examined the relative importance of diffusion and hydrodynamic shear in particle mobilization. A diffusion model provided a good fit of the average concentrations of the small and large particles as a function of time. Further experiments incorporated a preferential flow path of coarse-grained sand in the homogeneous columns. When the column was initially wet, there was little transport of water and particles to the preferential flow path. When the column was initially dry, substantial water and particle transport was observed through the preferential flow path. This suggests that the presence of preferential flow paths may influence particle transport in heterogeneous soils.; Model system results were tested on soil cores collected from metal-contaminated soils from Leadville, CO. Particle mobilization was consistent with a model that accounted for diffusion from the soil matrix to preferential flow paths. Almost an order of magnitude increase in rainfall intensity (0.64--5.06 mm/min) did not change average particle concentrations. Particle size analysis revealed that particles were significantly larger during the second rainfall event. This work indicates that particle mobilization is not significantly impacted by rainfall intensity, but was significantly impacted by the frequency of rainfall events.; Facilitated transport of contaminant metals by particles in the vadose zone may contribute to shallow groundwater contamination in some systems. During two rainfall simulations spaced 14 days apart, little of the metal present in the <63 mm size fraction of the soil was leached during simulated rainfall. On average, the particle-bound fraction of mobile Zn was 20%, Cu was 55%, and Pb was 90%. Particle size distribution, pH, and dissolved Ca concentration was not correlated to Cu, Mn, Pb, and Zn distribution coefficients. |
