The Mobilization and Transport of Particles and Particle-Associated Contaminants in the Unsaturated Zone

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9909553RyanMany of the hazardous contaminants introduced into the natural environment bind strongly to soil surfaces; hence, their mobility is limited by sorption. Clay-and silt-sized particles in soils are primarily responsible for the sorption of sorption of contaminants owing to their high surface areas. In the unsaturated zone, these particles are susceptible to mobilization and transport by infiltrating rainfall. If these particles are mobilized, the migration of the associated contaminants will increase. For transport of contaminants associated with particles to be significant, three prerequisites must be met: (1) there must be a source of particles (mobilization), (2) contaminants must sorb to the remain sorbed to the particles (sorption), and (3) the particles must migrate downward. This research plan focuses on improved understanding of the source, or mobilization, of particles in the unsaturated zone.We hypothesize that two major factors control the mobilization particles in the unsaturated zone: (1) heterogeneity in soil structure and (2) characteristics of rainfall events. Soil structure heterogeneity leads to preferential flow paths, or routes of high permeability through a matrix of lower permeability. In soil, these preferential flow paths are often referred to as macropores. Macropores are typically formed by soil desiccation, root growth, and burrowing fauna. The flow of infiltrating water is funneled through macropores at velocities much greater than those occurring in the surrounding soil matrix. The greater velocities lead to greater shear on attached particles and greater particle mobilization. For rainfall, the intensity, duration, and frequency are important. The intensity of rainfall will control the velocity of water flow through the soil and, hence, the extent of particle mobilization. The duration of rainfall events will affect the kinetics of particle mobilization, causing a change from a rapid particle release during the initial pulse of infiltrating water to a slower steady-state rate. The frequency of rainfall events will control the regeneration of the particle supply.The research plan outlines tests of these hypotheses in model systems and real soils. The model systems will consist of particles and media of known size and geometry. The real soil will extracted from sites at which particle-facilitated transport of contaminants may be significant. Measurements focus on the amount, size, and composition of the mobilized particles. Knowledge of these key particle properties will aid in assessment of contaminant sorption and particle transport.