is not describable by Gaussian plumes. Also, such approaches may be sufficient to demonstrate that it is not necessary to go to the expense of employing more-complex models.

Deposition on and Accumulation in Soil

Soil is formed from the weathering action of climate on rocks and minerals and from the actions of living organisms. It is a mixture of minerals, water, air, and organic substances. The proportion of these components and the characteristics of the contaminants of concern determine, to a large extent, how such a contaminant is transported or transformed in soil. A contaminant can enter soil water, soil solids (mineral and organic phases), and soil air. Soils are characteristically heterogeneous in the vertical direction, so that a trench dug into soil typically reveals several horizontal layers that have different colors and textures.

Studies of radioactive fallout in agricultural land-management units have revealed that, in the absence of tilling, particles deposited from the atmosphere initially accumulate in and are resuspended from a surface-soil layer that is 0.1-1 cm thick (Whicker and Kirchner 1987). Over the long term, there is mechanical transport deeper into the soil (e.g., by earthworms, ants, rabbits, anything else that burrows, and by frost heave and wetting/drying cycles). Particles in the surface layer can be transported mechanically in the horizontal direction by runoff to nearby surface waters or be blown by wind. Surface-soil contaminants can be transported (on particles) by wind erosion, by volatilization to the atmosphere, by diffusion, leaching, and mechanical movement deeper into the soil, by erosion (attached to particles) or dissolution in runoff, and may be transferred to plant surfaces by rain splash or via resuspension and deposition. They can also be transformed through photolysis by sunlight, through chemical degradation, and through degradation by microorganisms (biodegradation).

The roots of most plants are typically confined within the top 3 ft (about 90 cm) of soil. Contaminants in this root-zone soil, below the surface layers, are transported upward by vapor-and liquid-phase diffusion, root uptake, and by capillary motion of water; they are transported downward by vapor- and liquid-phase diffusion and leaching; and chemically transformed primarily by biodegradation, hydrolysis, and other liquid and solid phase chemical reactions.

Deposition on and Uptake by Plants

By mass, the dominant component of the terrestrial biota is land plants. Plants generally have contact with two environmental media —air and soil. Uptake of contaminants by plants can occur directly from air via particle deposition or by foliar uptake of contaminant vapors. Particle deposition and foliar vapor uptake can also take place from contaminated soil (itself contaminated through various pathways from air contamination), through evaporation or suspension, or



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