carried along by the water as it flows through (infiltration) or above (runoff) the soil. Dispersion is the spreading of matter about a mean position, such as the center of mass; spreading is caused by molecular diffusion and nonuniform flow fields. Interphase transfers —such as sorption, liquid-liquid partitioning, and volatilization—involve the transfer of matter in response to gradients in chemical potential or, more simply, concentration gradients. Transformation reactions include any process by which the physicochemical nature of a chemical is altered; examples are biotransformation (metabolism by organisms) and hydrolysis (interaction with water molecules). Additional information regarding the factors and processes that influence the transport and fate of contaminants in the environment can be found in Pepper et al. (1997).

The fate of a specific pesticide in the environment is a function of the combined influences of those four processes. The combined impact of the four processes determine the “pollution potential” and “persistence” of a pesticide in the environment. The pollution potential characterizes, in essence, the “ability” of a pesticide to contaminate the medium of interest (soil, water, or air). Pesticides with larger pollution potentials are generally transported readily (for example, low sorption) and not transformed to any great extent (they are persistent). High rates of transport mean that a pesticide readily moves away from the site of application. A low transformation potential means that a chemical will persist, and thus maintain its hazard potential, for a longer time than one with a high potential. The risk posed by a specific pesticide to humans or other receptors is, of course, a function of its toxicity, as well as its pollution potential. It is therefore important to understand both types of properties. For example, pesticides that are very mobile, persistent, and highly toxic will generally be associated with the greatest risk.

Once a pesticide is applied to or spilled onto soil, it can remain in place, or transfer to the air, surface runoff, or soil-pore water. Transfer of pesticides to surface runoff during precipitation or irrigation is a major concern associated with non-point-source pollution. Once entrained into surface runoff, a pesticide can be transported to surface-water bodies. Consumption of contaminated surface water is a major potential route of exposure to pesticides. A recent discussion of pesticides and non-point-source pollution is presented in Loague et al., (1998).

The other major potential soil-water route of pesticide exposure of humans is consumption of contaminated groundwater. Once applied to soil, a pesticide can partition to the soil-pore water. It then has the potential to move down to a saturated zone (aquifer), thereby contaminating the groundwater. Whether that occurs, the time it takes, and the resulting degree of contamination depend on numerous factors. Major factors are the magnitude and rate of infiltration and recharge, soil type, depth to the

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