proaches—which include excavation, the pump-and-treat method, and soil-vapor extraction—is to remove the contamination from the subsurface. The purpose of in situ treatment, which includes in situ bioremediation and in situ chemical transformation, is to reduce or eliminate the mass of the hazardous chemical in place. Any of those methods can be successful for a specific site under specific circumstances. However, because of the complexity of the subsurface and the physiochemical nature of many contaminants, the effectiveness of any remediation method will be constrained in many cases. As a result, subsurface remediation is expensive, and there is no guarantee of complete success. Although advances in remediation and characterization technologies continue to be made, it remains true that pollution prevention is much cheaper than pollution remediation in the long term.

Pesticides enter the air by a variety of processes. Drift during application can result in the entry of small aerosol particles and vapors into the air and their movement downwind from the spraying operation. After application is completed, additional residue can enter the air by volatilization and wind erosion. The resulting residue, both particles and vapors, can travel downwind, where deposition can occur by the wet processes of rainfall, snowfall, and fog coalescence washout or the dry processes of vapor exchange with surfaces and fallout of particles. Degradation can accompany the downwind processes, and the products of decomposition can join the parent-chemical residue in further transport and deposition.

Airborne pesticides can undergo long-range transport to remote environments (Kurtz 1990). The presence of residues of organochlorine chemicals in the earth's polar regions, where they accumulate in the body fat of Inuits and in seals and polar bears, is evidence of the operation of a “cold condensation” mechanism that moves residues from temperate and tropical areas of use to colder regions (Wania and Mackay 1993). The finding of current-use chemicals in the alpine and subalpine regions of the Sierra Nevadas (owing to airborne movement from California's farming valleys) (Zabik and Seiber 1993) and in the polar regions is a cause of continuing concern about long-range movement of pesticides.

There are several reasons to be concerned about airborne residues (Seiber and Woodrow 1995). Airborne residues can represent a direct hazard to humans, wildlife, and vegetation. The human hazard might be most prominent for farm workers who apply chemicals or work in and around treated fields. But people who live, work, or play downwind can also be exposed. The recent hypothesis of possible endocrine system disruption by exposure to “background” chemicals has raised concern about the potential of adverse population-level effects of ecosystem-scale contamination, such as can be caused by airborne residues (Arnold et al.

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