National Water-Quality Assessment (NAWQA) Project
The atmosphere is an important component of the hydrologic cycle to consider when assessing the impact of pesticides on the environment. As part of the hydrologic cycle (see Figure 1), precipitation (rain and snow) replenishes both surface and ground waters. Precipitation cleans the atmosphere of airborne pesticide vapors and particles and deposits them to the earth's surface, including lakes, rivers, and streams. In addition, dry deposition in the form of gaseous vapor and particulate matter also deposits airborne pesticides to the earth's surface. Pesticides in both precipitation and dry deposition can reach surface waters by direct deposition or surface runoff and can reach ground water by infiltration through the soil.
Until the 1960's, atmospheric pollution from pesticide spray drift was generally thought of as a local problem. Long-range movement of long-lived pesticides through the atmosphere was believed to be minimal. The detection of DDT and other organochlorine compounds in Arctic and Antarctic fish and mammals have changed this notion. The atmosphere is now recognized as a major pathway by which pesticides can be transported and deposited in areas sometimes far removed from their sources. Long-range transport of pesticides can occur over hundreds to thousands of kilometers. Toxaphene, for example, which was used on cotton in the Southern United States and banned in 1982, is still being transported into the Great Lakes region by southerly winds from the Gulf of Mexico. Once deposited on the earth's surface, the pesticide can revolatilize, re-enter the atmosphere, and be transported and deposited downwind repeatedly until it is finally degraded, sometimes over decades. This same process can also occur for the degradation products resulting from chemical or biochemical transformations of pesticides. Some pesticide degradation products are more toxic than the original compound.