National Water-Quality Assessment (NAWQA) Project
Determining the environmental significance of pesticides in air, rain, snow, and fog is difficult and there are no existing national standards or guidelines for these media. For drinking water, the U.S. Environmental Protection Agency establishes MCLs (maximum contaminant levels) for public water systems and determines exposure guidelines for adults and children. There are also U.S. Environmental Protection Agency and National Academy of Sciences water-quality criteria for aquatic organisms, which are often more sensitive to pesticides than humans are.
Only one fourth of the pesticides that have been analyzed in the various atmospheric matrices have been assigned MCL values, about half have exposure limits for children, and about one third have aquatic-life criteria. In most cases the measured pesticide concentrations in rain were much lower than MCL and human exposure guidelines. There are several instances, however, where the concentrations in rain have exceeded MCL values for alachlor, atrazine, and 2,4,-D. In general, the highest concentrations measured in rain occurred infrequently, but have occurred several times and will likely occur again. Atmospheric deposition of pesticides may adversely affect stream water quality during periods when direct precipitation and surface runoff are the major sources of streamflow.
Most of what is known about the significance of atmospheric deposition of pesticides to water quality comes from studies done in the Great Lakes area on organochlorine insecticides such as DDT and toxaphene. Even though concentrations in the atmosphere are low, historic and continued deposition have resulted in sufficient loading to cause adverse effects on fish and wildlife due to food chain accumulation.