National Water-Quality Assessment (NAWQA) Program
Steven J. Larson and Paul D. Capel, U.S. Geological Survey, Minneapolis, Minnesota.
A comprehensive review was undertaken to assess current understanding of the occurrence and distribution of pesticides in surface waters of the United States. Studies from the late 1950's to early 1990's were reviewed, ranging from small-scale studies of individual rivers and lakes to regional and national-scale studies. Of the 118 pesticides or pesticide transformation products targeted in the reviewed studies, 76 have been detected in one or more surface water bodies. Concentrations generally ranged from nanograms to micrograms per liter. Organochlorine insecticides continue to be detected in surface waters 20 years after their use was banned or severely restricted. A number of currently-used pesticides, particularly the triazine and acetanilide herbicides. occurred as seasonal pulses of elevated concentrations in rivers draining agricultural areas in the central United States. For most pesticides, data from the reviewed studies are not sufficient to assess trends in occurrence, because few studies sampled the same sites consistently over a number of years. Where long-term data do exist, year-to-year fluctuations due to variability in weather made trends difficult to detect. The significance of the observed concentrations of pesticides in terms of human and ecosystem health is not known, due to a lack of data relating environmental exposures to toxicological effects, and the lack of established standards or criteria for many pesticides. Annual mean concentrations of pesticides in surface waters used as sources of drinking water rarely exceeded maximum contaminant levels (MCLS) established by the United States Environmental Protection Agency, although peak concentrations of several currently used herbicides commonly exceeded MCLS for periods of days to weeks in streams of the central United States.