Nonpoint-source contamination of surface and ground water from herbicides has been a major water-quality issue during the past decade in the Midwestern United States. Investigations by the US Geological Survey have included rivers, reservoirs, ground water, and precipitation. Initially, a reconnaissance study of 147 rivers was conducted to determine the geographic and seasonal distribution of herbicides. This study showed that high concentrations of herbicides were flushed from cropland and transported through the river system as pulses in response to spring and summer precipitation, often exceeding annual health standards of US EPA. The study also revealed the persistence of herbicides and their metabolites in rivers, which led to the next study of 76 reservoirs located in 11 midwestern States.
The persistence of high concentrations of herbicides and their metabolites could be related to reservoir and drainage-basin characteristics, water and land use, herbicide use, and climate. Again, concentrations sometimes exceeded US EPA annual health standards. The importance of multiple herbicides in a single sample was also noted and the issue that health standards do not apply to these mixtures was emphasized. Next, reconnaissance studies of ground water identified the relationship between land use, ground-water age, and concentration and occurrence of herbicides and their metabolites. The frequency of herbicide detection in relation to analytical reporting limits, ground-water age, and the relatively high occurrence and persistence of metabolites were noted. The last study investigated precipitation and included 26 States in the upper Midwest, northeast to the Atlantic Ocean, and northward to the Canadian border with over 10,000 assays.
The results identified the occurrence and temporal distribution of triazine and chloroacetanilide herbicides and their metabolites in rainfall. The highest concentrations in precipitation occurred after herbicide application to cropland. Finally, the result of these studies is that a clear understanding exists of the aquatic transport and fate of the triazine and chloracetanilide herbicides in the environment. The increased knowledge concerning these compounds in the environment (i.e. dose information) gives toxicologists the data necessary to estimate toxic exposure both to man and the aquatic environment.
The following ethical questions will be raised at this seminar. 1. Does an inexpensive and high quality food supply justify the environmental risk of herbicides? 2. Do water-quality issues play a role in bringing increased pressure for new technologies creating a Catch-22 situation, i.e. GM foods and new low-use herbicides of high toxicity? 3. Has scientific technology and exponential population growth brought us to the brink of what may be called the 3rd Agricultural Revolution?
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