River Basin. Overall, there has been a steady increase in monitoring of pesticides in
surface waters over the last several decades.
In recent years, the herbicides alachlor, atrazine, and simazine have frequently
exceeded their MCLs in individual samples. A number of studies have shown that
procedures commonly used at most water treatment plants have little effect on con-
centrations of these herbicides in water. Thus, drinking water derived from some sur-
face water sources in the central United States likely contains concentrations of one
or more of these compounds above the MCL for part of the year because of seasonal
patterns. Annual mean concentrations, however, rarely exceed the MCL.
Our ability to assess the occurrence of pesticides in surface waters is limited by sev-
eral factors. First, water quality criteria have not been established for most pesticides
and pesticide transformation products, and existing criteria may be revised as more is
learned about the toxicity of these compounds. Second, criteria are based on tests of
individual pesticides and do not account for possible cumulative effects if several dif-
ferent pesticides are present. Finally, many pesticides and most transformation prod-
ucts have not been widely monitored in surface waters. These factors, and the lack of
data about long-term trends, show significant gaps in our understanding of the extent
and significance of pesticide contamination on surface waters. The results of this anal-
ysis indicate a need for long-term monitoring studies using a consistent study design
and targeting more of the currently used pesticides and their transformation
products.^32
The Atrazine Danger
Atrazine’s extensive use, persistence in soil, and mobility in water make it the most
frequently detected pesticide in ground and surface water across the United States. As
a result, drinking water is a common source of atrazine exposure, especially in agri-
cultural regions. For example, testing has found atrazine in finished water from 97
percent of surface water–supplied drinking-water systems in Iowa. In addition, a
recent survey of nearly 1,500 groundwater wells around the country detected atrazine
in 23 percent of the samples, and found it to be among the most common pollutants
detected.^33
Health Effects of Atrazine
A growing body of toxicological and epidemiological evidence has raised concerns
that chronic atrazine exposure may cause a variety of adverse human health effects.
One epidemiological study found an association between maternal exposure to tri-
azine herbicides in drinking water and increased incidence of developmental effects in
newborns, including low birth weights. Reduced sperm counts, decreased sperm mo-
tility, and prostate inflammation have been observed in male laboratory rats exposed
to atrazine. Endocrine disruption by atrazine and other triazine herbicides has also
been reported in laboratory studies. Researchers have observed chromosomal damage
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