From 2002 to 2012, concentrations of atrazine decreased in more than one-half of 60 U.S. streams and rivers studied and increased in only about one-third of these streams, according to a new study by the U.S. Geological Survey National Water Quality Program. The streams and rivers are in a range of land-use settings.
Atrazine is the second-most widely used weed killer in the United States, after glyphosate. The overall trend patterns indicate that regulatory changes starting in the early 1990s intended to decrease concentrations of atrazine in streams and encourage the breakdown of atrazine have had some success.
In contrast, more streams had an upward trend in concentrations of deethylatrazine (DEA), a major atrazine breakdown product, than a downward trend. This could be the result of successful actions to prevent atrazine transport by rainfall runoff, leading to more breakdown of atrazine into DEA in soils.
The study used Structural Equation Modeling (SEM) to test hypotheses concerning the causes, in addition to atrazine use, of the upward and downward trends in concentrations of atrazine and DEA at the sampled sites. Changes in corn acreage were important in explaining trends in atrazine concentration, but the presence of tile drains, amount of groundwater supply, changes in moisture supply, and corn acreage all contributed to explaining trends in DEA.