The amount of pesticides in streams nationwide has generally decreased, according to an assessment released by the U.S. Geological Survey (USGS) in March. But urban streams are carrying more and more pesticides, and the number of different chemicals present simultaneously has gone up, raising concerns among some researchers.
The picture is mixed over the period covered by the report, from 1992 to 2001, with some positive results and much uncertainty, according to Robert Gilliom, the head of the National Water Quality Assessment (NAWQA), within USGS. Land use is one of the most important influences on sources of pesticides, he said at a congressional briefing on March 3.
Using samples from more than 5,000 wells across the nation, USGS scientists tested for 83 pesticides, including DDT and its degraded forms. They tested for levels that have potential health impacts on people, finding few areas in which levels exceeded federal regulation limits.
Each pesticide, Gilliom said, had its own unique story, according to how and when it was applied and where. For example, atrazine, a common pesticide used mostly to control weeds in corn crops, is applied only during certain time periods and at high concentrations, with 80 percent of it applied in the nations corn belt.
That usage pattern makes it possible to use modeling to predict pesticide concentrations in streams where measurements are lacking, he said, particularly when modelers can also include water flow and precipitation patterns. But atrazine was easiest, Gilliom said. The rest of the contaminants studied get harder.
The cause of the rise in pesticides in urban streams, in particular, remains unclear. Although USGS scientists can show that urban streams contain more pesticides than they have in the past, we have nothing in terms of data to show who is applying the pesticides and how, Gilliom said.
Other trends include the slow decline in the presence of three very toxic pesticides: DDT, banned in 1972, as well as chlordane and dieldrin, after the Environmental Protection Agency (EPA) required the phaseout of their use by the late 1980s. Still, the report shows that a variety of pesticides in some form are almost always present in streams (at least 90 percent of the time) and, to a lesser extent, in shallow groundwater.
Although individual pesticide concentrations in streams did not often reach levels of concern to human health, the pesticides effects on other stream ecosystems inhabitants, such as fish, plants and insects, are generally less known. Especially of concern is that the presence of various mixtures of pesticides in streams is almost universal.
Such pesticide mixes have not been very successfully addressed yet, Gilliom said, to determine the impacts of different chemical combinations on humans and other animals. With this in mind, Gilliom cautioned that the NAWQA assessments of toxicity and environmental impacts could be overestimating or underestimating the hazards of the chemical concentrations.
Jim Jones, director of EPAs pesticide program, said in the same March presentation that the science is going to take some time before we have our arms around it. EPA is currently conducting evaluations for new and old pesticides, to establish benchmarks for their presence in streams with regard to human and ecosystems health, and will begin work soon on mixtures. The NAWQA survey was not able to document all variations because of a lack of enough sites.
The work represents a huge and commendable effort, says Donald P. Weston, a toxicologist at the University of California in Berkeley. However, Weston points out that pesticide use patterns can change faster than monitoring efforts of this scale can keep up with them. Thus, the NAWQA report demonstrates an unfortunate time lag between when data are collected and when they becomes publically available.
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