PANNA: Pesticides in Water Threaten Aquatic Life


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Pesticides in Water Threaten Aquatic Life
7, 2003

A recent U.S. Geological Survey (USGS) study of water samples collected in California’s San Joaquin Valley reveals pesticide concentrations more than 10 times higher than proposed limits. USGS researchers gathered samples of rainfall, storm-water runoff, and river water during the winter of 2000 through 2001 to determine concentrations and water travel patterns for two common organophosphate pesticides, diazinon and chlorpyrifos. They found concentrations of these pesticides at levels that are dangerous to aquatic life and clearly demonstrate the ability of pesticides to move far beyond the scope of their initial application.

These same pesticides are prone to airborne pesticide drift and have been found in unhealthy concentrations in air samples collected up to 72 feet from the application site (see the PANNA report, “Secondhand Pesticides: Airborne Pesticide Drift in California,” Pesticide drift also contributes to high concentrations of pesticides in rainfall as pesticides volatize into the lower atmosphere, are washed back to earth with rain, and run-off into rivers. USGS scientists attributed 68 percent of diazinon concentrations found in river water samples to rainfall.

The two pesticides monitored in the study, chlorpyrifos and diazinon, are insecticides most commonly used during the dormant winter season and are applied via crop dusting planes or tractors equipped with spray mechanisms. These pesticides were once popular for home use, but the U.S. Environmental Protection Agency (U.S. EPA) recently initiated phaseouts because studies showed that existing exposures levels were a high risk to children’s health. Both chemicals are neurotoxins, inhibiting proper transmission of nerve impulses. Both are highly toxic to aquatic life.

Chlorpyrifos and diazinon are frequently found in toxic levels in streams, in part because their application period coincides with the rainy season. Surveys from previous years found similarly toxic levels of chlorpyrifos and diazinon in streams and groundwater throughout the nation, and especially in California’s waters. A 1992-95 survey of ground and surface waters of the San Joaquin-Tulare Basins revealed concentrations of diazinon and chlorpyrifos at levels doubling those considered safe for aquatic life and, in the case of diazinon, also exceeding safe drinking-water levels.

Unfortunately there is no single, regulatory organization determining health and safety limits on the concentrations of these pesticides in water, making monitoring and restrictions difficult to implement. The California Department of Fish and Game has set concentration baselines necessary to maintain a healthy aquatic environment. Of the water samples collected, a total of 78 contained concentrations exceeding these limits, 60 samples for diazinon and 18 for chlorpyrifos. In all, researchers collected 220 samples.

The California Department of Pesticide Regulations (CA DPR) has initiated their own concentration limits through the proposed Dormant Spray Water Quality Initiative. When the USGS study results are compared to these proposed guidelines, pesticide concentrations in water samples exceeded limits by up to a factor of 10 for diazinon and 7.4 for chlorpyrifos. Although the CA DPR states that these levels are of no consequence to human health, they do pose grave threats to aquatic life. In addition to establishing concentration limits, the CA DPR initiative would require mandatory controls to limit drift and water run-off of these pesticides and provide ongoing monitoring of concentration levels.

The USGS report is part of an ongoing effort to monitor pesticide levels in water. Initiated in 1991, the National Water-Quality Assessment (NAWQA) program conducts surveys of various water quality indicators, including pesticide levels. A recent NAWQA study found 90 percent of water and fish samples collected contained at least one pesticide, some of which have not been used in decades, testimony to the persistence of these chemicals in the environment.

Sources: Diazinon and Chlorpyrifos Loads in Precipitation and Urban and Agricultural Storm Runoff during January and February 2001 in the San Joaquin River Basin, California, Celia Zamora, Charles R. Kratzer, Michael S. Majewski, and Donna L. Knifong, USGS, 2003,; The Quality of Our Nation’s Waters: Nutrients and Pesticides, USGS NAWQA, 1999,; Secondhand Pesticides: Airborne Pesticide Drift in California, Susan Kegley, Anne Katten, & Marion Moses, Californians for Pesticide Reform, 2003,, Water Quality in the San Joaquin-Tulare Basins, California, 1992-95, Neil M. Dubrovsky, Charles R. Kratzer, Larry R. Brown, Jo Ann M. Gronberg, Karen R. Burow, USGS, 1998,

Contact: PANNA.

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