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Pesticides and groundwater

Some pesticides will take longer to break down than others; the 
outcome will depend on both the pesticide and the environment

Canada uses over 35 million kilograms of pesticide active ingredients per year,” says Dr. Annemieke Farenhorst, a soil science professor at the University of Manitoba. Farenhorst studies the persistence and transport of agrichemicals — including pesticides — in soil, water, and air.

Some active ingredients break down more quickly than others.

Pesticide break down

How quickly a pesticide will break down depends on both the pesticide and the environment.

Exposed to sunlight and other microorganisms, some pesticide molecules break down in a few hours. But, Farenhorst says, “Other pesticide molecules are persistent in the environment. Some have been known to persist for decades.”

These long-lasting pesticides can wind up in the air, water, soil and living organisms. “This distribution results in some pesticides being widely present in the environment,” Farenhorst says. “This is one reason why it’s difficult deal with contamination.”

To stop contamination applicators need to keep the pesticide where it’s applied.

Pesticides can be most difficult to contain when they enter ground water, where they have an opportunity to travel to other locations.

Of course, even without traveling, pesticides are not wanted in ground water. “About one in four Canadians rely on groundwater for their drinking water,” said Farenhorst.

As well, in surface waters, Farenhorst says, “pesticides are retained onto suspended or bottom sediments. This can prolong their persistence, as once they’re retained by solids, it’s more difficult for microorganisms to break down the chemicals.

Preventing contamination

There are many ways to prevent water contamination, such as reducing soil erosion and creating buffer strips to help prevent water and sediments from reaching surface waters.

“Building up soil organic matter levels helps retain pesticides into the soil,” says Farenhorst. There, they can be broken down, reducing the risk that they’ll reach ground water.

And of course, using fewer pesticides also lowers the risk of contamination.

“Avoiding pesticide over-application and practices that lead to weed resistance will generally reduce the release of pesticides in and risks to the environment.”

While there are several actions famers can take, Farenhorst says, “Pesticide contamination can’t be solely controlled by specific on-farm practices, even if a farmer is following all the recommended practices.

“For example,” she says, “spray drift and post-volatilization losses (the movement of pesticides into the air following application) from other farms will allow pesticide residues to enter the atmosphere. These pesticide residues can be transported short or long distances and then be re-deposited on other soils or surface waters.”

According to Dr. Jeanette Gaultier, pesticides minor use and regulatory specialist, Manitoba Agriculture, Food, and Rural Initiatives (MAFRI), leaching of pesticides is not a major problem on Prairie soils, “due to our low average annual precipitation.”

The widespread adoption of conservation till and no-till by Prairie farmers is one of the best management practices to minimize the risk of spring surface runoff of pesticides from fields, said Gaultier. Crop residues act as a barrier to slow water and soluble compounds movement from fields.

“Crop residues also reduce soil loss by wind and water erosion, which may contain soil-bound herbicide residues,” said Gaultier. “Producers in the Prairie pothole region can also use two-to-five metre wide buffer strips around riparian and other sensitive areas to reduce soluble and soil-bound pesticide runoff.

Gaultier also notes the shift from to relatively non-residual pre- and post-emergent pesticides. “Some of the more residual products are still used in specialty crops or to combat known populations of herbicide-tolerant weeds. However, producers in flood prone areas tend to avoid their use, as pesticide loss from their fields is also lost dollars from their pocket book.” †

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