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by Barbara Duckworth
Western Producer Newsfeature Service
Research at Lethbridge, Alta., and Outlook, Sask., reveals trace amounts of farm chemicals do appear in groundwater, especially after spraying season.
Water tables in these irrigated areas are as high as one to three metres from the soil surface. It's at these high levels where traces of chemical might show.
For researchers at the National Institute of Hydrology at Saskatoon and the Agriculture Canada research station at Lethbridge, a number of questions remain about leaching.
Wally Nicholaichuk wants to know what happens to these chemicals once they are in the soil and if they build up with repeated applications.
His team knows pesticides travel through the soil at different rates because soil is not homogeneous, said researcher Kelly Best. Movement through the soil is easier when soil fractures appear. These could be channels as small as a hole drilled by an earthworm.
With differing soil conditions in a single field, herbicide movement varies, so making predictions and recommendations about applications is difficult.
Water in Canadian aquifers hasn't been systematically checked for pesticide residues mostly because of the high cost involved, said Jane Elliott of the hydrology institute. During a presentation in Red Deer, she said residues have been detected in almost every province. Certain types of pesticide are more guilty of leaching into the soils.
Carbofuran seeps into the soil while glysphosate doesn't appear to, she said. The more soluble the chemical, the more likely it will leach into the soil. Very deep aquifers don't appear to be affected. At Lethbridge, Bernie Hill has detected some pesticide residues in shallow groundwater reserves.
While researchers aren't trying to scare people or outlaw chemicals, caution when using pesticides is advised.
"We're not in this to get these things banned or create a panic," Hill said. "We're in this to keep agriculture sustainable and, if we can, give a warning to the producer about how prudent he wants to be about exposure."
A leaching problem does exist. When scientists have looked for residues over the last three years they've found them, he said.
"The levels are anywhere from one to 30 percent of the drinking water guidelines. So we still have a cushion there," Hill said. "There's no sense hiding the fact that there are small levels in the shallow groundwater in an actively farmed field."
Residues appear to peak in the spraying season. The worst-case scenario comes after heavy rain falls or irrigation watering one to three days after spraying.
Rather than giving the herbicide a chance to absorb and be degraded it tends to get flushed through cracks and into the groundwater.
"In that situation the levels can rise up to and slightly exceed the drinking water guidelines," said Hill.
In Canada the allowable herbicide residue guidelines are 100 parts per billion (ppb) for 2,4-D, nine ppb for diclofop-methyl, 230 ppb for triallate and five ppb for bromoxynil.
Once chemicals have hit groundwater they don't degrade because the environment is cool. On the surface there is sunlight, exposure to air and microbes to break them down sooner, Hill said.
Tests at Lethbridge are done on a single plot where the scientists have a 20-year history of what was done to that land. They have also conducted some small experiments on sites within a 160-kilometre radius and have found some residues from herbicide spraying.
No one is at the research stage where they can offer advice on when to spray. Part of the leaching problem is caused by unpredictable weather, said Hill.
"I would think if someone has a shallow groundwater well and they're using the water for cattle or for human consumption it would be prudent to avoid using that water for a couple of weeks after spraying if they could," said Hill.
Copyright © 1993 REAP Canada
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