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Year-long Investigation Finds Chemical Herbicide In Compost

Last year, hundreds of gardeners in northern Vermont were shocked to learn that the compost they bought to nourish their plants had instead damaged them with trace amounts of chemical herbicides.

A year after the widespread contamination, officials have tracked down the main source of the problem to horse manure from a farm in Colchester.

The investigation involved scientific detective work and led to the discovery that persistent herbicides are found in both compost and the human food supply.

Tom Moreau is an avid gardener. So when he dug a new bed for tomatoes last year he layered in some gardener’s gold – nutrient rich compost.

Moreau knows his compost. As the manager of the Chittenden Solid Waste District, he oversaw a $2.3 million public investment in a new composting facility in Williston. So what happened next affected him both personally and professionally.

“All I can say is tomato plants looked like fiddleheads; they were all curled up tightly,” Moreau recalled. “And I’ve never seen that before, gardening for, you know, decades.”

Moreau suspected insects were feasting on his tomato plants so he treated them with an organic pesticide. But they continued to wilt.  

On a Monday morning last June he brought one of his sickly plants in to work to show to his compost manager, Dan Goossen.

“And just sheer coincidence, a woman who’s a master gardener from Williston, says, ‘I think you guys have an herbicide problem,’” Moreau said.

A year later, Moreau and Goossen have learned a great deal about persistent herbicides. Along with officials at the state Agency of Agriculture, they identified the chemicals that affected several hundred gardens and forced a halt of the district’s compost sales last year and this year.

The detailed trace-back investigation featured false leads, conflicting lab results, and the involvement of some of the world’s largest chemical companies. The year-long detective work also eventually led to regulatory changes that Moreau and state specialists say will protect compost in the future.

The work continues. In a new greenhouse near the district’s composting facility, Moreau showed off a healthy clover plant with deep green leaves.

“This is the first non-aminopyralid sample of compost that we’ve produced in the last eight months. So we had eight months of problems. We identified that aminopyralid was the main culprit. It is a Dow AgroSciences herbicide, persistent herbicide,” he said. “We’re finding that it can impact plants at below 1 part per billion, perhaps as low as .2 parts per billion.”

Persistent herbicides are called persistent for good reason. The chemical that stunted Moreau’s tomatoes was aminopyralid, a Dow product that’s used to kill weeds on hay fields. The hay was later fed to horses. The state investigation has focused on a horse farm in Colchester that bought its hay from several suppliers. The chemical remained potent, despite going through the horses’ digestive tract. Composting the manure also failed to neutralize it.

Aminopyralid is deadly to sensitive plants in minute quantities. One part per billion is equivalent to one drop of water in five Olympic swimming pools. Moreau, who has a background in chemistry, was frustrated. He said detecting the chemical at levels that damaged plants was extremely challenging science.

The first tests run by a lab the district hired did not identify aminopyralid but instead found evidence of two other persistent herbicides. A second lab hired by Dow tested the same samples and didn’t find those two chemicals, but instead pinpointed a third product, made by DuPont, a Dow competitor. Moreau at the time thought the findings made no sense.

“The results were absolutely wrong. The irony was, you can imagine the sensitivity when we get Dow’s first results back from their contract lab (showing) it wasn’t any of their product, it was their competitor’s brand new product,” he said. “Obviously, that was pretty dicey.”

It turned out that the only lab capable of finding Dow’s aminopyralid in compost was Dow’s own testing facility.  

That’s where state officials played an important role. Cary Giguere is the agrichemical section chief for the Agency of Agriculture who also chairs a national board of state regulators. Moreau had been frustrated trying to get Dow’s help. But Giguere has worked closely with the industry over the years. So when he called, Dow listened.

The company agreed to run many samples of the Chittenden compost at its own corporate lab. Dow also agreed to change its label – which has the force of law. It no longer sells aminopyralid in the region for use on hay and pasture. Giguere says Dow acted responsibly.

“Dow has been on-board from the beginning. They’ve provided a tremendous amount of product stewardship in this case,” Giguere said. “And their attempt to solve this in the Northeast, was to make all pasture use in New York and New England, illegal.” 

But there was another persistent herbicide found in the Chittenden waste district’s compost. The chemical is called clopyralid. And it’s much more widespread because it’s widely used on feed and grain raised for both animals and people. Tom Moreau began sending samples of horse feed out to labs for testing, and they all came back positive for clopyralid.

“Then we started thinking, ‘well, if it’s in horse feed grains, could it be in food grain, human grains?’ And the answer was: yes. And part of the dilemma for the regulators are: here’s something that has to have a label and warn people downstream, however it’s a product that’s in our food,” he said..

Food scraps from Burlington schools – leftover pizza and pasta – showed minute levels of clopyralid.

The amounts are orders of magnitude below what’s considered hazardous to human health. But the herbicide can be a problem for composters. Moreau said it’s a manageable one, because compared to aminopyralid, at very low levels clopyralid is less toxic to plants.

“With clopyralid, it becomes an algebra problem. Dan (Goossen) has to figure out at what concentration will it not impact plants,” he said.

At the waste district’s facility in Williston, a giant blender stirs a slurry of food waste, leaves and other organic material.

Compost manager Dan Goossen is using the algebraic formula to manage the amounts of clopyralid in his feedstock. He also tests the compost blends in the greenhouse to make sure that they don’t damage plants.

Goossen said it’s been a tough year for all commercial composters in Vermont.

“And very eye-opening as we’ve learned that pretty much all compost with a manure component has persistent herbicides in it,” he said. “I don’t think anybody really thought that was going to be the case. So luckily it’s clopyrlaid, the lesser of the two evils. And if you get a low enough concentration you’re probably in the clear. So that’s where everybody is now, but it’s been a pretty scary season for everybody involved in the composting community.”

The Chittenden district is not accepting horse manure for now. The district is also not selling its earthy product to gardeners this year.

Goossen and Moreau say they want to wait and keep testing to make absolutely sure the compost is good before they put it back out on the market, or in their own gardens. 

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