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Compost Contaminants Linked to Farm Horses

Colchester, Vt. — A year after compost used by hundreds of Vermont gardeners was found to be contaminated with trace amounts of persistent herbicides, officials have traced the main source of the contamination to manure from a Colchester farm.

Last year, the bagged and bulk compost from the Williston-based Chittenden Solid Waste District caused some plants grown in it to wilt.

The year-long investigation has turned up incorrect leads, and tests of the compost produced conflicting results.

With help from the Vermont Agency of Agriculture, the district has identified the two herbicides that caused it to stop selling its compost. One is aminopyralid, which is applied to hay fields to kill weeds. In this case, officials believe hay treated with the herbicide was fed to horses and the chemical was not neutralized in the manure used in the compost.

The other herbicide is clopyralid, which is used on feed and grain grown for both animals and people. District Manager Tom Moreau said food waste from Burlington schools, such as pizza and pasta, turned up tiny levels of the herbicide.

The amounts are far below what’s deemed a threat to human health. And clopyralid is less of a problem for gardeners because at low levels it’s less toxic to plants than aminopyralid is, Moreau said.

At this point, the district is not accepting horse manure and still is not ready to sell compost to gardeners. The work is ongoing.

“This is the first non-aminopyralid sample of compost that we’ve produced in the last eight months,” he said, pointing to a healthy clover plant.

Moreau and compost manager Dan Goosens said the district wants to keep testing and be certain that the compost is good before it’s sold again and added to gardens.