DEP Proposes Chemical Treatment to Control Invasive Weed in Reservoir

A dense mat of hydrilla in the Croton River. The invasive weed has spread to the Croton Reservoir. Credits: Department of Environmental Conservation

YORKTOWN, N.Y.-Looking to stop the spread of a weed that has made its way into the Croton Reservoir, the New York City Department of Environmental Protection is seeking the town’s approval to treat it with a chemical-based product.

Much of the reservoir, which supplies drinking water to the city, is infested with hydrilla, a federally listed noxious weed, which means it’s illegal to import or transport between states without a permit.

“It can affect water quality and taste and odor of the water as the plant decomposes,” said Meredith Taylor, an invasive species biologist for the Department of Environmental Protection.

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Taylor met with the Town Board last week because the city is launching a pilot program to control the hydrilla, which was first discovered in 2013 in the state-owned Croton River. Water flows from that river into the Amawalk Reservoir, which supplies a portion of Yorktown’s drinking water. The New York State Department of Environmental Conservation has already started treating the infestation with fluridone.

Hydrilla is an aquatic weed from Asia that, according to the Department of Environmental Protection, is one of the most difficult aquatic invasive plants to control and eradicate in the United States. The plant was discovered in the Croton Reservoir in 2014, and the city hopes to treat it with the same chemical, Taylor said.

“It’s mostly in the downstream portion of the reservoir, but the density and spread of infestation is increasing,” she said. “We really are running against the clock here to control it. So, we’re hoping to move forward with a pilot project this year.”

One of those pilot areas is in Yorktown, which would need to grant a wetland permit before the city can begin treatment in May or June. If successful, the city would launch a full-scale control program later this year.

“If we don’t do the control work, we risk letting the plant spread into our unfiltered drinking water supply, where we have fewer options for dealing with it,” Taylor said.

Taylor said experts agree that fluridone treatment within the reservoir is “safe, effective and the only option we have for controlling hydrilla long-term.” The chemical has been used in other drinking supplies throughout the country, including in California.

“We hope that this project shows that the treatment can be done effectively, [and] that the chemical is not spreading into our intakes and into distribution,” Taylor said.

The fluridone product the city would use is called Sonar H4C. Taylor said it is a granular product, and the pellets, which look like rice, would be spread across the sediment within the reservoir.

“They don’t dissolve very quickly, so they don’t release very much chemical at a time,” she said. “It maintains a very low dosage in the water.”

Taylor said the product has a “very low toxicity” and comes with no water-use restrictions for swimming, boating or fishing. She said that it is not possible to physically remove the weed.

“The plant is very fragile, so as soon as you go to remove it, it breaks into pieces and each piece is reproductively active, so you end up spreading it further,” she said.

Supervisor Ilan Gilbert advised Taylor and the city to move forward with applying for a wetland permit, which requires a public hearing.

Councilwoman Alice Roker said she wants to do her own research on the chemical.

“I think it’s important that people in this town understand what’s going to happen,” Roker said.

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