Stonington — Gardeners and farmers who collect eel grass from a borough beach to use as fertilizer may want to think twice after a sewer backup sent wastewater onto the beach, town officials said this week.

A grease buildup in a pipe blocked water going into in a nearby pump station and caused wastewater to trickle out of a manhole on Ash Street on Tuesday, according to Water Pollution Control Authority Executive Director Douglas Nettleton.

WPCA staff fixed the blockage, notified the state Department of Energy and Environmental Protection and applied bleach to the surrounding area within hours, Nettleton said, but not before several hundred gallons of wastewater had leaked out of the manhole.

They didn't realize that water had trickled through a stone wall and onto the small public beach facing Little Narrangansett Bay at the end of street.

The next day, a neighbor called to say the wastewater leak may be a problem for another reason: for the past several years, local gardeners and farmers have collected eel grass that accumulates on the beach, gathering it using pitchforks, garbage bags and trucks to spread on their gardens as a natural fertilizer.

Eel grass, an underwater plant, and cladophora, a type of algae, have collected on the beach for the past several years and made it unusable, said Karen McGee, who lives near Ash Street and founded a group called the Coalition for the Management of Seaweed in Stonington Borough in 2014 to address the problem.

Town officials have tried several methods of controlling the sea grass buildup — including once collecting it in a truck and delivering it to a nearby farm — to no avail.

"It was back in a week," McGee said.

The dead eel grass makes a nutrient-rich mulch that she encourages local gardeners to collect and apply to their gardens, while also making a dent in the pileup of the sea grass on the beach.

"They come, and they come with trucks," said Alice Despard, who lives in a house beside the beach. "In the spring they're here every day."

After learning that the beach is a source of fertilizer, Nettleton said water authority staff posted a sign warning people that the seaweed could be contaminated.

"There's always a potential that there's some contact there," he said. "You want to refrain from using this on any vegetable garden."

Ryan Hammond, the supervisor of the Ledge Light Health District's environmental health department, said Friday that exposure to several days of sunlight and a heavy rainfall would probably clear out any pathogens in the water that came in contact with the eel grass.

"I can't say there is no risk — there's always a risk," he said. But, he added, "if they're taking it and not consuming it ... the risk is fairly low. I'd probably recommend that they wear gloves and wash their hands after they come in contact with it."

McGee said she hopes Stonington gardeners will keep using the sea grass, putting productive use to the beach nuisance.

"It's really a good mulch," she said. "It's great because the water drains through it."

Plus, she added, "it's free."

m.shanahan@theday.com