On a cold morning this week, a team of scientists hammered a pole into the ground along the shores of Edgartown Great Pond and pulled up enough murky groundwater to fill a test tube.

Kelsey Chenoweth and Owen Porterfield aid in the research. — Ray Ewing

Though the act was small, the researchers from the Marine Biological Laboratory in Woods Hole and Great Pond Foundation hope it can unlock the source of nitrogen that has been increasingly choking the 890-acre coastal pond along the Island’s south shore. 

For the next several months, the team will be collecting groundwater samples from all around Meshacket Cove and Slough Cove in a hurried search for nitrogen plumes.

The Island’s great ponds have seen high levels of nitrogen for years and warnings over algal blooms have become regular events. While Edgartown Great Pond is not the most polluted pond on the Island, Great Pond Foundation executive director Emily Reddington said she’s seen a rapid decline of its ecosystem. 

“There used to be hundreds of acres of submerged aquatic vegetation, mostly eelgrass, some widgeon grass,” Ms. Reddington said. “In 2022 we went from having almost full coverage of the pond . . . to just little, tiny pockets.”

Two years ago, the foundation and MBL traced most of the nitrogen influx to wastewater. But where that was coming from was still in question, with the possible culprits ranging from the town’s wastewater plant to the hundreds of unsewered homes flanking the pond. 

Javier Lloret with the Marine Biological Laboratory. — Ray Ewing

The new research from the Marine Biological Laboratory and the Great Pond Foundation looks to track the groundwater through the watershed to pinpoint its origin. So far, according to MBL scientist Javier Lloret, upwards of 70 per cent of the nitrogen in Edgartown Great Pond is entering at Meshacket Cove and Slough Cove.

Increased human development along the Island’s watersheds has contributed to pollution in all 16 ponds. 

“The rain falls on the land and it carries all the nutrients and all the things that we put in that water, including the fertilizer, outflow of our septic system and the fertilizers that we put on the land,” Mr. Lloret said while pumping groundwater from Meshacket Cove Tuesday. “Any kind of pollution that is in the atmosphere settles down on the surface of the soil, and the rainwater carries it into the ground.”

When nitrogen enters a pond, it feeds the algae, which then can create harmful blooms that make ponds unlivable for plants, fish, shellfish, marine mammals and birds. These blooms can also make the ponds unswimmable for homeowners who hold the pond — and their property values — close. 

The groundwater samples will help inform researchers on nitrogen-loaded “hot-spots” in the pond linked to septic systems, agricultural runoff and other sources, so they can prevent any further pollution. The study is an iterative process, with each round of sampling helping to better identify the nitrogen source.

Emily Reddington, the executive director of the Great Pond Foundation. — Ray Ewing

Mr. Lloret will be analyzing the samples for the next several months and constructing a model informing the foundation on where the plumes of nitrogen are located in comparison with the land uses. 

Within the Meshacket Cove watershed is a town wastewater treatment facility, which Ms. Reddington said is a likely source of nitrogen.

Bill Burke, the facilities manager for the Edgartown wastewater department, said the state requires that the facility only discharge a maximum of 2,200 kilograms of nitrogen per year. In 2023, the facility only discharged 587 kilograms, he said.

Mr. Burke added that the town treats all the wastewater that comes to the facility and has a nitrogen removal rate of more than 90 per cent.

The pond has seen nitrogen enter the pond at Meshacket Cove and Slough Cove. — Ray Ewing

Ms. Reddington said homeowners can take precautionary measures to help as the research is ongoing. The tall grasses absorb excess nitrogen from the ground and use it for their own growth, she said. If people planted more of this type of vegetation, more nitrogen would seep into their roots instead of leaking into the pond.

“This is a place people turn to to feed themselves in different ways, whether it’s their bodies, their minds or their spirits,” she said. “I think it’s impossible to overstate the importance of living water to the Island community.”