Scientists Will Study How Sewering Has Impacted Little Pond In Falmouth

The following article by Gilda Geist was published in The Falmouth Enterprise and online on 2/23/2024.

The Town of Falmouth spent $44 million to build a sewer system in the Little Pond watershed in 2016, with the goals of reducing nitrogen in the pond and returning the impaired pond ecosystem to a healthy state. Now, a team of local scientists is working to find out if the project did indeed meet those goals.

“We know it’s very expensive to build this kind of wastewater infrastructure,” said Kenneth H. Foreman, a scientist on the project and a member of the water quality management committee. “People have a right to know that it’s going to pay off.”

“We have thousands of examples of what happens when the system becomes polluted,” said Matthew H. Long, another scientist on the project. “But this is a really unique opportunity in that we can see what happens to one of those polluted ecosystems when we rectify the problem.”

Dr. Long works at the Woods Hole Oceanographic Institution and Dr. Foreman works at the Marine Biological Laboratory. The two scientists, along with MBL scientist Ketil Koop-Jackobson, will be measuring and assessing how nitrogen flows into and through Little Pond.

“Here was a chance to look at how an ecosystem would respond to a pretty significant—like 80 percent or so—reduction in the nutrient inputs,” Dr. Foreman said. “We want to know, what did we get for $44 million?”

The History

Construction on the Little Pond sewers began in 2015 and was completed in 2016; homeowners hooked up to the sewers from 2016 to 2019. The sewer area includes the Maravista peninsula and much of the land between Route 28 and Little Pond, for a total of about 1,400 homes and businesses.

Little Pond is primarily fed by groundwater, which in Falmouth is typically nitrogen-rich due to inputs from the leaching of septic systems, Dr. Foreman said. The sewer project was supposed to divert much of that nitrogen away from the groundwater and to the Wastewater Treatment Facility, where it could be removed.

To see how much that nitrogen was actually diverted, Dr. Foreman and some students and interns installed 12 monitoring wells around Little Pond in 2015. Groundwater samples were drawn from each well around the pond at several different depths. Dr. Foreman and his team collected those samples once or twice a year from 2016 to 2022.

The samples revealed that indeed, the concentration of nitrogen in the groundwater surrounding Little Pond had decreased significantly following the sewer installation and hookups.

“That data has showed, pretty convincingly, that nutrient levels have dropped by at least half, and that the loading reduction targets—which were on the order of 5,000 kilograms for the system—have been reached,” Dr. Foreman said. “Less clear is how long it might then take for the ecosystem to return to a fully healthy state.”

Measuring Metabolism

Even though nitrogen concentrations have been reduced in the groundwater, Little Pond has still not quite made a full recovery. That is because groundwater is not the only source of nitrogen for a pond—organic matter that has accumulated in the sediments at the bottom of a pond will release nitrogen as it decomposes, which fosters the overgrowth of algae and phytoplankton.

“Just like when you add fertilizer to your lawn, it [nitrogen] is fueling the growth of the plants,” Dr. Foreman said.

In simple terms, the overgrowth of algae and phytoplankton are telltale signs of an unhealthy pond system, while a healthy pond system is marked by the presence of eelgrass.

Exactly how much nitrogen is being released from the bottom of the pond is what Dr. Foreman and his colleagues seek to find out. The team will collect sediment cores and bring them back to the lab to study how nutrients move through the bottom of the pond. Some of this research will be done right in the pond, Dr. Foreman said, without having to transport samples back to the lab.

“The collaboration with [Dr. Long] grew out of the fact that he has some really cool instruments that allow us to make measurements of water column and benthic metabolism in the pond,” Dr. Foreman said.

Metabolism, in simple terms, describes how an organism or ecosystem creates and uses energy. Benthic refers to the bottom of a water body. So Dr. Long’s instruments will be able to gather information about how energy and oxygen are created and consumed in the water and in the sediments of Little Pond. Dr. Long will then be able to synthesize this information to make determinations about the health of the pond.

“Right now, it’s a eutrophic [meaning nutrient-rich], muddy pond. There’s algal blooms which cause…low oxygen conditions, which effectively excludes other organisms from living there, because they all need oxygen,” Dr. Long said. “In the best-case scenario, we would have an eelgrass bed there with relatively clear water. So that’s sort of the transition we’re trying to capture.”

Dr. Long will use a method called eddy covariance to measure how oxygen moves at the bottom of the pond and another method to measure how oxygen moves in the water column using a device he developed in his lab.

“It’s called CRISPEE,” Dr. Long said. “Continuous Reconnaissance In-Situ Photosynthetic Ecosystem Explorer.”  CRISPEE looks like a metal cylinder with a pump attached. The device takes in a water sample and a series of sensors measure changes in oxygen.

“If the water column was all clear and nice, like what a natural estuary should be, there actually won’t be a lot of oxygen production or consumption in the water column,” Dr. Long said.

When it comes to the bottom of the pond, “if it’s a healthy seagrass bed, it’ll be producing lots of oxygen,” Dr. Long said. “If it’s a degraded, muddy pond, it’ll be consuming a lot of oxygen.”

Tidal Exchange

The project will also look at the tidal exchange between Little Pond and Vineyard Sound. Nitrogen can be flushed out of Little Pond through tidal exchange, Dr. Foreman said. The team will use tools to measure how much flushing is actually taking place. This will be especially useful to measure because there is some concern that the narrow mouth of the inlet will hinder the nitrogen reduction process.

“That’s one of the big issues—even if you remove the nutrients, is that going to be sufficient?” Dr. Foreman said. “Because there’s such limited tidal exchange, and that retains nutrients and phytoplankton in the pond.”

Some of this work will require the use of the MBL’s 14-foot aluminum skiff, which is powered by an electric motor, Dr. Foreman said. An abutter to the pond allowed the team to dock the boat at his property, he said.

The team will also make use of a device called a multiparameter sonde, which can measure changes to oxygen levels, salinity, pH and more, Dr. Foreman said. The device takes measurements every 15 minutes and when the scientists are ready, they can bring it back into the lab and download the data that it has been collecting.

In addition to answering big questions about the efficacy of the Little Pond sewer project, Dr. Foreman said he hopes the project can help create a set of best practices for determining whether a water body is healthy.

This project will cost about $300,000, Dr. Foreman said. The project is being funded by WHOI Sea Grant and a number of nonfederal match funds. Most of the funding pays for personnel time, Dr. Foreman said.