Sewering Little Pond Reduced Nitrogen in Groundwater

The following article was published by The Falmouth Enterprise on August 26, 2022:

Data Show Sewering Falmouth’s Little Pond Reduced Nitrogen In Groundwater

The sewering of Little Pond watershed in 2016 appears to have significantly reduced the amount of nitrogen in the surrounding groundwater, committee member and Marine Biological Laboratory scientist Kenneth Foreman told the Water Quality Management Committee in its Monday, August 22, meeting.

Dr. Foreman, MBL researcher Rich McHorney, and students from the Woods Hole Partnership Education Program have been monitoring the groundwater in wells around Little Pond since sewer hookups for the area started in 2016. Dr. Foreman said that there has been a 56 percent reduction in nitrogen in the groundwater since the monitoring began. Samples were taken from three to five depths at the 12 groundwater monitoring wells around the pond once or twice per year from 2016 to 2022.

The town’s goal in constructing this sewer system was to reduce the amount of nitrogen by about 80 percent, Dr. Foreman explained in his presentation. Approximately 1,400 homes and businesses in the area hooked up to the sewer, mostly from 2017 to 2019. The sewering project cost about $37 million.

Even though nitrogen inputs from wastewater from a particular home or business stop as soon as the sewer is connected, nitrogen may linger in the groundwater for years. Dr. Foreman calls this “a legacy of contaminated groundwater” and said that groundwater flows about one foot per day on average.

Even with this residual contamination, however, the average concentration of nitrogen in groundwater went from 2.7 mg/L in 2016 to 1.2 mg/L in 2022.

Nitrogen occurs naturally in the environment, including in human urine. Traditional septic systems do not remove nitrogen from effluent, so it ends up flowing into the groundwater. When there are many septic systems in an area, like Falmouth, the groundwater can become overloaded with nitrogen.

When this excess nitrogen eventually flows into local estuaries, it can have an adverse effect on the ecosystem. Nitrogen serves as a nutrient for plants, including aquatic ones like algae. When there is too much nitrogen, algae can grow out of control and create algal blooms and dead zones that are harmful to fish and shellfish.

The Town of Falmouth has a documented nitrogen issue due to a high number of septic systems. Sewer systems, on the other hand, use bacteria that help to denitrify wastewater before it goes back into the environment. Dr. Foreman said that “without question,” the sewing of Little Pond is what resulted in less nitrogen in nearby groundwater.

The Water Quality Management Committee has been working on plans to sewer other parts of Falmouth to meet water quality goals. Great Pond is the next watershed to be sewered.

Dr. Foreman said he and other team members will continue to sample the Little Pond groundwater wells once or twice per year to see what happens to the nitrogen levels next. They will also begin looking at the exchange of water and other materials between Little Pond and where it empties out into the Vineyard Sound, Dr. Foreman said.