About 74% of residents in Long Island, N.Y., discharge wastewater into septic tanks and cesspools instead of into a sewer. Many of these tanks and pools were not designed to filter out nutrients and allow nitrate to seep into the region’s coastal aquifer.
Nitrate itself isn’t toxic. But when too much flows underground and out to sea, it feeds harmful algal blooms that suck oxygen from the water, killing thousands of fish.
A mulch-filled trench dug along the coastline, called a permeable reactive barrier (PRB), can neutralize up to 100% of the nitrate before it reaches the ocean, researchers reported at AGU’s Water Science Conference 2024 in Saint Paul, Minn.
“I think it’s incredibly important research,” said Matt Charette, senior scientist at Woods Hole Oceanographic Institution who was not involved in the study. His team is doing similar work in Cape Cod, Mass. “We’ve sort of kicked the can down the road in some of these areas, like Long Island and here on Cape Cod, in terms of managing our wastewater.… The cost of fixing it now by traditional means has gotten a little bit out of hand,” Charette said.
Legislators have proposed a $4 billion sewer expansion plan for Long Island to slow the influx of nitrate into the area’s groundwater. “We want to obviously minimize the inputs of new nitrogen, but that will take a while, and we have to deal with legacy nitrogen,” said one of the new study’s researchers, biogeochemist Nils Volkenborn of Stony Brook University in New York.
PRBs could supplement such a sewer upgrade. Each unit would cost about $100,000 and last for 20–30 years, according to the researchers.
Residents may see improved water quality sooner with PRBs than if they installed only a sewer system, Charette said. The reason is that there is so much nitrate already stored in the aquifer that it could take as long as a decade to flush it out into the ocean naturally.
Working with Nature
These biofilters depend on naturally occurring microbes called denitrifiers. When oxygen is absent, they use the oxygen in nitrate molecules to digest organic carbon in the wood chips. This process converts nitrate into harmless nitrogen gas.
But once these microbes break down all of the nitrate, other classes of bacteria come in to continue decomposing the mulch. Charette said this means PRBs could have “unintended consequences.” Some bacteria can mobilize harmful elements and compounds such as arsenic and methane.
“We want to have a [permeable reactive barrier] that is thick enough to remove pretty much all of the incoming nitrate, but not thicker than that.”
To prevent that, Volkenborn and his colleagues searched for the “sweet spot” PRB design. “We want to have a [PRB] that is thick enough to remove pretty much all of the incoming nitrate, but not thicker than that,” Volkenborn said.
The team tested hardwood mulch against pine mulch and simulated how much nitrate would be removed at three different temperatures to resolve seasonal changes. They also tested grids of holes filled with mulch—known as column arrays—against a continuous trench design lodged into the shoreline. They trialed the trench design with mulch 2.5 feet (0.8 meter) thick and 5 feet (1.5 meters) thick.
After sampling upstream and downstream wells for 3 years, Volkenborn and his colleagues found that denser hardwood filtered more nitrate than pine did. The barriers also appeared to work better in warmer water, removing 7 times the nitrate than was removed in cold water.
The column arrays removed 84.5% of the nitrate. The 2.5-foot-thick (0.8-meter) trench removed 99.7% of the nitrate, whereas the 5-foot-thick (1.5-meter) trench neutralized 100%.
At this particular site in Hampton Bays, where the groundwater flows at 2 feet (0.6 meter) per day and there are 5 milligrams of nitrate per liter of water, the scientists think a 1.7-foot-thick (0.5-meter) PRB might be the sweet spot that would remove all incoming nitrate for most of the year. They estimate they can scrub 60 kilograms of nitrate per year with a 100-foot (30.5-meter) PRB. The researchers did not detect excess methane in the air above their trial PRB, and they are designing experiments to look at the fate of iron. They predict that some iron will mobilize but that it will resolidify and sink when it meets oxygenated water. “This happens subsurface and we do not expect significant amounts of [iron] entering the water,” Volkenborn wrote in an email.
“PRBs are very effective and have been demonstrated as such for removing nitrate,” said Paul Dombrowski, an engineer at In-Situ Oxidative Technologies Inc. (ISOTEC) and a lecturer at Tufts University who was not involved in the study. He has installed different kinds of permeable reactive barriers for more than 20 years. “I think the biggest question is, Where are the best places to put them?” Dombrowski said.
Finding the right sites to install PRBs is the tricky part, Volkenborn said, because they’re used to treat historic nitrate deposited when little land use data were collected. The barriers work best where groundwater flows quickly and the nitrate concentration is high. To find those spots, the researchers have to drill monitoring wells and test for up to a year, Volkenborn said.
In addition to PRBs, the team is working to install biofilters that remove nitrate from individual septic tanks with the hope of preventing Long Island’s nitrate legacy from living on.
—Alix Soliman (@alixoutdoors), Science Writer
Citation: Soliman, A. (2024), Reactive barriers could keep nitrate out of the Atlantic, Eos, 105, https://doi.org/10.1029/2024EO240296. Published on 15 July 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
Related