Adi Talwar

A CSO/Wet Weather Discharge Point on the West coast of the Bronx River near Tremont Avenue.

After a rainy summer and an even wetter fall, New Yorkers are becoming familiar with showery forecasts. With droughts and wildfires in the West and Southwest, rain might seem like a welcome reprieve—but not in New York City, where the rain washes pet waste, plastic utensils and other street grime into an aging sewer system that often can’t handle it.

Every year the city releases 100 billion liters of untreated water sewage and stormwater runoff into its waterways. Whenever it rains heavily, the surge of stormwater forces the sewers to hit maximum capacity, causing wastewater to flow from the 100-year-old system into nearby rivers.

The city is working on various ways to improve water quality but those efforts might not be enough. Recently, researchers at Queens College found that pollution from the overflow might actually be contributing to greenhouse gases in nearby marshes.

“The take home message [was] carbon additions increased both carbon dioxide and methane production in wetland soils,” said lead study author, Dr. Brian Brigham.

Typically wetlands serve as carbon sinks, areas that can absorb carbon dioxide from the atmosphere. They trap and store the carbon through a process called biological carbon sequestration. Wetland vegetation and soil accumulate organic matter, which is anything that includes carbon, before decomposing and sometimes emitting it through natural respiration. This is all part of the carbon cycle, a set of processes by which natural systems absorb and emit carbon. It’s estimated that wetlands store about 35 percent of all land-based carbon.

But the U.S has lost more than half of its original wetland areas because of agricultural and urban developments. When wetlands are drained or otherwise disturbed, that stored carbon can be released back into the atmosphere. Every time it rains a lot, and human pollutants are carried by the excess stormwater into surrounding waterways, wetlands lose their ability to contain the carbon, allowing more greenhouse gases to enter the atmosphere.

In order to show this, Brigham and his team added certain types of carbon and nitrogen commonly found in sewage-polluted environments to three different marshes along the Hudson River. In each sample, they took mud, soil and microbes and simulated in the lab what would happen if sewage had been added. They found that the added carbon increased carbon dioxide production rates 1.4 to 2 times more in the treated soils than in the controls. The added carbon also increased methane production rates in all three sites, depending on their salinity.

Salty environments inhibit methane production. The less salinity a marsh has—like the brackish waters of the Piermont and Iona Island Marshes—the more methane is produced. Extra carbon caused significantly greater methane production in Piermont and Iona Island compared to the saltier waters of Staten Island’s Saw Mill Creek Marsh. Every influx of stormwater carrying human pollutants from the sewers brings along excess carbon, which fuels microbial respiration, producing more methane.

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The researchers concluded that inorganic nitrogen was not a major driver in carbon dioxide and methane production rates.

“The most important thing about this study is that it points to the fact that there are a lot of underappreciated impacts from water pollution,” said Dan Shapley, New York Riverkeeper’s Water Quality Program Director, who said it could shake the ground for managing greenhouse gases and preventing climate change.

Other environmental scientists have also been studying this area and have found similar results.

Siobhan Fennessy is a professor of biology and environmental studies at Kenyon College and she thought the Queens College researchers’ results were generally consistent with how carbon cycles function in wetlands. One difference, she noted, between what happens in natural sites compared to the lab study is that they added acetate as a carbon source. Acetate is a chemical compound that Brigham’s team used because it’s likely to exist in sewage-polluted environments.

“So the huge response they got in the lab study may be more,” Fennessey said, “than what they would’ve gotten if they had used the same sort of carbon sources that are in the CSO.”

Climate change is leading to more intense rainstorms and as a result there’s a peak load problem. The NYC Panel on Climate Change expects up to an 11 percent increase in precipitation by the 2050s.

This excess amount of water is predicted to stress the city’s sewer system even further, causing wastewater that is normally treated by treatment plants to flow into nearby rivers and wetlands.  

Under a 2012 Consent Order, the city committed to spend a total of $4.2 billion to mitigate combined sewer overflows. It promised to invest in improvements like wastewater treatment plant upgrades, storage tanks and sewer separation as well as allot $1.5 billion to green infrastructure projects like rain gardens and green roofs.

The city’s green infrastructure addresses 90 percent of rain events for the year but it’s not designed for very large storms, said DEP Managing Director of the Office of Ecosystem Services, Green Infrastructure and Research, John McLaughlin.

“It’s all very storm-specific,” he said, if it rains three inches in four hours, the rain gardens and green roofs probably won’t be able to handle that volume of water but they probably can take on three inches over the course of a day. If the rainfall is spaced out enough and the subsurface infiltration is great enough, green infrastructure is equipped to handle stormwater catchment, he explained.

Some still think the city could be a lot more ambitious about how they’re reducing CSOs. Korin Tangtrakul, a Soil and Water Conservation Stormwater Technician with New York SWIM Coalition, believes having a more robust and effective green infrastructure plan will help reduce the overflow.

“To think that sewage is causing more greenhouse gas emissions in wetlands—it’s a pretty terrifying prospect,” she said.

As the city’s population increases, the amount of sanitary waste pouring into the system also skyrockets, potentially adding to the greenhouse gas emissions. According to the Mayor’s Office of Climate Policy and Programs, the city is leading in the fight against climate change with a commitment to reduce these emissions 80 percent by 2050.

But Dr. Greg O’Mullan believes the problem is a lack of understanding of how CSOs and greenhouse gas emissions relate to one another. An environmental microbiologist who worked on the study with Brigham, O’Mullan has been studying bacteria indicators in the city’s surrounding waterways for the past 15 years.

“The CSOs are a design feature that has very negative consequences,” said O’Mullan. When city planners discuss CSOs, he said they should look beyond just the health factors like reducing pathogens and infection risk.

“We don’t account for what’s released into the environment and turned into greenhouse gases,” he said, because carbon can be delivered in one form but there are alterations that happen in the environment, which turn it into carbon dioxide and methane.  

“New York City has reduced sewer overflows by 80 percent and the Harbor is cleaner today than it has been since the Civil War,” according to a spokesperson for the mayor’s office, and the city “continues to invest billions of dollars to reduce overflows and clean up all of the waterways.”

But Tangtrakul said the study’s findings further demonstrate the need to take action to reduce combined sewer overflows. New Yorkers should start conserving water during rainstorms to help reduce the amount of sewage contributing to a CSO, she said. That means waiting to do laundry or dishes until after the storm passes, she said.

“If every resident in New York did that,” she said, “we would significantly reduce how much sewage ended up in the waterways.”