New York’s Sewer Overflows Could be Contributing to Climate Change

Sourced from CityLimits

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.

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.

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