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Study: Retention ponds emit carbon dioxide and methane into the environment

Retention pond
Retention ponds that capture, or sequester, the most carbon are ringed or even filled with the types of plants they are using in the Everglades to filter polluted water.

Most stormwater runoff ponds have been found to constantly emit greenhouse gases that contribute carbon dioxide to global warming.

Once considered the perfect way to let nature process rain that has run along the ground and picked up pollutants, most stormwater runoff ponds have now been found to emit greenhouse gases that contribute to global warming.

The most common stormwater control measure in America today, there are at least 76,000 of the ubiquitous ponds throughout the Sunshine State. Placed next to one another, stormwater ponds in Florida alone would encompass nearly 250 square miles, which is roughly the size of Chicago.

Now that there is widespread acceptance of global warming caused by greenhouse gases such as carbon dioxide and methane, new research out of Manatee County has discovered most stormwater retention ponds emit more of the gases into the atmosphere than they store, or sequester, down in the mud or sediment at the bottom.

The development of stormwater containment ponds as a method for handling runoff, which is filled with a mix of everything water can wash away, whether inert or dangerous, has evolved over several decades.

In new subdivisions, a series of ponds to catch excess nitrogen and phosphorus from fertilizers and animal waste are typically part of the design. One pond each at the four corners of a bridge corrals rainwater pouring from the road surface filled with motor oil, rubber from tires, assorted pieces of glass, plastic, fabric, paint, and anything else that ends up on the roadbed.

The ponds are often filled with the types of plants they are using in the Everglades restoration to filter polluted water from Lake Okeechobee before releasing it south to its historic flow toward Florida Bay.

The thinking was, and still is in many cases, that the stormwater ponds are a great solution that uses nature, not electricity, to clean polluted runoff as the water percolates back down toward the aquifer.

But in a study in “Communications Earth & Environment,” a journal with research on Earth, environmental, and planetary sciences, climate change researchers tested several stormwater ponds in Manatee County and discovered they emit far more carbon dioxide into the atmosphere than they may trap in the sediments below.

"In the process of preventing urban sediment from moving into downstream waters, constructed stormwater ponds have emerged as significant reservoirs of sediment carbon," the climate change scientists wrote. "The growing body of research on artificial waters and urban ponds has shown that they are capable of emitting substantial quantities of carbon dioxide and methane, greenhouse gases, and potentially more so than natural ecosystems."

By comparing the amount of carbon sequestered at the bottom of the ponds with the level of emissions of carbon dioxide and methane, both potent greenhouse gases, it became evident to the researchers that many stormwater ponds emit more carbon than they store.

 Retention ponds without carbon-processing plants around the edges emit the gas into the atmosphere
Retention ponds without carbon-processing plants around the edges emit the gas into the atmosphere

The variables that can affect whether stormwater ponds are effective carbon sinks are almost limitless: how deep they are, how long water stays in the ponds, whether the ponds regularly dry up between rainy periods, if fish have been stocked in them, the make-up of sediments and pollutants in the runoff entering the ponds, whether the ponds are treated with chemicals to control algae growth, if there are crumbling sidewalks near the ponds contributing elements of concrete, the type and abundance of vegetation, or lack thereof, surrounding the ponds, whether muck removal is part of the maintenance of the ponds, whether there are fountains or bubblers in the ponds to add oxygen into the water, how the ponds were constructed … the list goes on and on.

In the short term, the authors suggest that the role of stormwater ponds should be considered in regional and global carbon models.

But the most important variable is how long a pond has been there. The good news is the longer the ponds are in place, the more effective they become at sequestering planet-warming gases.

Environmental reporting for WGCU is funded in part by VoLo Foundation, a non-profit with a mission to accelerate change and global impact by supporting science-based climate solutions, enhancing education, and improving health. 

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Tom Bayles