Environmental

ORNL researchers work to improve understanding of GHG emissions from reservoirs

ORNL researchers work to improve understanding of GHG emissions from reservoirs
(photo courtesy ORNL)

Researchers with Oak Ridge National Laboratory are gathering data on emissions of methane and carbon dioxide from Douglas Lake as part of a broader study across five southeastern U.S. states.

For the work, researchers set traps at different depths and locations on the lake in east Tennessee to capture bubbles and examine the amount of methane and carbon dioxide released. The aim is to better understand and predict how much of these climate-warming gases are coming from reservoirs across the U.S.

Scientists are working to better understand emissions sources and the factors that influence them, with a view toward crafting potential mitigation strategies. Achieving a clearer picture of emission rates starts with gathering more data. Emissions vary widely with season, geography, reservoir operations and a changing climate. They are affected by many factors, including temperature, algae growth and water depth and flow. Emissions are sensitive to natural and human-driven changes, such as nutrient pollution, droughts and storms. Releasing water from reservoirs for flood control, irrigation, hydropower generation and recreation also affects emission rates.

All inland waterways emit carbon dioxide and methane. Most of the methane emissions come from shallow areas in lakes and reservoirs.

Across the U.S., there are more than 90,000 dams. Many of these dams store water in reservoirs for flood control, irrigation, water supply or generating electricity. These reservoirs serve multiple purposes, which can make it difficult to attribute emissions sources.

Data on emissions are only available for a handful of hydropower reservoirs, and many of those data are incomplete. The sparse data and the variability of reservoir levels, temperatures and algae growth make it difficult to use models to predict potential emissions. That’s why ORNL scientists are collecting key data from reservoirs in five southeastern states.

Aquatic Ecologist Natalie Griffiths and her colleagues use aquatic drones and an array of other tools to measure the gases that bubble to the surface.

The ORNL study is one of the first to measure all three emission pathways simultaneously. The team’s recent sampling campaigns showed high variability in emissions over time both within a reservoir and across reservoirs. In the summer, the sampled reservoirs act as carbon sinks – absorbing carbon dioxide from the atmosphere – mostly due to algae growth. In the spring and fall, the reservoirs become carbon sources.

An important factor affecting emissions on seasonal timescales is reservoir operations, such as release of water for flood control, that reduce lake levels and surface area. Lower lake levels create shallow zones where methane can more easily bubble to the surface.

Reservoir geometry along with seasonal changes in the layers of temperatures, known as thermal stratification, also strongly influence emission rates. The cooler denser waters at the bottom of the lake often contain no dissolved oxygen, allowing methane to build up as microbes break down organic matter. If this methane-rich water is pulled through deep-water intakes, methane is released as water exits the dam. More data are needed to understand the interplay of these factors and to represent them in models that predict future emissions.

“Reservoirs are complex ecosystems,” Griffiths said. “As we collect more data, we will better understand the factors affecting emissions across space and time and may be able to inform if and how reservoir operations can reduce emissions.”