New Monitoring Method Improves Ability to Measure Methane Emissions from Reservoirs
Published May 18, 2021
Reservoirs, or human-made lakes, are built to store clean water, generate electricity, and provide recreation, but they can also emit methane, a greenhouse gas (GHG) that contributes to climate change. One source of methane emissions from reservoirs is algal blooms, which can occur when excess nutrients enter water bodies as runoff from chemicals in agriculture and industry. When algae die, it creates an oxygen-poor environment that enables methane-producing organisms to form in the sediment of lakes. These organisms can create bubbles in the soil that rise to the surface filled with methane.
Scientists recognized the importance of including reservoir emissions in the nation’s GHG inventory to better understand their climate impacts in the U.S. and globally. It’s currently estimated that surface water reservoirs emit about 18 million metric tons of methane into the atmosphere each year. However, this estimate is highly uncertain, and researchers are actively working to better understand what factors can impact the amount of methane reservoirs emit.
One issue with measuring methane emissions from reservoirs is the lack of year-round data. Traditional monitoring techniques require researchers to deploy instruments for short periods of time (i.e., minutes) at different locations within a water body. These methods are labor intensive, and for safety and convenience, are largely restricted to daylight hours during the warmer months. As a result, researchers have very little information regarding GHG emissions during the night or the winter months.
In a recent project, EPA researchers adapted a monitoring technique called eddy covariance to measure emissions from reservoirs. Eddy covariance measures vertical fluxes of methane from the bottom of reservoirs to the atmosphere and can be used to make continuous measurements of emissions over long periods with minimal upkeep. This method has been widely used in terrestrial ecosystems – such as forests, farmlands, and wetlands – but few researchers have used it to measure emissions from water surfaces.
Using the eddy covariance method, EPA researchers collected two years of methane emission data at Acton Lake in Ohio, which is a small reservoir impacted by agricultural runoff. Traditional methods were also used to complement the eddy covariance monitoring. The two techniques work well together: whereas traditional methods provide “snapshots in time” at different regions of the reservoir, eddy covariance produces a “high-resolution movie” of one area.
The “high-resolution movie” provided valuable insight into seasonal and daily patterns in methane emissions. For example, the researchers found that nearly 90 percent of annual emissions occurred during the summer months, with winter emission rates close to zero. Daytime and nighttime emission rates varied. They found that higher daytime emissions were linked to windier conditions, since wind can cause underwater mixing, which can release methane. Alternatively, the researchers found that higher nighttime emissions were linked to lower air pressure, which can release methane from reservoir sediment.
“Greenhouse gas emissions from reservoirs are notoriously variable, both in space and time, leading to considerable uncertainty in published estimates,” says EPA scientist Jake Beaulieu, who is leading the research. “The eddy covariance method is our best method for integrating across this variability, yielding the most accurate emission estimates to date.”
These findings are extremely valuable because few investigators have measured emissions year-round and the relative importance of summer and winter emissions is not well known. This information is critical for using field measurements to determine annual methane emission estimates for national greenhouse gas inventories.
A preprint version of their work describing this research and the eddy covariance method has been published online in Biogeosciences – Discussions. The findings from this study are being used to include reservoirs in the Inventory of U.S. Greenhouse Gas Emissions and Sinks for the first time. This research is also helping EPA and other organizations understand the role of reservoirs in air quality and a changing climate.
Connections Between Methane Rates and Environmental Conditions
The high-resolution methane emission measurements also allowed the researchers to identify connections between methane emission rates and three important environmental conditions: the formation of nuisance algal blooms, the temperature of the sediment at the bottom of the reservoir, and the amount of pressure from the air and water above the sediment.
A possible explanation for the connection between algal blooms and methane emissions is that algae is a source of high-quality carbon that can be rapidly converted to methane by microorganisms in sediments. This important finding suggests that algal blooms not only degrade water quality and impair the recreational value of lakes and reservoirs, but also contribute to a warming climate.
While the relationship between sediment temperature and methane emissions is well established, the results of this study provided more nuanced insights. Combining the eddy covariance data with traditional measurements revealed variation in this relationship across different areas in the reservoir. In deeper areas with cooler sediments, methane emissions did not respond as much to increasing sediment temperatures as emissions did in shallower areas with overall warmer sediments.
The scientists also observed higher emissions during periods when the pressure on the sediment from the overlying water and air was decreasing. This is likely because atmospheric pressure serves as a lid holding gases in the water and sediments. Falling atmospheric pressure ‘removes the lid,’ allowing gases to escape into the atmosphere, similar to the release of gas that occurs when a can of soda is opened.
The newly adopted method is contributing to better estimates of methane emissions in reservoirs and providing new information that can be used in reservoir management to improve water quality and decrease greenhouse gas emissions—a win-win for the environment and public health.