Source: Journal of Geophysical Research: Biogeosciences
Scientists know that streams and rivers can contribute significant quantities of greenhouse gases to the atmosphere. One way these bodies of water come to contain greenhouse gases is via groundwater, which picks up carbon and nitrogen as it seeps and flows through rock and sediment near rivers. Much research into greenhouse gas emissions from rivers assumes that before being released into the atmosphere, the gases in this groundwater mix with the currents of rivers and streams. But during low-flow conditions, groundwater can seep out along stream banks at or above the river surface, creating a pathway for greenhouse gases to escape directly from groundwater.
Bisson et al. set out to estimate the magnitude of emissions from groundwater rising directly to the surface, known as groundwater discharge. They measured greenhouse gas emissions along riverbanks at three locations in the Farmington River watershed in Connecticut and Massachusetts, concentrating on areas that had groundwater discharge above the waterlines during a typical summer flow season.
At each stream, the team used handheld thermal infrared cameras to identify stream banks with and without areas of exposed groundwater discharge. Once these stream banks were located, the team measured fluxes of the greenhouse gases carbon dioxide (CO2), nitrous oxide (N2O), and methane, as well as groundwater discharge rates along the stream banks. They also collected subsurface groundwater samples and analyzed the samples for concentrations of dissolved organic carbon, oxygen, and nitrogen.
At one site, the researchers found that CO2 concentrations were 1.4–19.2 times higher in groundwater discharge than in surface water and N2O concentrations were 1.1–40.6 times higher. In comparison, stretches of stream with no groundwater seeps acted as N2O sinks. They also found that groundwater emissions of CO2 and N2O were 1.5 and 1.6 times higher than surface water emissions, respectively. On average, 21% of emissions from the groundwater seeps were released into the atmosphere before they could mix with surface waters.
The authors note that their work shows that exposed groundwater discharge along stream banks can be a significant, often unaccounted-for, source of river corridor greenhouse gas emissions. They add that more work should be done to better understand potential emissions from river corridors where groundwater discharge is abundant. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2024JG008395, 2025)
—Sarah Derouin (@sarahderouin.com), Science Writer
