Greenhouse Gas Releases from Exposed Groundwater Discharged During Low Stream Flow Conditions: New Research Quantifies Releases

     New research shows that seeping groundwater can be a significant source of greenhouse gases, including CO2, methane, and nitrous oxide (N2O). Groundwater flowing through sediment and rock near rivers accumulates carbon and nitrogen. Past research showed that groundwater connected to rivers usually mixes with the river water, limiting gas releases. However, during low-flow conditions, groundwater can seep out along stream banks at or above the river surface.

     The research involved measuring greenhouse gas emissions along riverbanks at three locations in the Farmington River watershed in Connecticut and Massachusetts, focusing on areas above the water level during the summer flow season. The research team utilized handheld infrared cameras to identify streambanks with exposed groundwater discharges, or seeps. They then measured changes in the concentrations of CO2, methane, and N2O, as well as discharge rates. They also collected groundwater samples and analyzed them for concentrations of dissolved carbon, oxygen, and nitrogen.

     The abstract from the paper in the March 2025 issue of American Geophysical Union’s JGR Biogeosciences, Preferential Groundwater Discharges Along Stream Corridors Are Disregarded Sources of Greenhouse Gases, is given below.

 

Abstract

Groundwater delivery of greenhouse gases (GHGs) to stream banks and riparian areas, before mixing with surface waters, has not been well quantified. We measured preferential groundwater delivery of GHGs to stream banks within three stream reaches, and found that stream banks with discharging groundwater emitted more CO2 and were sources of N2O compared to stream banks without actively discharging groundwater, which emitted less CO2 and were N2O sinks. At one of our stream reaches, groundwater CO2 and N2O concentrations were 1.4–19.2 and 1.1–40.6 times higher than those in surface water, respectively, and groundwater delivery rates of CO2 and N2O were 1.5 and 1.6 times higher than surface water emissions per unit area. On average, 21% (range 0%–100%) of CO2 and N2O were emitted at the stream bank before mixing with surface waters. Preferential groundwater GHG emissions may contribute substantially to stream corridor emissions and may be underestimated when using a channel-centric approach to estimate riverine GHG budgets.

     The researchers noted that estimates of riverine greenhouse gas emissions should be updated to account for these above-water-level groundwater seeps.

“Overall, our results indicate that groundwater discharge occurring along stream banks has the potential to contribute greatly to river GHG emissions and is unaccounted for in current approaches that measure GHG emissions from surface water channels.”

 

Key Points

·        Stream banks with actively discharging groundwater acted as localized N2O sources, while adjacent nondischarging stream banks acted as sinks

·        On average, 21% of groundwater discharge CO2 and N2O are emitted from the stream bank before mixing with surface water

·        Preferential groundwater discharges have higher concentrations and areal delivery rates of CO2 and N2O than adjacent surface water

  

 

References:

 

Seeping groundwater can be a hidden source of greenhouse gases. Sarah Derouin. Phys.org. March 31, 2025. Seeping groundwater can be a hidden source of greenhouse gases

Alaina M. Bisson et al, Preferential Groundwater Discharges Along Stream Corridors Are Disregarded Sources of Greenhouse Gases, Journal of Geophysical Research: Biogeosciences (2025). Preferential Groundwater Discharges Along Stream Corridors Are Disregarded Sources of Greenhouse Gases