Quantifying CO2 Degassing and d13C Using a Floating Chamber in a Gaining Headwater Stream

Scientists still debate the sources and sinks of carbon on the earth’s surface. Headwater streams, a carbon source often overlooked in the carbon balance equation, cover more than 50% of the total stream length in the conterminous U.S. Shallow groundwater discharge is the main contributor to the baseflow and CO₂ concentrations in headwater streams. Groundwater datasets at the Konza Prairie Long-Term Ecological Research Site (Konza) demonstrate an increase in groundwater pCO₂ by approximately 29% from 1990 to 2009. This continuous increase suggests that headwater streams, recharged by shallow groundwater, could be a significant carbon source.

We used a floating chamber to measure CO₂ degassing rates in a headwater stream reach at Konza. Temperature contrasts, seen with an infrared camera, between discharging groundwater and the stream, located groundwater seeps. We found that groundwater discharge occurs primarily along the stream bank or in fractured bedrock on the stream bottom. At each seep, we collected water samples for major-ion chemistry, δ¹³C of CO₂, and δ¹³C of dissolved inorganic carbon (DIC). Because the limestone aquifers crop out in the stream, we also collected groundwater samples from nearby wells completed in those limestones.

We compare stream water and groundwater to assess the changes that occur during groundwater discharge to a headwater stream.  Preliminary results show stream water degassing at rates between 1.82 and 58.4 g C m^2-day^1-. The CO₂ fluxes and pCO₂ decreased with distance from the seeps (~2 meters). Decreases in fluxes and pCO₂ were accompanied by heavier carbon isotopic ratios, with δ¹³C—CO₂ increasing downstream of groundwater seeps. Furthermore, groundwater and stream water chemistry were similar for major anions, major cations, and alkalinity. This study provides additional understanding of groundwater and surface water interactions and evidence of a significant contribution to atmospheric carbon from headwater streams in the region.