QUANTIFYING CO2 DEGASSING AND δ13C USING A FLOATING CHAMBER IN A GAINING HEADWATER STREAM

Headwater streams comprise more than half of the total stream length in the conterminous U.S.A., and despite their high rates of CO₂ degassing, these stream reaches are often overlooked when accounting for carbon contributions to the atmosphere. Shallow groundwater is the main contributor to the baseflow in headwater streams and the source of the CO₂. At the Konza Tallgrass Prairie Long-Term Ecological Research Site (Konza), CO₂ in shallow groundwater has been increasing continuously over the past 25 years of observation, suggesting that the contribution of headwater-stream CO₂ may also be increasing.

We used a floating chamber to measure CO₂ flux in a headwater stream located on the Kings Creek watershed divide at Konza during the summer of 2015. We identified exact locations of groundwater seeps using an infrared camera, because of temperature difference between the discharging groundwater and stream water. We collected stream water samples and groundwater samples from nearby wells completed in the same merokarst limestones that crop out in the streambed, and determined major-ion chemistry as well as δ¹³C of the CO₂ and of dissolved inorganic carbon (DIC). Preliminary data show that three different groundwater seeps, located along the stream bank or in fractures in the bedrock on the stream bottom, were degassing with CO₂ concentrations between 5500 and 500 ppm. Locations downstream from the seeps had slower CO₂ degassing rates and lower CO₂ concentrations. We compare the CO₂ concentrations, degassing rates, and water elemental and isotope chemistry of the stream water with groundwater in order to assess the changes that occur during groundwater discharge to a headwater stream and the potential contribution of headwater streams in the region to atmospheric CO₂.