CARBON DIOXIDE EFFLUX FROM A GROUNDWATER-FED STREAM, KONZA PRAIRIE LTER SITE, NORTHEASTERN KANSAS, USA

The upper 10.6 km² of the Kings Creek (USGS gauging station 06879650) drainage basin is entirely confined by the Konza Prairie Long-Term Ecological Research site (Konza), an unplowed tallgrass prairie with woody riparian zones. This fifth-order stream is an ideal location to study groundwater-stream water interactions that have been minimally disturbed by human activities. The shallow aquifers at Konza comprise Quaternary alluvium and the relatively thin (1-2 m) Permian-aged bedrock limestones that are separated by thicker (2-4 m) shales. In the upper reaches (first through fourth order) of the watershed, the Kings Creek channel is underlain by bedrock and discontinuous alluvium. Bedrock also outcrops adjacent to nearly continuous alluvium in the lower reaches. Here we show evidence of direct groundwater addition to the stream in a time of very low base flow through thermal imaging in a lower reach of Kings Creek. Because groundwater contains CO₂ at partial pressures greater than the atmosphere, the CO₂ degasses when groundwater discharges to the stream. This efflux is an important component of the global carbon cycle, and there have been very few direct measurements of efflux rates in upland streams. We have measured the efflux of CO₂ from Kings Creek, and the rate (around 8 gms m^-2 d^-1) compares favorably to efflux rates calculated by other investigators in larger streams and lakes using gas transfer coefficients. Using thermal imaging to identify specific locations of groundwater addition to stream water improves the quantification of the process of CO₂ efflux, and provides a valuable tool for improving input to global carbon cycle models.