Headwater Springs and Streams as Important Sites for Carbon Dioxide Degassing in the South Carolina Piedmont

Previous studies have demonstrated that surface waters are important sources of CO2 to the atmosphere. Because first-order streams account for about half of total channel length within a river network, such streams could account for a large proportion of all CO2 released by the network to the atmosphere. However, few studies have measured CO2 degassing from first-order streams near the stream sources, where CO2 evasion rates are likely to be highest. We measured CO2 concentrations in six first-order streams from both urban and forested watersheds and 38 springs under summer baseflow conditions in the South Carolina Piedmont. In all six streams, dissolved CO2 concentrations declined within the first 100 m downstream of the source. Stable carbon isotopic compositions, which were measured in one stream, became more enriched with 13C over the same sampling interval, consistent with CO2 degassing. A Rayleigh isotope fractionation model indicated that degassing was a kinetically controlled process that decreased in intensity as distance from the source increased. Our results suggest that rapid CO2 degassing from headwater streams within the first 100 m downstream of the source is a common feature of first-order streams in temperate environments, similar to what has been found in tropical headwater streams.