@article{fdi:010072673,
  title = {{C}arbon dioxide degassing at the groundwater-stream-atmosphere interface : isotopic equilibration and hydrological mass balance in a sandy watershed},
  author = {{D}eirmendjian, {L}. and {A}bril, {G}wena{\¨e}l},
  editor = {},
  language = {{ENG}},
  abstract = {{S}treams and rivers emit significant amounts of {CO}2 and constitute a preferential pathway of carbon transport from terrestrial ecosystems to the atmosphere. {H}owever, the estimation of {CO}2 degassing based on the water-air {CO}2 gradient, gas transfer velocity and stream surface area is subject to large uncertainties. {F}urthermore, the stable isotope signature of dissolved inorganic carbon (delta {C}-13-{DIC}) in streams is strongly impacted by gas exchange, which makes it a useful tracer of {CO}2 degassing under specific conditions. {F}or this study, we characterized the annual transfers of dissolved inorganic carbon ({DIC}) along the groundwater-stream-river continuum based on {DIC} concentrations, stable isotope composition and measurements of stream discharges. {W}e selected a homogeneous, forested and sandy lowland watershed as a study site, where the hydrology occurs almost exclusively through drainage of shallow groundwater (no surface runoff). {W}e observed the first general spatial pattern of decreases in p{CO}(2) and {DIC} and an increase in delta {C}-13-{DIC} from groundwater to stream orders 1 and 2, which was due to the experimentally verified faster degassing of groundwater {C}-12-{DIC} compared to {C}-13-{DIC}. {T}his downstream enrichment in {C}-13-{DIC} could be modelled by simply considering the isotopic equilibration of groundwater-derived {DIC} with the atmosphere during {CO}2 degassing. {A} second spatial pattern occurred between stream orders 2 and 4, consisting of an increase in the proportion of carbonate alkalinity to the {DIC} accompanied by the enrichment of {C}-13 in the stream {DIC}, which was due to the occurrence of carbonate rock weathering downstream. {W}e could separate the contribution of these two processes (gas exchange and carbonate weathering) in the stable isotope budget of the river network. {T}hereafter, we built a hydrological mass balance based on drainages and the relative contribution of groundwater in streams of increasing order. {A}fter combining with the dissolved {CO}2 concentrations, we quantified {CO}2 degassing for each stream order for the whole watershed. {A}pproximately 75% of the total {CO}2 degassing from the watershed occurred in first- and second-order streams. {F}urthermore, from stream order 2-4, our {CO}2 degassing fluxes compared well with those based on stream hydraulic geometry, water p{CO}(2), gas transfer velocity, and stream surface area. {I}n first-order streams, however, our approach showed {CO}2 fluxes that were twice as large, suggesting that a fraction of degassing occurred as hotspots in the vicinity of groundwater resurgence and was missed by conventional stream sampling.},
  keywords = {{G}roundwater-stream interface ; {H}eadwaters ; {C}arbon stable isotopes (delta {C}-13-{DIC}) ; {CO}2 degassing ; {C}arbonate weathering ; {FRANCE}},
  booktitle = {},
  journal = {{J}ournal of {H}ydrology},
  volume = {558},
  numero = {},
  pages = {129--143},
  ISSN = {0022-1694},
  year = {2018},
  DOI = {10.1016/j.jhydro1.2018.01.003},
  URL = {https://www.documentation.ird.fr/hor/fdi:010072673},
}