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Poster communications

Spatial Localization of Stream Reaeration Inferred by Gaseous Tracer Injections and In-Situ Measurements

Abstract : Characterizing gaseous exchanges occurring at the stream-atmosphere interface is crucial to understand stream biogeochemistry and to infer the position of streams in the atmospheric greenhouse gases budget. However, stream reaeration coefficients are often estimated using one of the many existing empirical equations, which leads to major uncertainties. Here we set up a method to accurately measure reaeration coefficients in headwater streams and we propose an explanation for the diversity of empirical equations found in the literature. Our experimental work was based on four stream reaches (1st to 3rd order) located on a fractured bedrock aquifer in Brittany, France. Continuous- and point- injections of salt and helium were performed, and dissolved gases were continuously measured downstream using a membrane inlet mass spectrometer (MIMS). Measurements were then calibrated thanks to grab samples analyzed by gas chromatography. Point injections allowed us to develop a new analytical method to calculate reaeration coefficients based on the shapes of salt and helium breakthrough curves. This method was validated by its consistency with the classical continuous injection method. Since a point injection is much quicker to implement than a continuous injection, we hope this method will make it possible to broadly measure gas exchanges between streams and atmosphere. During continuous injections, dissolved gases and conductivity were measured at several distances from the injection site by gradually moving the pump upstream. This allowed us to map the helium decay along the stream. We identified reaches where reaeration was very strong (highly turbulent zones such as cascades), while the major part of the stream showed low reaeration rates. This strong variability of degassing along a single stream reach probably explains the difficulty of defining a general equation to predict degassing and shows that fine-scale streambed morphology is a major control of reaeration processes. We conclude that empirical equations can give an honest estimation of degassing that in areas with moderated turbulence, nevertheless, in highly turbulent zones, degassing need to be measured. The high localization degree of reaeration has major implications for studies relying on gaseous tracer measurements in streams and rivers.
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Poster communications
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https://hal-insu.archives-ouvertes.fr/insu-01927779
Contributor : Isabelle Dubigeon <>
Submitted on : Tuesday, November 20, 2018 - 10:15:59 AM
Last modification on : Friday, April 5, 2019 - 8:18:07 PM

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  • HAL Id : insu-01927779, version 1

Citation

Ronan Abherve, Camille Vautier, Eliot Chatton, Thierry Labasque, Christophe Petton, et al.. Spatial Localization of Stream Reaeration Inferred by Gaseous Tracer Injections and In-Situ Measurements. American Geophysical Union 2018, Fall Meeting, Dec 2018, Washington, United States. pp.H11S-1713, 2018. ⟨insu-01927779⟩

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