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Mobilization of DOM during flood events: impact of water table dynamics

Abstract : dissolved organic matter (DOM) over hydrological year. During these hot moments, the increased flow at the outlet is accompanied with an increase of DOM concentration and a modification of DOM composition, implying the mobilization of additional DOM sources regard to base-flow conditions. Molecular analysis have been performed on outlet samples of a French agricultural catchment (Kervidy-Naizin) which belongs to the Critical Zone Observatory AgrHyS (SOERE Réseau des Bassins Versants). Previous results shown a clear modification in the distribution of lignin compounds during flood events and evidence an heterogeneity in the intensity of molecular changes. This DOM, less biodegraded, was assumed to be produced by partition between particulate and dissolved phases when the soil/water ratio is low (soil particles isolated in water). However, in the outlet, modifications of DOM composition is still recorded even after the return to pre-event turbidity. To explain these results we hypothesize that molecular changes could come from DOM produced in soil solutions. So how soil erosion could induce production of less biodegraded DOM? Is the intensity of molecular composition changes impacted by intensity of the flood event? Those questions were investigated during flood events, by sampling with high frequency stream DOM at the outlet of the watershed and soil solutions in riparian soils equipped with zero-tension lysimeters. Samples were filtered, analyzed for DOC and freezedried for molecular analysis (thermally assisted hydrolysis and methylation–gas chromatography/mass spectrometry). The hydraulic gradient was monitored every 15 minutes, using piezometers implemented along the slope. The long-term monitoring program of ORE AgrHyS on this site allowed a good understanding of hydrological functioning of this catchment, which is essential to link hydrology and biogeochemistry. At the beginning of the event, hydraulic gradient increased rapidly and stayed high during several days. Modification of DOM composition in soil solutions were recorded during the hydraulic gradient rise with an increase in the proportion of less degraded molecules than during base-flow conditions. In river, modification of DOM quality is also recorded short time after its appearance in soil solutions. Intensity of the flood events seems also to impact intensity of the modification with intense flood events responsible for high variations in DOM composition. For the first time, composition of soil DOM has been investigated during flood events. Results evidence the concomitant evolution of hydraulic gradient, DOM concentration and composition in soils solutions and stream outlet. Results highlight the contribution of sub-surface erosion in the production of storm stream DOM, and the key role of hydrological conditions in these modifications
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https://hal-insu.archives-ouvertes.fr/insu-01388057
Contributor : Isabelle Dubigeon <>
Submitted on : Wednesday, October 26, 2016 - 2:11:52 PM
Last modification on : Monday, July 6, 2020 - 3:38:32 PM

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  • HAL Id : insu-01388057, version 1

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Marie Denis, Laurent Jeanneau, Gérard Gruau, Anaëlle Murzeau, Patrice Petitjean, et al.. Mobilization of DOM during flood events: impact of water table dynamics. 25 ème Réunion des sciences de la Terre (RST 2016), Société Géologique de France, Oct 2016, Caen, France. pp.99. ⟨insu-01388057⟩

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