Elemental and Isotopic composition of River water during a flood event in agricultural watershed: Insight of sources and pathways of water and terrestrial derived matter Roman TEISSERENC, Désirée EL AZZI, Issam MOUSSA, Luc LAMBS, Maritxu GUIRESSE, Jean-Luc PROBST Université de Toulouse (INPT, UPS) and CNRS ; Laboratoire Écologie Fonctionnelle et Environnement (ECOLAB); ENSAT, Avenue de l’Agrobiopole, 31326 Castanet Tolosan – France. Save catchment Sampling Larra River length : 145 km Catchment area: 1110 km² Population density: 39,3 h/km2 75% Agricultural land • Multi proxies analysis : • - d2H and d18O for water sources and fluxes • - 13CID : Inorganic carbon sources • DOC and POC: • C, N elemental analysis • 13C, 15N Organic matter sources During storm period, surface and subsurface runoffs contribute to river flow. Integration ofwater pathway complexity and spatial heterogeneity of contaminant inputs in the watershed need to be assessed by biogeochemical proxy measured at watershed outlet in the main river channel. High resolution sampling has been conducted during the flood event Agricultural watershed dominated by Corn, Wheat and Sunflower. Important use of pesticides and fertilizers. Hydrological regime is dominated by recurrent flood event (64% od water discharge) that represent 71% and 94% dissolved and suspended matter respectively d18O and d-excess variations during flood event d2H and 18O relationship Decrease in δ18O values betray a change in water reservoir supply to total river flow with an isotopic signature of cooler rain water coming from upstream. The increase of discharge can be related to a quick transfer of water from runoff. This watershed has a quick response to runoff, implying important transfer of terrestrial matter from surface soil horizons. While the rising limb of the hydrograph is clearly supported by rain water, we observe a more complex water isotopic signature during the recession limb. Revealing a increasing influence of subsurface and ground water on river discharge. LMWL : d2H=7.7*d18H+9.9 GMWL Interpretation of d-excess is more tricky: before the flood d-excess is close to the GMWL (10‰,no evaporation process). It increases during the flood, then remain close to 20‰after the flood. As the meteorological data indicate only North-West wind, i.e. no Mediterranean rainfall entry, the only explanation is high water recycling due to mist/fog formation during these cold days. d13CID variation during flood event Contaminant variation during flood event During the storm event, we observe increased concentration of pesticides and metallic contaminants such as metolachlor and copper. Whereas metolachlor is clearly leached from the surface runoff in the watershed, we observe that copper concentration increases with subsurface runoff. During the storm event, δ13DIC increase can be associated with an increasing leaching of soil carbonate accumulated in upper part of soils. Indeed d13C carbonate signature in the watershed is 7.9 ‰ AWP δ13DIC plotted against NO3-/(Na2++Mg2+) molar ratio is used as a proxy between natural weathering (NWP) and anthropogenic weathering pathway (AWP) (trough N-fertilizers). We observe that during the flood event, change of soil horizon leaching will change weathering pathways. NWP Conclusions These results improve our comprehension of water pathways during a flood event in agricultural watershed. Furthermore, it provides information about key processes that occurs in the watershed such as the change of weathering pathways or contaminants fate. Comprehension of watershed – aquatic system dynamics is crucial in order to well understand the pollutants fate in agricultural watershed during flood event.