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Origine de l'eau et des éléments dissous par traçage isotopique (H, O, S, Mg, Sr) sur le bassin amont de la Moselle :

Abstract : The objective of this work was to identify and quantify, in time and space, the origin of water and of dissolved elements on the Moselle river basin, using three types of isotopic tracers. This basin is remarkable for its very contrasted lithology and land use at a small distance scale. A strong contrast appears between the Vosges mountains with silicate rocks and mainly forested soils upstream and the Lorraine plateau with carbonate/evaporite rocks and dominated by agriculture downstream. The first objective of this work was to assess some hydrological processes occurring on the basin. The isotopic composition of H and O of water permited to describe water dynamics and reponse delay of water to meteoric events depending on the water reservoir studied and the sampling period. Response delay estimated vary from few days for soil water collected by drains during high flow period, to several weeks for river water, to several months for soil water collected by porous cups and spring water. Secondly, we documented variations of the isotopic composition of S and O of dissolved sulfates in time and space. Three main sources have been identified: atmospheric inputs, weathering of evaporites and leaching of S-fertilizers. Sulfates inputs from S-fertilizers showed a clear seasonality and appeared during high river flow period (October 2003 to March 20004), when the leaching of soils is intensive and would represent up to 13% of the sulfates exported each year by the Madon river at Mirecourt. The isotopic composition of dissolved Sr in rivers on the Moselle basin correspond to a simple mixing between the silicate end-member and the carbonate end-member defined locally, suggesting a relative simplicity of the system studied. On this basis, we tested the potential of Mg as a new isotopic tracer in river systems. Mg in river waters draining silicates is systematically depleted in 26Mg ( 26Mg=-1.2 and -0.7 ) relative to corresponding rocks and soils ( 26Mg=0.2 and 1.7 ), suggesting that isotope fractionation occurred during weathering. 26Mg values of dissolved Mg in rivers flowing on sedimentary rocks (-1.6 to -1.1 ) are significantly lower than those for rivers flowing on silicate rocks. In addition, 26Mg values are clearly correlate with 87Sr/86Sr ratio and Sr/Mg ratio, showing that Mg isotopes are able to trace the origin of dissolved Mg for these rivers. Finally, an important excess of Mg and Sr relative to Ca appeared in rivers draining sedimentary rocks on the Lorraine plateau and we demonstrated that the removal of Ca by calcite precipitation is the most likely process to explain this excess
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Agnès Brenot. Origine de l'eau et des éléments dissous par traçage isotopique (H, O, S, Mg, Sr) sur le bassin amont de la Moselle :. Autre. Institut National Polytechnique de Lorraine, 2006. Français. ⟨NNT : 2006INPL003N⟩. ⟨tel-01752506⟩

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