Isotopic characteristics of the Garonne River and its tributaries

The Garonne is the largest river in the south-west of France, and its drainage basin stretches between the Pyrenees and the Massif Central mountains. Until now, no water stable isotope study has been performed on the whole Garonne river basin which is composed of different geological substrata, and where the water resources are limited during the dry summer period. This study focuses on the Garonne river and its tributaries from the Pyre´ne´es foothill upstream to its confluence with the Lot River downstream. The aim of the study is to determine the origins of the surface waters using their chemical and stable isotopic compositions (18O, D and 13C), to better understand their circulation within the drainage basin and to assess the anthropogenic influences. The Garonne displays a specific 18O seasonal effect, and keeps its Pyre´nean characteristics until its confluence with the Tarn River. The difference in the dissolved inorganic carbon (DIC) comes mainly from the change in lithology between the Pyre´ne´es and the Massif Central mountains. Agriculture activity is only detected in the small tributaries

Stable carbon isotope evidence for nitrogenous fertilizer impact on carbonate weathering in a small agricultural watershed

The isotopic signature of Dissolved Inorganic Carbon (DIC), δ13CDIC, has been investigated in the surface waters of a small agricultural catchment on calcareous substratum, Montoussé, located at Auradé (south‐west France). The Montoussé catchment is subjected to intense farming (wheat/sunflower rotation) and a moderated application of nitrogenous fertilizers. During the nitrification of the NH4 +, supplied by fertilization, nitrate and H+ ions are produced in the soil. This anthropogenic acidity is combined with the natural acidity due to carbonic acid in weathering processes. From an isotopic point of view, with ’natural weathering’, using carbonic acid, δ13CDIC is intermediate between the δ13C of soil CO2 produced by organic matter oxidation and that of the carbonate rocks, while it has the same value as the carbonates when carbonic acid is substituted by another acid like nitric acid derived from nitrogen fertilizer. The δ13CDIC values range from −17.1‰ to −10.7‰ in Montoussé stream waters. We also measured the δ13C of calcareous molassic deposits (average −7.9‰) and of soil organic carbon (between −24.1‰ and −26‰) to identify the different sources of DIC and to estimate their contribution. The δ13CDIC value indicates that weathering largely follows the carbonic acid pathway at the springs (sources of the stream). At the outlet of the basin, H+ ions, produced during the nitrification of N‐fertilizer, also contribute to weathering, especially during flood events. This result is illustrated by the relationship between δ13CDIC and the molar ratio NO3 –/(Ca2+ + Mg2+). Consequently, when the contribution of nitrate increases, the δ13CDIC increases towards the calcareous end‐member. This new isotopic result provides evidence for the direct influence of nitrogen fertilizer inputs on weathering, CO2 consumption and base cation leaching and confirms previous results obtained using the chemistry of the major ions present in the field, and in soil column experiments