Origines du carbone inorganique dissous dans les eaux de la Garonne. Variations saisonnières et interannuelles. / Sources of dissolved inorganic carbon in the Garonne river water. Seasonal and inter annual variations

Abstract

Sources of dissolved inorganic carbon in the Garonne river water. Seasonal and interannual variations The dissolved inorganic carbon transported by rivers as bicarbonate ions mainly originates from two sources : i) the dissolution of carbonate minerals which outcrops in the substratum of the drainage basin, and ii) the dissolved CO₂ in the soil leaching water which participates as H₂CO₃ to the weathering of most of the minerals. In order to distinguish these two sources in the bicarbonate flux transported by the Garonne river, we have applied a model of decomposition of dissolved major elements (MEGA model) to the monthly fluxes of elements transported in solution by the Garonne river from 1971 to 1991. The mean annual flux of soil C02 consumed by rock weathering amount to 411 10³ moles/km².yr. which corresponds to two times the annual average flux estimated for the whole continental area. The soil C02 contribution to the total bicarbonate flux varies from 47% in winter to 57% in summer. These variations are related to the contribution of two different hydro-logical reservoirs which supply the Garonne discharge : i) the non-saturated zone and the karst zone during period of high water level (winter), and ii) deeper groundwater during period of low water level (summer). The modelling results show that rock weathering in the Garonne basin is disturbed by the spreading of nitrogen fertilizers of which nitrification releases protons in soil solutions, leading to an average increase of 16% in the dissolution of carbonate minerals.