Water Geochemistry of Three Mountain Streams from Carbonate Watersheds in the Southern French Alps
Résumé
The chemical composition of water from three streams flowing through a carbonate watershed was investigated. Although the study area is not spatially very large (a few km(2)), local inhomogeneities within the lithology appreciably affect the chemical composition of the water and the geochemical gradient as a function of elevation. The main chemical process which leads to the observed water chemistry is the dissolution of calcium carbonate by atmospheric and metabolic CO2. In the stream La Sigouste, the water dissolves nearly pure calcite and this reaction proceeds until an equilibrium with respect to calcite is reached. In Le Lauzon stream, local inhomogeneities in the lithologic composition prevent the establishment of an unambiguous weathering budget while, for Le Rif de l'Arc a stoichiometric model shows that the weathering process is adequately represented by the dissolution of calcite associated with a minor incongruent dissolution of chlorite present in marls. For the two last streams supersaturation with respect to calcite is observed. For both streams, supersaturation with respect to atmospheric CO2 is nearly permanent. The observed altitudinal gradients of sulfate are interpreted through a set of biogeochemical redox reactions.