Water Geochemistry of Three Mountain Streams from Carbonate Watersheds in the Southern French Alps

Abstract

The chemical composition of water from three streamsflowing through a carbonate watershed wasinvestigated. Although the study area is not spatiallyvery large (a few km²), local inhomogeneitieswithin the lithology appreciably affect the chemicalcomposition of the water and the geochemical gradientas a function of elevation. The main chemical processwhich leads to the observed water chemistry is thedissolution of calcium carbonate by atmospheric andmetabolic CO₂. In the stream La Sigouste, thewater dissolves nearly pure calcite and this reactionproceeds until an equilibrium with respect to calciteis reached. In Le Lauzon stream, local inhomogeneitiesin the lithologic composition prevent theestablishment of an unambiguous weathering budgetwhile, for Le Rif de l'Arc a stoichiometric modelshows that the weathering process is adequatelyrepresented by the dissolution of calcite associatedwith a minor incongruent dissolution of chloritepresent in marls. For the two last streamssupersaturation with respect to calcite is observed.For both streams, supersaturation with respect toatmospheric CO₂ is nearly permanent. The observedaltitudinal gradients of sulfate are interpretedthrough a set of biogeochemical redox reactions.