Water-rock interaction in the Bisagno valley (Genoa, Italy): Application of an inverse approach to model spring water chemistry

The chemical evolution of the Bisagno valley spring waters, ranging from in itial Ca-HCO3 towards final Na-HCO3 composition is reconstructed with a rea ction progress scheme, defined by a fractional degree of advancement (zeta) of the irreversible mass-transfer process and attaining the continuum limi t during water-rock interaction. The system is solved in terms of a transpo sed reaction rate vector, introducing the experimental kinetic rate constan ts and solving for the surface areas of dissolving albite and K-feldspar an d precipitating carbonates. This is also done by considering the activities of dissolved silica and Al3+ ion as constrained by the instantaneous equil ibrium with quartz and kaolinite, respectively. The results of this inverse geochemical model are fully consistent with those obtained using the EQ3NR -EQ6 forward code, provided this last is run in iterative way, changing pro gressively the surface areas of solid reactants and f(CO2) values. Accounting for the compositional dependence of the kinetic rate constants o f plagioclase and alkali feldspar allows explanation of the progressive alb itization of the Mt. Antola Formation through a kinetic phenomenon involvin g nowadays circulating fluids. The comparison of computed surface areas with estimates based on the modal mineralogy, grainsize and intergranular porosity of the arenitic siliciclas tic beds outlines the importance of fracture-driven flow in agreement with available Lugeon flow tests. Copyright (C) 2000 Elsevier Science Ltd.