Irreversible water-rock mass transfer accompanying the generation of the neutral, Mg-HCO3 and high-pH, Ca-OH spring waters of the Genova province, Italy

In a recent survey of the spring waters of the Genova province, many neutral Mg-HCO3 waters and some high-pH, Ca-OH waters were found in association with serpentinites. All the springs are of meteoric origin as indicated by the stable isotopes of water and dissolved N-2 and Ar. Interaction of these meteoric waters with serpentinites determines a progressive evolution in the chemistry of the aqueous phase from an immature Mg-rich, SO4-Cl facies of low salinity to an intermediate Mg-HCO3 facies (pH 7.0-8.5, P-CO2 10(-3.5)-10(-2.5) bar, Eh 150-250 mV), and to a mature Ca-OH facies (pH 10-12, P-CO2 10(-9.4)-10(-10.6) bar, Eh-390 to-516 mV). The irreversible water-rock mass transfer leading to these chemical changes in the aqueous phase was simulated through reaction path modeling, assuming bulk dissolution of a local serpentinite, and the precipitation of gibbsite, goethite, calcite, hydromagnesite, kaolinite, a montmorillonite solid mixture, a saponite solid mixture, sepiolite, and serpentine. The simulation was carried out in two steps, under open-system and closed-system conditions with respect to CO2, respectively. The calculated concentrations agree with analytical data, indicating that the computed water-rock mass transfer is a realistic simulation of the natural process. Moreover, the simulation elucidates the role of calcite precipitation during closed-system serpentinite dissolution in depleting the aqueous solution of C species, allowing the concurrent increment in Ca and the acquisition of a Ca-OH composition. Calcium-OH waters, due to their high pH, tend to absorb CO2, precipitating calcite. Therefore, these waters might be used to sequester anthropogenic CO2, locally preventing environmental impact to the atmosphere. (C) 2002 Elsevier Science Ltd. All rights reserved.