Irreversible water–rock mass transfer accompanying the generation of the neutral, Mg–HCO 3 and high-pH, Ca–OH spring waters of the Genova province, Italy

Abstract

In a recent survey of the spring waters of the Genova province, many neutral Mg–HCO 3 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, SO 4–Cl facies of low salinity to an intermediate Mg–HCO 3 facies (pH 7.0–8.5, P CO 2 10 −3.5–10 −2.5 bar, Eh 150–250 mV), and to a mature Ca–OH facies (pH 10–12, P CO 2 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 CO 2, 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 CO 2, precipitating calcite. Therefore, these waters might be used to sequester anthropogenic CO 2, locally preventing environmental impact to the atmosphere.