Investigation of rock-to-water release and fate of major, minor, and trace elements in the metabasalt–serpentinite shallow aquifer of Mt. Reventino (CZ, Italy) by reaction path modelling

Abstract

Abstract The progressive dissolution of metabasalts and serpentinites hosting the shallow aquifer of Mt. Reventino was simulated by means of the EQ3/6 software package, version 8.0, adopting both the Ideal Solid Solution Approach (ISSA) and the Double Solid Reactant Method (DSRM), which provided comparable results. A detailed field and laboratory study was performed on rock samples and local groundwaters to constrain and validate reaction path modelling. The prevalence of Ca–HCO3 over Mg–HCO3 compositions suggests that groundwaters interact chiefly with metabasalts and secondarily with the less abundant and less permeable serpentinites. The most important and active Cr source is a tremolite-rich amphibole, whose role as supplier of dissolved Cr has rarely been recognized in previous studies carried out in areas where ophiolitic rocks crop out. Speciation calculations indicate that hexavalent dissolved Cr is mainly present as chromate ion, followed by the neutral complexes CaCrO 4 o (14–32 mol%) and MgCrO 4 o (2–12 mol%), which are more mobile and more bio-available than charged solutes. All dissolved trace elements are supplied to shallow groundwaters by gradual dissolution of local rocks and, therefore, contributions linked to anthropogenic sources can be ruled out. In particular: (i) Ni is chiefly contributed to the aqueous phase by the tremolite-rich amphibole; (ii) different amounts of Sr, Ba, and Pb are provided by calcite dissolution (with Sr ≫ Ba > Pb), whereas the solid solution of orthorhombic carbonates acts as sink for these trace elements (with Xstrontianite ≫ Xwitherite > Xcerussite); (iii) the principal source of Cu and Zn is again calcite, whereas the solid solution of trigonal carbonates represents their major sink.