Water-rock interactions and trace elements distribution in dolomite aquifers: The Sassolungo and Sella systems (Northern Italy)

摘要

Water-rock interaction processes and trace elements distribution were studied in the Triassic dolomite aquifers of Sassolungo and Sella group (Dolomite mountains, northern Italy). These systems consist of almost pure dolomite and constitute an extremely favourable situation to study water-rock interactions and trace elements mobility in aquifers hosted by dolomite rocks. The chemical and isotopic composition of 26 springs was analysed together with mineralogical and geochemical data of the host rocks from the Sciliar Dolomite Fm. (Sassolungo) and the Dolomia Principale Fm. (Sella Group). Rock samples were found to be formed by almost ordered and stoichiometric dolomite (dolomite wt.% > 93) and minor phases, such as calcite, Mg-calcite and, to a smaller extent, albite and gypsum. The sampled waters are characterised by a Ca-Mg bicarbonate composition and aqueous speciation calculations, coupled with the analyses of isotopic data, reveal that congruent dissolution of dolomite is the main process governing the composition of groundwater. For most of the samples, dolomite dissolution occurs in an open system with PCO_2 between 10~(-2) and 10~(-3) bar. The study of trace elements distribution shows that during dolomite dissolution alkaline metals, alkaline earth elements larger than Ca and some transition metals are relatively enriched in the solutions while the solutions result to be depleted in Fe, Al, Y and REE.