Isotopic Signature of Naturally Cr(VI) Contaminated Spring Waters from Western Tuscany (Italy)

Weathering of serpentinites produces soils and sediments with high Cr concentrations. High Cr (VI) contents (up to 50 μg/l) have also been found in some spring waters spilling out from serpentinite bodies that outcrop in Western Tuscany. A section of a multidisciplinary research program (RESPIRA) is aimed to enhance the understanding of serpentinite rocks weathering processes in order to assess the mobility and bioavailability of Cr. Petrographic and minero-chemical analyses of both rocks and soil samples highlight the occurrence of minerals able to release Cr (III) containing significant Cr amounts, such as chlorites (Cr2O3 up to 8 wt%). The absence of Mn-oxides, permitting to rapid oxidise Cr(III), implies that local presence of Cr (VI) in waters have to be ascribed to other processes. Sr-Pb isotopes of serpentinites suggest an interac tion with recent, low-T, waters, whereas most soils display larger Sr-Pb isotopic ranges indicating a significant contribution of both elements from crustal sedimentary rocks. All spring waters display Mg-HCO3 chemical composition and a Sr-Pb isotopic signature fitting within the serpentinites and soils range. Cr isotopes are generally used to investigate Cr(VI) reduction occurred in contaminated ground waters, during biotic and abiotic processes, which produces a strong positive fractionation of residual unreduced Cr(VI) as well as a powerful tool for to reconstruct the redox state of ancient sea water. Nevertheless, little is know about fractionation effects accompanying Cr (III) oxidation although little positive fractionation has been experimentally demonstrated. The spring waters preliminary investigated for Cr isotopes are strongly positively fractionated ($53Cr values between +1 and +3 ‰) as bserved in other naturally Cr (VI) contaminated ground waters [1]. The observed strong positive fractionation can be the result of both Cr oxidation and partial back reduction of soluble Cr (VI). Further investigations on serpentinite spring waters can contribute to better understand Cr isotopes behaviour during natural Cr redox reactions.