Major and trace elements (a) in groundwater, ultramafic rocks from natural outcrops and soil samples from cultivated sites of Central Euboea and Assopos basin, and (b) in experimentally produced laboratory water leachates of rocks and soils were investigated by SEM/EDS, XRD and ICP/MS. In addition, stable chromium isotopes (expressed as δ53Cr values) were measured in groundwater and leachates in order to identify potential sources for Cr-contamination.
The higher Cr(VI) concentrations in soil leachates compared to those in the rock pulp leachates potentially can be explained by the presence of larger amounts of Fe (Fe(II)) and Mn (Mn-oxides acting as oxidizing catalysts). Assuming that redox processes produce significant Cr isotope fractionation (groundwater δ53Cr values range between 0.8 and 1.98‰), the compilation of the obtained analytical data suggests that the dominant cause of Cr isotope fractionation is post-mobilization reduction of Cr(VI). However, the lack of a very good negative relationship between Cr(VI) concentrations and δ53Cr values may reflect that sorption, precipitation and biological processes (fractionation during uptake by plants) complicate the interpretation of the Cr isotope signatures.
The variation in δ53Cr values (0.84 to 1.98‰ in groundwater from Euboea, and from 0.98 to 1.03‰ in samples from the Assopos basin) imply initial oxidative mobilization of Cr(VI) from the ultramafic host rocks, followed by reductive processes that lead to immobilization of portions of Cr(III). Using a Rayleigh distillation model and different fractionation factors of Cr(VI) reduction valid for aqueous Fe(II) and Fe(II)-bearing minerals, we calculate that more than ~ 53%, but maximum ~ 94%, of the originally mobile Cr(VI) pool was reduced to immobile Cr(III) in the waters investigated. This indicates that efficient processes in the aquifers may facilitate natural attenuation of the toxic Cr(VI) to less harmful Cr(III).