Isotopic evidence of possible Cr(III) oxidation in an early contaminated site from NE Italy.

Trivalent chromium [Cr (III)] is an essential human and animal nutrient at the proper amount of ingestion; hexavalent chromium [Cr (VI)] is toxic and causes a wide range of health effects. Under the redox and pH conditions usually found in nature, Cr (VI) has the tendency to be reduced to Cr (III), e.g.in the presence of aqueous Fe (II), and is scavenged from solution by adsorption, in particular onto clay mineral surfaces. Only few oxidants are present in natural systems which can oxidize Cr (III) to Cr (VI). It has been demonstrated that Cr isotopes fractionate during the Cr (VI) to Cr (III) conversion under reducing conditions [1]. In the present study, preliminary ?53Cr data were obtained on groundwaters draining a phreatic aquifer in the High Friuli Plain (NE Italy), an area which underwent extensive Cr (VI) contamination from manufacturing during 1985, creating a plume of Cr (VI)- contaminated groundwaters. Later, the industrial activity was made environmentally protective and a natural reduction of contamination occurred over the years. Recently, new spikes in the Cr (VI) content, reaching 1600 ?g/L, have been observed in a number of wells, with concentration generally decreasing downgradient from the original site of contamination. The measured ?53Cr on the Cr (VI) contaminated waters ranges between -0.50 and -2.96. These data are tentatively interpreted, within a Rayleigh distillation model, as possible evidence of the oxidation of the trivalent chromium originated by reduction of the anthropogenic Cr (VI), released during the '80s, which accumulated onto alluvial sediments. [1] Berna et al. (2010) Environ. Sci. Technol. 44, 1043-1048