Comparison of δ53CrCr(VI) values of contaminated groundwater at two industrial sites in the eastern U.S. with contrasting availability of reducing agents

Natural attenuation of groundwater contaminated by toxic hexavalent chromium [Cr(VI)] was studied at two industrial sites in the eastern U.S. Concentration and isotope composition of the residual Cr(VI) were determined at site A in Connecticut whose aquifer was polluted by electroplating solutions between 1930 and 1995, and at site B in New Jersey whose aquifer was polluted by soluble chromium-processing wastes between 1911 and 1963. Site A was characterized by lower mean Cr(VI) concentrations (2 mg L−1) and lower mean δ53CrCr(VI) values (1.84‰) in groundwater than site B (200 mg L−1 and 2.89‰, respectively). δ53CrCr(VI) values were strongly positively correlated with dissolved organic carbon (DOC; p = 0.002), but not with dissolved ferrous iron [Fe(II)], or divalent manganese [Mn(II)], indicating that DOC may have played a major role in spontaneous reduction of the mobile carcinogenic Cr(VI) to immobile non-toxic Cr(III). At site B, the supply of DOC may be nearly unlimited, due to the presence of a buried organic meadow mat at a depth of six meters below the ground surface. At site A, contaminated groundwater has been extracted and treated since 1995. The mean δ53CrCr(VI) values of the residual aqueous contamination at site A significantly decreased over the past 15 years (p < 0.05), possibly suggesting re-oxidation and remobilization of previously precipitated pollutant Cr.