2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 91-1
Presentation Time: 8:00 AM


WADHAWAN, Amar R., Geosyntec Consultants, 10220 Old Columbia Road, Suite A, Columbia, MD 21046 and BOUWER, Edward J., Geography and Environmental Engineering, Johns Hopkins University, 3400 N. Charles Street, Ames Hall, Room 313, Baltimore, MD 21218, awadhawan@geosyntec.com

Assessing aquatic and human health risks of chromium (Cr) necessitates characterization of its speciation as either relatively non-toxic CrIII or as toxic CrVI. Additionally, management of Cr-contaminated environments requires elucidating the long-term influence of biogeochemical changes on Cr redox behavior and fate. Baltimore Harbor is an estuarine system where Cr has accumulated in sediments as a legacy contaminant and where the potential of CrVI persistence is a serious concern. Characterization of sediments collected from various sites in the harbor revealed that reducing conditions are dominant and Cr is mainly present as CrIII. Total Cr concentration exceeded the effects range low (ERL) criteria (81 mg/kg) in all samples and the effects range medium (ERM) criteria (370 mg/kg) in 4 of the 11 samples. While reduction of CrVI to product CrIII is dominant under the quiescent anoxic conditions of the sediments, the potential of product CrIII oxidation to cause CrVI reoccurrence during sediment resuspension resulting from dredging, bioturbation, and flood events poses a significant threat.

Laboratory-scale batch experiments were performed with diluted suspensions of grab sediment samples wherein the anoxic suspensions were spiked with aqueous CrVI, which reduced to product CrIII under anaerobic conditions. While conditions remained anaerobic no CrVI reoccurrence was observed. However, aeration caused CrVI reoccurrence from the abiotic oxidation of product CrIII. Between 1 and 15 % of the product CrIII was oxidized for the various sampling locations. During aeration, the oxidation of reduced iron and sulfur species lowered the sediment reductant capacity while the oxidation of dissolved manganese (MnII) resulted in the formation of MnIII,IV(hydr)oxides that in turn caused product CrIII oxidation to CrVI. Aeration-driven CrVI reoccurrence decreased with sediment loading and a negative correlation existed between CrVI formation and the acid volatile sulfide (AVS) concentration. The study concluded that even though sediment resuspension and oxygenation may create temporary conditions conducive to CrVI formation in surficial sediments, the abundance of reductant capacity is expected to support monitored natural recovery (MNR) as a remedy for managing Cr contamination in reducing environments.