CHROMIUM FATE AND TRANSPORT IN ESTUARINE SEDIMENTS: ROLE OF GEOCHEMICAL PROCESSES IN SUPPORTING MNR

Assessing aquatic and human health risks of chromium (Cr) necessitates characterization of its speciation as either relatively non-toxic Cr^III or as toxic Cr^VI. 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 Cr^VI 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 Cr^III. 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 Cr^VI to product Cr^III is dominant under the quiescent anoxic conditions of the sediments, the potential of product Cr^III oxidation to cause Cr^VI 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 Cr^VI, which reduced to product Cr^III under anaerobic conditions. While conditions remained anaerobic no Cr^VI reoccurrence was observed. However, aeration caused Cr^VI reoccurrence from the abiotic oxidation of product Cr^III. Between 1 and 15 % of the product Cr^III 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 (Mn^II) resulted in the formation of Mn^III,IV(hydr)oxides that in turn caused product Cr^III oxidation to Cr^VI. Aeration-driven Cr^VI reoccurrence decreased with sediment loading and a negative correlation existed between Cr^VI formation and the acid volatile sulfide (AVS) concentration. The study concluded that even though sediment resuspension and oxygenation may create temporary conditions conducive to Cr^VI 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.