Geological Society of America

T91 Fate and Remediation of Organic and Metal Contaminants in Wetlands and Sediments

Monday, 2 November 2015
8:00 AM - 12:00 PM
Room 343

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

Paper number: T91-1
Program: Technical Sessions
Time: 8:00 AM – 8:15 AM

Authors

Amar R. Wadhawan Presenting Author
Geosyntec Consultants, Columbia, MD
Edward J. Bouwer
Johns Hopkins University, Baltimore, MD

Abstract

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.