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Predicting the aquatic and human health impacts of chromium (Cr) necessitates one to determine its speciation as either relatively nontoxic Cr(III) or toxic Cr(VI) and elucidate the influence of biogeochemical changes on its behavior and fate. In the Baltimore Harbor, Cr predominantly exists as Cr(III) associated with sediments. While reduction of Cr(VI) to Cr(III) is dominant in these anoxic sediments, the potential of Cr(III) oxidation and Cr(VI) reoccurrence during sediment resuspension and oxygenation resulting from dredging, bioturbation, and flood events poses a serious concern. In batch experiments, aqueous Cr(VI) spiked into continuously mixed anoxic suspensions was reduced to product Cr(III) under anaerobic conditions. No Cr(VI) reoccurrence was observed when conditions remained anaerobic. Aeration caused Cr(VI) reoccurrence from the abiotic oxidation of product Cr(III). Rates of aeration-driven Cr(VI) reoccurrence increased with pH, and Cr(VI) reoccurrence positively correlated with dissolved manganese (Mn) decline at pH ≥ 7. Aeration-driven oxidation of Mn(II) to Mn(III,IV)(hydr)oxides was the underlying mechanism causing product Cr(III) oxidation. Cr(VI) reoccurrence decreased with sediment loading and negatively correlated with the acid volatile sulfide (AVS) concentration. Although sediment resuspension and oxygenation may create temporary conditions conducive to Cr(VI) formation, long-term Cr(VI) persistence is unlikely in the presence of sediment reductants. While such natural attenuation in reducing environments mitigates the risk associated with Cr toxicity, this risk may still persist in Mn-rich and reductant-deficient environments.

Industrial activities in the Baltimore Harbor, Baltimore, Maryland, USA, have resulted in widespread chromium contamination of sediments. A comprehensive analysis of Cr speciation in sediment and porewater collected from 22 locations in the Baltimore Harbor was completed to understand Cr bioavailability and probability of toxicity due to Cr in sediments. The analysis employed a reverse-phase ion-pair high-performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) method. Sub-microgram-per-liter determination of Cr(III) and Cr(VI) in environmental samples was found, with method validation revealing broad method applicability of HPLC-ICP-MS to a wide range of sample types. The major limitation of the method was poor Cr species separation in high ionic strength solutions (greater than 0.1 M NaCl). Total Cr concentrations in Baltimore Harbor sediments ranged from 2.5 to 1,050 mg/kg with 11 of the 22 sites containing total Cr in excess of the 370 mg/ kg effects range-median (ER-M) sediment quality guideline. The Cr(VI) concentrations in sediments, however, were markedly lower, ranging from 0.10 to 0.38 mg/kg with Cr(VI) not detected in 14 of the 22 stations. Porewater concentrations, both for total Cr and Cr(VI), were quite low, with total Cr ranging from 0.20 to 2.16 microg/L and Cr(VI) ranging from 0.73 to 1.17 microg/L. The Cr(VI)-reducing capacity of the sediments, based on a sediment-spiking experiment, was found to be strongly correlated with the acid volatile sulfides content of the sediment. Overall, our results provide field validation of the hypothesis that Cr(VI) will not persist in sediments with excess acid volatile sulfides. Given the low concentrations of Cr(VI) in sediment and porewater, it appears unlikely that Cr in Baltimore Harbor sediments contributes appreciably to previously observed sediment toxicity.