Redox dynamics during recovery of an oil-impacted estuarine wetland.
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abstract
Redox potentials and sediment porewater parameters were measured around the periphery of a small cove along the San Jacinto River in Texas throughout a crude oil and chemical dispersant remediation study to distinguish normal dynamics from those caused as a response to stress from oil deposition and chemical treatment and subsequent recovery. Before the application of oil and treatments, sediments displayed average redox potentials of 0-350 mV when not submerged. Within 2 days of the applications, redox potentials in these plots decreased and exhibited a range from -200 to 0 mV for a duration of 5 weeks. Applied treatments significantly reduced the sediments of the wetland. Reduced redox potentials were indicative of the corresponding sulfate reduction that was also found to be significant following the oil application. GC/MS and MPN analysis indicates this reduction is due to biological oxidation of the crude oil components by alkane- and PAH-degraders in these surficial sediments and validates the usefulness of the redox measurement as an indicator for carbon oxidation. Increases in aqueous phase total organic and inorganic carbon coincided with a decrease in pH shortly after the applications, suggesting incomplete mineralization and the generation of organic acids. While dissolved ferrous iron and sulfide have been found to be good indicators of reductive processes in petroleum-contaminated aquifer sediments; that was not the case in this wetland study. Despite the disappearance of sulfate following the oil application, dissolved ferrous iron and sulfide concentrations remained at pre-application levels suggesting the formation of mackinawite and/or pyrite. The transient exposure of surface sediments to oxygen complicates the consideration of potential solid phase pathways since aqueous iron may be removed by precipitation when oxidized or reduced, making porewater iron a poor indicator for terminal electron accepting processes in wetland sediments.
