Magnetic evidence of iron mineral transformation in hydrocarbon-contaminated aquifer
Date
2021
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Publisher
University of Delaware
Abstract
Iron mineral transformation occurring in hydrocarbon-contaminated sites is linked to the biodegradation of hydrocarbons in these environments. Magnetic susceptibility (MS) has been proposed as a method for the continuous monitoring of the natural attenuation of hydrocarbons related to iron cycling. However, at the National Crude Oil Spill Fate and Natural Attenuation Research Site in Bemidji, MN, US, elevated MS signals were found to be transient, thereby making long-term MS monitoring uncertain. Furthermore, the iron mineral phases acting as transformation reactants and products associated with this long-term decrease in MS remain largely unknown. To address these ambiguities, we collected detailed mineral magnetism measurements, including hysteresis loops, backfield curves, and isothermal remanent magnetizations on core samples retrieved from different depths at the site and fresh magnetite packs installed within the contaminated and uncontaminated aquifer. Our results show that the magnetite packs display decreases in saturation magnetization (Ms) with time, where samples within the oil pool showed a 90% decrease over an eleven-month period compared to 20% for samples outside the oil pool. This loss in magnetization was accompanied by increases in bulk coercivity (Bc) indicative of possible decrease in grain sizes or partial oxidation. Low temperature magnetometry on all samples displayed behavior consistent with magnetite, as indicated by a prominent Verwey transition. However, samples within the plume also showed smearing of the Verwey transition and a higher remanence recovery during low temperature thermal cycling, which we interpret as evidence for maghemitization. This interpretation is supported by the occurrence of shrinkage cracks on the surface of the grains imaged during SEM analyses. The precipitous decrease in Ms within the water table fluctuation zone at the center of the plume is due to a combination of mineral dissolution and maghemitization. However, the center of the plume is strongly anoxic and would not support conventional abiotic oxidation. Here we propose that maghemitization is occurring within the anoxic portions of the plume via microbially mediated anaerobic oxidation. Microorganisms capable of such anaerobic oxidation have been identified within other areas at the Bemidji site, but additional microbiological studies are needed to link specific iron oxidizers with this loss of magnetization.
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Keywords
Anaerobic oxidation, Dissolution, Hydrocarbon-contaminated, Maghemite, Maghemitization, Magnetite