Using geophysical methods to understand, monitor, and assess a hydrocarbon-contaminated area
Creators: name: Yokus, Selcen Zehra
Publisher: name: No Publisher Supplied
Year: 2022

description: Hydrocarbon contamination is one of the most significant pollution problems globally. At hydrocarbon-contaminated sites, iron-reducing bacteria may use hydrocarbon as a carbon source and reduce Fe+3 to Fe+2 to form, for example, magnetite. At an oil spill site near Bemidji (MN, USA), hydro ferric oxide (HFO) transformation to magnetite and, subsequently, magnetite to siderite has been recorded. Being able to detect these transformations with geophysics could help with the monitoring of the progress of biodegradation. Therefore, this study uses two geophysical methods: spectral induced polarization (SIP) and magnetic susceptibility (MS). SIP is known to be sensitive to specific iron minerals and, therefore, possibly to the transformation of iron minerals. In this study, the sensitivity of SIP to different percentages by volume (between 0.09 % - 5 %) of magnetite, siderite, and HFO was investigated for artificial sand-iron mineral mixtures saturated with groundwater from this hydrocarbon-contaminated site. Additionally, experiments were conducted to understand how the SIP response depends on the radius of magnetite particles (0.7 mm, 0.4 mm, 0.1 mm, 0.05 mm, 0.02 mm). Our results suggest that the SIP method might be used to differentiate the iron minerals involved in hydrocarbon degradation because the technique is sensitive to iron mineralogy. The SIP response of variations in magnetite concentration and grain size is strong, but it is small (negligible) for the siderite and HFO. The minimal concentrations of magnetite, siderite and HFO likely to de detectable in the laboratory based on the results were respectively 0.09%, 3% and 3% by volume. SIP might be used to monitor the long-term progress of biodegradation by capturing iron mineral transformations in the future. Magnetic susceptibility was examined in both laboratory and field conditions. To understand the sensitivity of MS to different percentages by volume (between 0.0007 % - 100 %) of magnetite, siderite, and HFO, artificial sand-iron mineral mixtures were prepared in the laboratory, and several measurements were recorded. In addition, experiments were conducted to understand if the MS response depended on the radius of magnetite particles (0.7 mm, 0.4 mm, 0.1 mm, 0.05 mm, 0.02 mm). Using these experimental results, synthetic mineral packets were prepared in the laboratory, installed in a hydrocarbon-contaminated area, and monitored monthly using MS. Additionally, MS was applied to cores retrieved from the hydrocarbon-contaminated area in previous years. As the percentage concentrations increased, an increase was also observed in the MS for all iron minerals in the laboratory study. The minimal concentrations of magnetite, siderite and HFO likely to de detectable in the laboratory were respectively 0.09%, 5% and 12 % by volume. The MS response of variations in grain size is almost the same for all magnetite grain sizes; therefore, no relationship could be established between grain size and MS response. For synthetic mineral packets, while the MS fluctuated every month, possibly due to magnetite formation and oxidation, in the contaminated zone, a consistent increase in MS was observed in the uncontaminated area in the groundwater fluctuation zone. A decrease in MS was recorded in the mineral packet above the groundwater fluctuation zone in the uncontaminated area. The results obtained from cores retrieved show that the MS value has decreased over the years. These experiments suggest a variation in the MS in the contaminated area that depends on the water level. As the groundwater level increases, the MS value increases, and in its absence, the up and down fluctuating motion of the MS is observed. The results suggest that the MS method might be used to differentiate iron minerals involved in hydrocarbon degradation because the technique is sensitive to iron mineralogy.