High-resolution magnetic susceptibility measurements for investigating magnetic mineral formation during microbial mediated iron reduction

Abstract

Disimilatory iron-reducing bacteria play an important role in the reduction of Fe(hydr)oxides and the production of secondary solid-iron mineral phases that can have magnetic properties. Magnetic susceptibility can therefore play an important role in identifying zones where microbial-mediated iron reduction is occurring. We investigated the magnetic susceptibility variations in a hydrocarbon-contaminated aquifer where methanogenesis and iron reduction are the main biogeochemical processes. Our objectives are to (1) determine the variability of magnetic susceptibility, (2) determine the hydrobiogeochemical controls on the magnetic susceptibility variability, and (3) evaluate the use of magnetic susceptibility as a viable technique for identifying zones where the coupling of iron and organic carbon cycling is occurring. Magnetic susceptibility data were acquired down 11 boreholes within contaminated and uncontaminated locations. We show that magnetic susceptibility values for boreholes within the free phase plume are higher than values for boreholes within the dissolved phase plume and background. Magnetic susceptibility values are highest within the zone of water table fluctuation with peaks predominantly occurring at the highest water table marks and are also coincident with high concentrations of dissolved Fe(II) and organic carbon content, suggesting that the zone of water table fluctuation is most biologically active. High magnetic susceptibility values within the vadose zone above the free phase plume are coincident with a zone of methane depletion suggesting aerobic or anaerobic oxidation of methane coupled to iron reduction. Our results suggest that magnetic susceptibility can be used as a viable tool in iron and carbon cycling studies.

Multiscale geophysical imaging of the critical zone

“…Atekwana et al . [] show the microbial degradation of hydrocarbon by iron reducing bacteria results in a unique magnetic susceptibility signature. An investigation of the controls of magnetic susceptibility on soils showed that parent material and drainage are the primary controls while biological activity is observed to be a second‐order effect [ Blundell et al ., ].…”

“…Nonetheless, we suggest that under the right conditions, a similar approach to Atekwana et al . [] could be used to monitor long‐term natural biological activity in the CZ driven by natural forcing.…”

Geophysical signatures of disseminated iron minerals: A proxy for understanding subsurface biophysicochemical processes

“…This is supported by the study of Atekwana et al . [] where scanning electron microscopy images (see Figures a and c) show magnetite coating sand grains (millimeter scale) within the zone of enhanced complex conductivity response observed by Mewafy et al . [].…”

“…The IP method has been applied to the study of metal‐sand mixtures [ Gurin et al ., ; Mansoor and Slater , ; Slater et al ., , ; Wu et al ., ], monitoring of biotic [ Flores Orozco et al ., ; Ntarlagiannis et al ., ; Williams et al ., , ; Slater , ; Personna et al ., ] and abiotic sulfide precipitation [ Ntarlagiannis et al ., ]. More recently, the magnetic susceptibility method has been used to map magnetic iron biominerals at hydrocarbon contaminated sites where iron reduction is coupled with the oxidation of organic carbon [ Rijal et al ., , ; Mewafy et al ., , ; Atekwana et al ., ]. In addition, laboratory studies have documented changes in magnetization associated with microbial precipitation of magnetite [ Porsch et al ., ; Klueglein et al ., ].…”

Reactive transport modeling of geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN

“…To represent the possible sinks of reduced forms through precipitation, we include the mineral phases of siderite (FeCO 3 ) and rhodochrosite (MnCO 3 ). We simplify the model implementation by omitting magnetite, which is limited to locations within the oil body [Atekwana et al, 2014]. Siderite has been detected in the reduced zones of the Bemidji plume [Baedecker et al, 1992;Tuccillo et al, 1999;Zachara et al, 2004], but saturation indices of siderite and rhodochrosite, using the PHREEQC-2 database dissolution log K values of 210.89 and 211.13 for siderite and rhodochrosite, respectively, are often above zero within the plume (Figure 3).…”