In Situ Magnetic Susceptibility Variations at Two Contaminated Sites: Brandywine, MD and Bemidji, MN

Geophysical methods are widely used monitoring tools for investigating subsurface processes. Compared to more traditional methods, they are low cost and minimally invasive. Magnetic susceptibility (MS) is a geophysical technique particularly sensitive to the presence of ferri/ ferro-magnetic particles such as iron oxides (e.g., magnetite, hematite and goethite). These oxides can be formed through microbially mediated redox reactions, inducing changes in the soil properties that can be observed by MS measurements. Monitoring MS changes over time provides indications of iron mineral transformations in the ground. These transformations are of particular interest for the characterization of contaminated sites. We acquired borehole MS measurements from two contaminated sites: Brandywine, MD and Bemidji, MN. Active remediation was applied at Brandywine, whereas natural attenuation has been geophysically monitored at Bemidji since 2011 using MS log measurements. High MS values were observed at both sites within the contaminated area only. We hypothesize that this is due to iron reducing bacteria reducing Fe-(III) to Fe-(II) and utilizing contaminants and/or amendments injected as a carbon source. At Bemidji, elevated MS readings were observed in the smear zone and correlate to the presence of magnetite. Furthermore, time-lapse MS observations at Bemidji indicate a decay in signal amplitude over time suggesting further redox transformation into less magnetic particles. For both field examples presented here, we observe variations in magnetic susceptibility within the contaminated areas that can be linked with redox reactions and mineral transformations occurring during the degradation of organic contaminants.