Characterization of Iron Oxides in the Streambed Using Magnetic Susceptibility

Rust staining of sediments is frequently observed in shallow streams and wetlands due to the precipitation of iron (III) oxides and/or hydroxides (Fe oxides). These Fe oxides are often the result of redox reactions occurring when anoxic groundwater containing Fe (II) ions discharges into shallow oxic sediments and surface water. Characterization of Fe oxide distributions could potentially help identify anoxic groundwater discharges associated with contamination. Additionally, it provides a better understanding of biogeochemical and sorption reactions occurring along river corridors. Conventional methods for studying Fe oxides in the streambed rely on core extraction and lab analysis, which are time consuming and typically result in low spatial resolution data. In contrast, a non-invasive, rapid and relatively high spatial resolution geophysical method, magnetic susceptibility (MS), could potentially be used to detect Fe oxides based on their paramagnetic properties. In this study, we evaluate the sensitivity of MS to quantify Fe oxides in both laboratory and field settings. At the laboratory scale, we established a quantitative relationship between MS and the concentration of typical Fe oxide species (e.g., hematite, goethite and ferrihydrite). At the field scale, we conducted in situ streambed MS measurements at discrete points with 1~5 m interval along two streams with known Fe-rich groundwater discharge. In general, increased streambed MS was observed in the areas with stronger anoxic groundwater discharges. The groundwater discharges were confirmed by measuring streambed temperature, dissolved oxygen and piezometer-based vertical head gradient.