Field-scale applications of magnetic susceptibility for monitoring iron transformations associated with hotspots of biogeochemical activity

Abstract:

The coupled oxidation and reduction reactions of iron, a key element for life, regulate important ecosystems and also are fundamental biogeochemical processes controlling the natural attenuation of organic contaminants. Such redox reactions often result in the precipitation and transformation of iron oxides at hotspots of biogeochemical activity in subsurface environments characterized by strong redox gradients. Improving understanding of the dynamics of iron cycling in such hotspots requires field-scale measurements that are sensitive to the precipitation, dissolution and/or transformation of iron oxides. We have utilized field-deployed magnetic susceptibility (MS) measurements in novel configurations to characterize and monitor hotspots associated with (1) localized discharges of Fe(II) rich anoxic groundwater into surface water bodies, and (2) the zone of water table fluctuation in a hydrocarbon-contaminated aquifer undergoing natural attenuation. In the first case, a commercially available field sensor that senses the MS of sediments approximately 0.3 m below the surface was deployed at the streambed to characterize solid phase Fe oxides associated with anoxic stream discharges along reaches of multiple streams where both natural (beaver activity) and anthropogenic (contaminant discharges) processes influence iron dynamics. In the second case, time-lapse MS measurements were acquired on iron mineral columns, constructed of alternating sequences of magnetite and ferrihydrite separated by sand, suspended within the zone of water table fluctuation of the hydrocarbon-contaminated aquifer and at multiple points along the long-axis of the contaminant plume. Although these packets introduced non-native iron into the aquifer, they are a powerful approach to examining the stability of iron minerals over time by repeated retrieval and sampling. Measurements were made with a MS core logger by periodically removing the mineral columns from the well. Supporting laboratory measurements on samples retrieved from both investigated hotspots indicate that field-scale MS sensing is a valuable proxy of iron transformations associated with (1) spatial variations in anoxic discharge along a stream reach, and (2) hydrocarbon degradation processes. Our findings highlight a need for new sensors that could support in situ, automated MS monitoring to capture iron transformations in biogeochemical hotspots at much finer temporal resolution than is possible with existing instrumentation.