Collaborative Research: Investigating how transient electrical and magnetic signals relate to changes in recharge-driven redox state and iron mineral transformations

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Hydrocarbon source zones resulting from oil spills and/or crude oil pipeline ruptures result in persistent, long-term source of contamination of the aquifers that store potable groundwater in the Earth. This project serves the national interest by advancing the science needed to understand the long-term fate of hydrocarbon contaminants in the Earth. Geophysical tools that have been traditionally implemented to locate oil reservoirs and mineral deposits will be used to explore how geophysical signals provide diagnostic information on the progress of contaminant transformations that are largely driven by microbes in the Earth. This exploration of the linkages between biogeochemical processes and geophysical signals over time at an oil contaminated site may provide the knowledge needed to reliably deploy relatively simple geophysical measurement systems to monitor the long-term fate of oil spills. In the same way that medical imaging uses non-invasive sensing of the human body, non-invasive geophysical sensing of contaminant plumes might ultimately be used to understand the subsurface Earth without the need to drill into it. A non-invasive approach to monitoring the health of the human-impacted subsurface Earth would limit exposure of humans and animals to contaminants and negate unwanted transport of contaminants along pathways caused by invasive drilling methods. The research will be performed by undergraduate students performing field-based research in collaboration with government scientists from the United States Geological Survey (USGS). The project will engage minority undergraduate geoscience students from urban, economically disadvantaged neighborhoods in northern New Jersey. Results of the research will be shared with other scientists and students by running a workshop on geophysical signals associated with contaminant plumes. Transitional environments such as hyporheic and water table fluctuation zones (WTFZ) are biogeochemical hotspots where hydrologic processes driven by recharge events cause electron donor/acceptor mixtures that enhance microbial metabolism. Hydro-biogeochemical processes in transitional environments are challenging to study using hydrological, microbial and geochemical proxies due to the spatio-temporal and dynamic nature of these systems. Geochemical and microbial processes/transformations occurring within the WTFZ at organic-rich contaminated sites give rise to magnetic susceptibility (MS) and self potential (SP) electrical signals that show evidence of being regulated by recharge events and changes in water level. Understanding of the biogeochemical factors resulting in the measured geophysical responses, as needed to apply these techniques to investigate hydro-biogeochemical processes at field sites, remains incomplete. This project will pursue interdisciplinary research at a highly characterized site where decades of hydrological, geochemical and microbiological data are available to interpret the driving mechanisms causing geophysical signatures. It will integrate undergraduate education with basic research to advance understanding of the origins of such biogeophysical signatures and how they are regulated by variable hydrologic conditions. Supporting laboratory studies will be performed to constrain the linkages between iron cycling and biogeophysical signatures within the WTFZ. Datasets will be acquired to address the following hypotheses: [1] Transient magnetic susceptibility profiles result from hydrologically-driven iron cycling in the source zone; [2] Magnetic susceptibility changes in hydrocarbon source zones result from the consumption of iron-oxyhydroxides initially present on the sediments; [3] Transient self potential signatures are associated with recharge-driven modifications of dissolved or gas phase electron acceptors; [4] A microbial-mediated Fe(II)/Fe(III) redox couple drives a biogeobattery causing an anomalous self potential profile through the WTFZ in the source zone. Hypotheses will be explored by a combination of field geophysical measurements, in situ geochemical measurements on sediment packets suspended in boreholes and laboratory simulations of the WTFZ zone.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.