Iron biomineralization controls on geophysical signatures of hydrocarbon contaminated sediments

The interpretation of geophysical data from mature hydrocarbon contaminated sites has relied on a conductive plume model where the conductivity of the subsurface contaminant volume is the result of microbial mediated changes in pore fluid chemistry. This conductive anomalous region is characterized by high total dissolved solids and occurs within the water table fluctuation zone where microbial activity is the maximum. Here we update this conductive plume model by providing new insights from recent laboratory investigations and geophysical data from hydrocarbon contaminated sites suggesting the unrecognized role of the impact that microbial-mediated metallic mineral precipitates have on geophysical signatures. We show that microbial redox processes (e.g., iron and sulfate reduction) during the biodegradation process involve mineralogical transformations and the precipitation of new minerals (e.g., magnetite, and pyrite) that can impact the electrical and magnetic properties of contaminated sediments. We provide examples from laboratory experiments and field studies and suggest that knowledge of the dominant redox processes occurring at hydrocarbon contaminated sites and the mineral phases formed is critical for a more robust interpretation of geophysical data associated with microbial-mediated changes at hydrocarbon contaminated sites. We also show that integration of both magnetic and electrical techniques may help reduce ambiguity in data interpretation.