Geophysical monitoring of microbial activity during stimulated subsurface bioremediation

Understanding how microorganisms alter their physical and chemical environment during bioremediation is hindered by our inability to resolve subsurface microbial activity with high spatial resolution. Here we demonstrate the use of a minimally invasive geophysical technique to monitor microbe-mediated iron and sulfate reduction during acetate amendment of a uranium-contaminated aquifer near Rifle, CO. During induced polarization (IP) measurements, spatiotemporal variations in the phase response between applied and measured voltages correlated with changes in groundwater geochemistry indicative of microbial iron and sulfate reduction and sulfide mineral precipitation. The enhanced sensitivity of the high and low frequency phase responses to accumulated aqueous iron and sulfide, respectively, provide the ability to discriminate the dominant subsurface biogeochemical process. The spectral effect was verified and calibrated using a biostimulated column experiment containing Rifle sediments and groundwater. Sediments and fluids recovered from regions of the field site exhibiting an anomalous phase response were enriched in sorbed Fe(II) and cell-associated 2-4 nm diameter FeS nanoparticles. These mineral precipitates and accumulated electroactive ions altered the ability of pore fluids to conduct electrical charge, accounting for the IP response. The results reveal the usefulness of multi-frequency IP measurements for discriminating mineralogical and geochemical changes during stimulated subsurface bioremediation.