Caroline A. Davis

Microbial-Induced Heterogeneity in the Acoustic Properties of Porous Media

Received 10 June 2009; revised 25 August 2009; accepted 13 October 2009; published 13 November 2009. [1] It is not known how biofilms affect seismic wave propagation in porous media. Such knowledge is critical for assessing the utility of seismic techniques for imaging biofilm development and their effects in field settings. Acoustic wave data were acquired over a two-dimensional region of a microbial-stimulated sand column and an unstimulated sand column. The acoustic signals from the unstimulated column were relatively uniform over the 2D scan region. The data from the microbial-stimulated column exhibited a high degree of spatial heterogeneity in the acoustic wave amplitude, with some regions exhibiting significant increases in attenuation while others exhibited decreases. Environmental scanning electron microscopy showed differences in the structure of the biofilm between regions of increased and decreased acoustic wave amplitude. We conclude from these observations that variations in microbial growth and biofilm structure cause heterogeneity in the elastic properties of porous media with implications for the validation of bioclogging models. Citation: Davis, C. A., L. J. Pyrak-Nolte, E. A. Atekwana, D. D. Werkema Jr., and M. E. Haugen (2009), Microbial-induced heterogeneity in the acoustic properties of porous media, Geophys. Res. Lett., 36, L21405, doi:10.1029/2009GL039569.

Geophysical and geochemical attenuated signatures associated with hydrocarbon contaminated site undergoing bioremediation

Previous geophysical investigations (1996, 2003, and 2004) conducted at the decommissioned Wurtsmith Air Force Base former Fire Training Cell (FT-02) showed a clearly defined high conductivity anomaly associated with hydrocarbon contaminants in the vadose zone and ground water near the source area. The source of the geophysical anomaly was attributed to biogeochemical modifications of the contaminated zone by intrinsic bioremediation. During previous surveys, ground penetrating radar (GPR) data showed a zone of attenuated GPR reflections extending from the vadose zone to below the water table. Self potential (SP) data defined a positive anomaly coincident with the hydrochemically defined plume, while electrical resistivity data showed anomalously high conductivity within the zone of impact. In 2007, another integrated geophysical study was conducted at of the site. GPR, SP, electrical resistivity, and induced polarization survey were conducted with expectations of obtaining similar results as the past surveys. However, preliminary assessment of the data shows a marked decrease in groundwater electrical conductivity and SP response over the plume. GPR data showed the attenuated signals, but the zone of attenuation was only observed below the water table. We attributed the attenuation of the observed geophysical anomalies to ongoing soil vapor extraction initiated in 2003. Significant removal of the contaminant mass by the vapor extraction system has altered the subsurface biogeochemical conditions and these changes were reflected in the 2007 geophysical data. The results show that the biological and physical attenuation of the contaminant plume is detectable with geophysical methods.

Temporal geophysical investigations of the FT-2- plume at the Wurtsmith Air Force Base, Oscoda, Michigan

The decommissioned Wurtsmith Air Force Base former Fire Training Cell (FT-02) facility has been the focus of several geophysical investigations. After several decades of fire training exercises, significant amounts of hydrocarbons and some solvents seeped into the subsurface contaminating the vadose and saturated zones in the source area. Groundwater geochemistry studies defined a contaminant plume that was approximately 125 m wide and > 300 m long. The boundary of the plume was further defined by using GPR, SP, and resistivity techniques. The source of the geophysical anomalies was attributed to biogeochemical modifications of the contaminated zone resulting from intrinsic bioremediation. In 2007, another integrated geophysical study of the site was conducted. GPR, SP, and electrical resistivity surveys were conducted with expectations of achieving similar results as the past investigations. However, there was a marked decrease in geophysical response from all of our geophysical techniques. The GPR anomaly has migrated deeper into the subsurface, the positive SP response was significantly attenuated, and the conductive resistivity anomaly has been replaced by background resistivity values. Also, six Geoprobe cores at three different locations were collected in order to conduct laboratory microbial counts and IP measurements. We attribute the attenuation of the observed geophysical anomalies to ongoing soil vapor extraction initiated in 2001. Significant removal of the contaminant mass by the vapor extraction system altered the subsurface biogeochemical conditions and these changes were documented by the 2007 geophysical data. The results of this study show that the attenuation of the contaminant plume is detectable with geophysical methods.

Effect of Microbial Metabolic Byproducts on Electrical Properties of Unconsolidated Sediments

Recent laboratory and field studies have documented enhanced electrolytic and interfacial electrical properties of unconsolidated sediments due to microbial activity. However the source mechanism of enhanced conduction and polarization is not very well understood. In this regard, laboratory measurements were conducted to investigate the effect of metabolic byproducts of microbial activity (i.e. organic acids and biosurfactants) on low frequency electrical properties of unconsolidated sediments. Uniform sand samples were saturated with different concentrations of individual organic acids (e.g., acetic, butyric, lactic, propionic) and electrical measurements were obtained in the frequency range 0.1-1000 Hz. The same procedure was repeated using different concentrations of natural ionic biosurfactants (aqueous solution of Rhamnolipids). It was observed that the electrolytic and interfacial electrical properties of unconsolidated sands increased with increasing concentrations of organic acids as well as the biosurfactants. A significant increase in the interfacial electrical properties of biosurfactants was observed at concentrations greater than the critical micelle concentration (CMC), which indicates that biosurfactants are highly polarized above the CMC. In addition, the observed magnitude of change in real conductivity was greater (two orders) than the imaginary conductivity component (one order). This indicates that the increase in concentrations of organic acids and biosurfactants increase the electrolytic conductivity to a greater degree than the interfacial conductivity. We conclude that metabolic byproducts of microbial activity can directly impact the electrical properties of unconsolidated sediments with implication of using electrical methods for quantitative assessments of microbial activity which are often required for evaluating the progress of bioremediation processes and microbial enhanced oil recovery.

Potential Application of Biogeophysics to EOR and Remediation Investigations

Summary In this work we discuss the potential application of biogeophysical studies to enhanced oil recovery (EOR) and remediation. We present the results of our recent biogeophysical laboratory studies that investigate the geophysical response of microbial-active media. We show that complex conductivity measurements are sensitive to metabolic byproducts (e.g. biosurfactants) and increases in microbial cell concentrations resulting from microbial growth. We suggest that geophysical measurements may be used to effectively characterize/monitor in situ biological activity during EOR and remediation.