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Acoustic and Electrical Property Changes Due to Microbial Growth and Biofilm Formation in Porous Media
Davis, C. A., L. J. Pyrak-Nolte, E. A. Atekwana, D. D. WERKEMA, AND M. E. Haugen. Acoustic and Electrical Property Changes Due to Microbial Growth and Biofilm Formation in Porous Media . Journal of Geophysical Research - Biogeosciences. American Geophysical Union, Washington, DC, 115:1-14, (2010).
Microorganisms have the ability to create micro-environments and niches in subsurface sediment environments by forming biofilms. Biofilms are created by the attachment, growth, and proliferation of microorganisms at mineral grain surfaces. These highly organized microbial systems consist of microbial cells, microbial byproducts, nutrients, substrates, and solid surfaces [Cunningham et al., 1991; Marshall, 1992]. An important aspect of microbial attachment and growth on mineral surfaces is the production of a “pseudo-glue” material consisting of exopolymeric substances (EPS), which helps to bind the microbes to surfaces. The EPS that connects microbes to mineral particles is a key factor in clogging of sediment pore spaces and fluid flow pathways [Baveye et al., 1998]. Studies have shown that biofilm development significantly reduces the porosity (by 50%-90%) and permeability (by 95%-99%) of porous media [e.g., Bouwer et al., 2000; Dunsmore et al., 2004]. Hence, microbial colonization of mineral surfaces and the proliferation of biofilms can have a profound effect on the physicochemical properties of subsurface environments, influencing fluid flow and transport properties [e.g., Cunningham et al., 1991; Baveye et al., 1998; Brovelli et al., 2009].
A laboratory study was conducted to investigate the effect of microbial growth and biofilm formation on compressional waves, and complex conductivity during stimulated microbial growth. Over the 29 day duration of the experiment, compressional wave amplitudes and arrival times for the control (non-biostimulated) sample were observed to be relatively uniform over the scanned 2D region. However, the biostimulated sample exhibited a high degree of spatial variability in both the amplitude and arrival times, with portions of the sample exhibiting increased attenuation (~ 80%) concurrent with an increase in the arrival times, while other portions exhibited decreased attenuation (~ 45%) and decreased arrival time. The acoustic amplitude and arrival times changed significantly in the biostimulated column between Days 5-7 of the experiment, consistent with a peak in the imaginary conductivity (σ”) values. The σ” response is interpreted as recording the different stages of biofilm development with peak σ” representing maximum biofilm thickness and decreasing σ” representing cell death or detachment. Environmental scanning electron microscope (ESEM) imaging confirmed microbial cell attachment to sand surfaces and showed apparent differences in the morphology of attached biomass between regions of increased and decreased attenuation. The heterogeneity in the elastic properties arises from the differences in the morphology and structure of attached biofilms. These results suggest that combining acoustic imaging and complex conductivity techniques can provide a powerful tool for assessing microbial growth or biofilm formation and the associated changes in porous media, such as those that occur during bioremediation and microbial enhanced oil recovery.