To investigate the potential role that indigenous micro-organisms and microbial processes may play in altering low-frequency electrical properties, induced-polarization (IP) measurements in the frequency range of 0.1 to 1000 Hz were acquired from sediment samples retrieved from a site contaminated by hydrocarbon undergoing intrinsic biodegradation. Increased imaginary conductivity and phase were observed for samples from the smear zone (contaminated with residual-phase hydrocarbon), exceeding values obtained for samples contaminated with diss-olved-phase hydrocarbons, and in turn, exceeding values obtained for uncontaminated samples. Real conductivity, although generally elevated for samples from the smear zone, did not show a strong correlation with contamination. Controlled experiments on uncontaminated samples from the field site indicate that variations in surface area, elec-trolytic conductivity, and water content across the site can-not account for the high imaginary conductivity observed within the smear zone.

We suggest that microbial processes may be responsible for the enhanced IP response observed at contaminated lo-cations. Scanning electron microscopy and IP measurements during acid leaching indicate that etched pits on mineral surfaces — caused by the production of organic acids or formed during microbial colonization of these surfaces — are not the cause of the IP enhancement. Rather, we postulate that the accumulation of microbial cells (biofilms) with high surface area at the mineral-electrolyte interface generates the IP response. These findings illustrate the potential use of electrical measurements to noninvasively monitor microbial activity at sites undergoing natural hydrocarbon degradation.

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