[1] The effect of microbial processes on electrical properties of unconsolidated sediments was investigated in a laboratory experiment consisting of biotic and abiotic sand columns. The biotic column (nutrient, diesel and bacteria) showed (a) temporal increase in the real, imaginary, and surface conductivity, and (b) temporal decrease in the formation factor. The abiotic columns (nutrient; and nutrient and diesel) showed no significant changes. Increase in microbial population numbers, decrease in organic carbon source, nitrate, and sulfate and increase in dissolved inorganic carbon and fluid conductivity were indicative of microbial activity in the biotic column. We also measure relative increase in the interfacial electrical properties that exceed relative increase in the electrolytic conductivity. Thus changes in the real and imaginary conductivity were induced by microbial processes. These results suggest that interpretation of geoelectrical data from near surface environments should consider effects of microbial processes.

We have derived a set of new relationships describing polarization parameters in porous materials with disseminated particles made of a semiconductor, such as pyrite or magnetite. We have compared various predictions of this model to a broad set of experimental data. The chargeability was found to be controlled only by the volume fraction of metallic particles in agreement with the experimental data. The relaxation time, defined from the peak frequency of the phase, was observed to be proportional to the square of the size of the metallic particles and was independent of the salinity of the pore water solution. The relationship between the peak frequency and the grain size could be used to determine the diffusion coefficient of the [Formula: see text]- and [Formula: see text]-charge carriers in the semiconductor. This diffusion coefficient was consistent with the mobility of the charge carriers derived from theoretical considerations or electric-conductivity measurements. The resistivity of a mixture of a porous matrix characterized by a low-chargeability and dispersed semiconductors does not depend on the content of metallic grains, as long as the grains are below a percolation threshold (< 22 vol.%). Various experiments were performed using magnetite and pyrite at different grain sizes, weight fractions, and with/without porous materials (i.e., suspended in agar gel). These data were used to test some additional aspects of the model. We found excellent agreement between the model predictions and these experimental data.

To investigate the potential role that indigenous microorganisms and microbial processes may play in altering lowfrequency 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 dissolved-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, electrolytic conductivity, and water content across the site cannot 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 locations. 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.