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IN SITU APPARENT CONDUCTIVITY MEASUREMENTS AND MICROBIAL POPULATION DISTRIBUTION AT A HYDROCARBON CONTAMINATED SITE
Citation:
Atekwana, E., D Werkema, J. W. Duris, S. Rossbach, E. A. Atekwana, W. Sauck, D. P. Cassidy, J. C. Means, AND F. Legall. IN SITU APPARENT CONDUCTIVITY MEASUREMENTS AND MICROBIAL POPULATION DISTRIBUTION AT A HYDROCARBON CONTAMINATED SITE. GEOPHYSICS 69(1):56-63, (2004).
Impact/Purpose:
Research is being conducted to improve and evaluate the resolution of the CR, EM, seismic, and GPR methods over complex geological formations (such as fractured geologies) and to evaluate the capability of these geophysical methods to delineate subsurface organic contaminants.
Description:
We investigated the bulk electrical conductivity and microbial population distribution in sediments at a site contaminated with light non-aqueous phase liquid (LNAPL). The bulk conductivity was measured using in situ vertical resistivity probes, while the most probable number method was used to characterize the spatial distribution of aerobic heterotrophic and oil degrading microbial populations, The purpose of this study was to assess if high conductivity observed at "aged" LNAPL-impacted sites may be related to microbial degradation of LNAPL. The results show higher bulk conductivity coincident with LNAPL-impacted zones, in contrast to geoelectrical models that predict lower conductivity in such zones. The highest bulk conductivity was observed to be associated with zones impacted by residual and free LNAPL. Data from bacteria enumeration from sediments close to the resistivity probes show that oil- degrading microbes make up a larger percentage (5-55 %) of the heterotrophic microbial community at depths coincident with the higher conductivity compared to ~5 % at the uncontaminated location. The coincidence of a higher percentage of oil degrading microbial populations in zones of higher bulk conductivity suggests that the higher conductivity in these zones may be due to increased fluid conductivity related to microbial degradation of LNAPL, consistent with geochemical studies that suggest that intrinsic biodegradation is occurring at the site. The findings from this study point to the fact that biogeochemical processes accompanying biodegradation o f contaminants c an potentially alter geoelectrical properties o f t he subsurface impacted media.