In-situ apparent conductivity measurements and microbial population distribution at a hydrocarbon-contaminated site

We investigated the bulk electrical conductivity and microbial population distribution in sediments at a site contaminated with light nonaqueous-phase liquid (LNAPL). The bulk conductivity was measured using in-situ vertical resistivity probes; the most probable num­ ber method was used to characterize the spatial distri­ bution of aerobic heterotrophic and oil-degrading mi­ crobial populations. The purpose of this study was to assess if high conductivity observed at aged LNAPLimpacted sites may be related to microbial degradation of LNAPL. The results show higher bulk conductiv­ ity coincident with LNAPL-impacted zones, in contrast to geoelectrical models that predict lower conductivity in such zones. The highest bulk conductivity was ob­ served to be associated with zones impacted by resid­ ual 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 popu­ lations in zones of higher bulk conductivity suggests that the higher conductivity in these zones may result from increased fluid conductivity related to microbial degra­ dation 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 biodegra­ dation of contaminants can potentially alter geoelectri­ cal properties of the subsurface impacted media.