International audienceThe induced polarization model developed recently by Revil and Florsch to understand the complex conductivity of fully saturated granular materials has been extended to partial saturation conditions. It is an improvement over previous models like the Vinegar and Waxman model, which do not account explicitly for the effect of frequency. The Vinegar and Waxman model can be considered as a limiting case of the Revil and Florsch model in the limit where the distribution of relaxation times is very broad. The extended model is applied to the case of unconsolidated sands partially saturated with oil and water. Laboratory experiments were performed to investigate the influence of oil saturation, frequency, grain size, and conductivity of the pore water upon the complex resistivity response of oil-bearing sands. The low-frequency polarization (below 100 Hz) is dominated by the polarization of the Stern layer (the inner part of the electrical double layer coating the surface of the grains in contact with water). The phase exhibits a well-defined relaxation peak with a peak frequency that is dependent on the mean grain diameter as predicted by the model. Both the resistivity and the magnitude of the phase increase with the relative saturation of the oil. The imaginary (quadrature) component of the complex conductivity is observed to decrease with the oil saturation. All these observations are reproduced by the new model

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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 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 LNAPLimpacted 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 result from 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 of contaminants can potentially alter geoelectrical properties of the subsurface impacted media.

The interdependence between geoelectrical signatures at underground petroleum plumes and the structures of subsurface microbial communities was investigated. For sediments contaminated with light non-aqueousphase liquids, anomalous high conductivity values have been observed. Vertical changes in the geoelectrical properties of the sediments were concomitant with significant changes in the microbial community structures as determined by the construction and evaluation of 16S rRNA gene libraries. DNA sequencing of clones from four 16S rRNA gene libraries from different depths of a contaminated field site and two libraries from an uncontaminated background site revealed spatial heterogeneity in the microbial community structures. Correspondence analysis showed that the presence of distinct microbial populations, including the various hydrocarbon-degrading, syntrophic, sulfate-reducing, and dissimilatory-iron-reducing populations, was a contributing factor to the elevated geoelectrical measurements. Thus, through their growth and metabolic activities, microbial populations that have adapted to the use of petroleum as a carbon source can strongly influence their geophysical surroundings. Since changes in the geophysical properties of contaminated sediments parallel changes in the microbial community compositions, it is suggested that geoelectrical measurements can be a cost-efficient tool to guide microbiological sampling for microbial ecology studies during the monitoring of natural or engineered bioremediation processes.

Groundwater flow advects heat, and thus, the deviation of subsurface temperatures from an expected conduction‐dominated regime can be analysed to estimate vertical water fluxes. A number of analytical approaches have been proposed for using heat as a groundwater tracer, and these have typically assumed a homogeneous medium. However, heterogeneous thermal properties are ubiquitous in subsurface environments, both at the scale of geologic strata and at finer scales in streambeds. Herein, we apply the analytical solution of Shan and Bodvarsson (), developed for estimating vertical water fluxes in layered systems, in 2 new environments distinct from previous vadose zone applications. The utility of the solution for studying groundwater‐surface water exchange is demonstrated using temperature data collected from an upwelling streambed with sediment layers, and a simple sensitivity analysis using these data indicates the solution is relatively robust. Also, a deeper temperature profile recorded in a borehole in South Australia is analysed to estimate deeper water fluxes. The analytical solution is able to match observed thermal gradients, including the change in slope at sediment interfaces. Results indicate that not accounting for layering can yield errors in the magnitude and even direction of the inferred Darcy fluxes. A simple automated spreadsheet tool (Flux‐LM) is presented to allow users to input temperature and layer data and solve the inverse problem to estimate groundwater flux rates from shallow (e.g., <1 m) or deep (e.g., up to 100 m) profiles. The solution is not transient, and thus, it should be cautiously applied where diel signals propagate or in deeper zones where multi‐decadal surface signals have disturbed subsurface thermal regimes.

[1] 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 (nonbiostimulated) sample were observed to be relatively uniform over the scanned 2-D 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 times. The acoustic amplitude and arrival times changed significantly in the biostimulated column between days 5 and 7 of the experiment, consistent with a peak in the imaginary conductivity (s″) values. The s″ response is interpreted as recording the different stages of biofilm development with peak s″ representing maximum biofilm thickness and decreasing s″ representing cell death or detachment. Environmental scanning electron microscope 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.

International audienceInduced polarization (more precisely the magnitude and phase of impedance of the subsurface) is measured using a network of electrodes located at the ground surface or in boreholes. This method yields important information related to the distribution of permeability and contaminants in the shallow subsurface. We propose a new time-lapse 3-D modelling and inversion algorithm to image the evolution of complex conductivity over time. We discretize the subsurface using hexahedron cells. Each cell is assigned a complex resistivity or conductivity value. Using the finite-element approach, we model the in-phase and out-of-phase (quadrature) electrical potentials on the 3-D grid, which are then transformed into apparent complex resistivity. Inhomogeneous Dirichlet boundary conditions are used at the boundary of the domain. The calculation of the Jacobian matrix is based on the principles of reciprocity. The goal of time-lapse inversion is to determine the change in the complex resistivity of each cell of the spatial grid as a function of time. Each model along the time axis is called a 'reference space model'. This approach can be simplified into an inverse problem looking for the optimum of several reference space models using the approximation that the material properties vary linearly in time between two subsequent reference models. Regularizations in both space domain and time domain reduce inversion artefacts and improve the stability of the inversion problem. In addition, the use of the time-lapse equations allows the simultaneous inversion of data obtained at different times in just one inversion step (4-D inversion). The advantages of this new inversion algorithm are demonstrated on synthetic time-lapse data resulting from the simulation of a salt tracer test in a heterogeneous random material described by an anisotropic semi-variogram

Disimilatory iron-reducing bacteria play an important role in the reduction of Fe(hydr)oxides and the production of secondary solid-iron mineral phases that can have magnetic properties. Magnetic susceptibility can therefore play an important role in identifying zones where microbial-mediated iron reduction is occurring. We investigated the magnetic susceptibility variations in a hydrocarbon-contaminated aquifer where methanogenesis and iron reduction are the main biogeochemical processes. Our objectives are to (1) determine the variability of magnetic susceptibility, (2) determine the hydrobiogeochemical controls on the magnetic susceptibility variability, and (3) evaluate the use of magnetic susceptibility as a viable technique for identifying zones where the coupling of iron and organic carbon cycling is occurring. Magnetic susceptibility data were acquired down 11 boreholes within contaminated and uncontaminated locations. We show that magnetic susceptibility values for boreholes within the free phase plume are higher than values for boreholes within the dissolved phase plume and background. Magnetic susceptibility values are highest within the zone of water table fluctuation with peaks predominantly occurring at the highest water table marks and are also coincident with high concentrations of dissolved Fe(II) and organic carbon content, suggesting that the zone of water table fluctuation is most biologically active. High magnetic susceptibility values within the vadose zone above the free phase plume are coincident with a zone of methane depletion suggesting aerobic or anaerobic oxidation of methane coupled to iron reduction. Our results suggest that magnetic susceptibility can be used as a viable tool in iron and carbon cycling studies.