Field Evidence for Geophysical Detection of Subsurface Zones of Enhanced Microbial Activity

Geochemical data from closely spaced vertical intervals in a hydrocarbon-impacted aquifer were used to assess the relationship between bulk conductivity and zones of enhanced microbial activity. The bulk conductivity was measured using in situ vertical resistivity probes. Microbial activity was verified using terminal electron acceptors (nitrate, sulfate, iron, and manganese), dissolved inorganic carbon (DIC), and major ion chemistry. Peaks in bulk conductivity in the aquifer overlapped with zones where nitrates and sulfates were depleted, total petroleum hydrocarbon, iron, manganese, dissolved ions, and DIC were elevated, suggesting a link between higher electrical conductivity and zones of enhanced microbial activity stimulated by the presence of hydrocarbon. Thus the subsurface expression of microbial activity is apparently recorded in the bulk conductivity measurements. Our results argue for combining geophysics with biogeochemistry studies to delineate subsurface zones of enhanced microbial activity

Effects of Microbial Processes on Electrolytic and Interfacial Electrical Properties of Unconsolidated Sediments

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

Hydrological Monitoring with Hybrid Sensor Networks

Existing hydrological monitoring systems suffer from short- comings in accuracy, resolution, and scalability. Their fragility, high power consumption, and lack of autonomy necessitate frequent site visits. Cabling requirements and large size limit their scalability and make them prohibitively expensive. The research described in this paper proposes to alleviate these problems by pairing high-resolution in situ measure- ment with remote data collection and software maintenance. A hybrid sensor network composed of wired and wireless connections autonomously measures various attributes of the soil, including moisture, temperature, and resistivity. The mea- surements are communicated to a processing server over the existing GSM cellular infrastructure. This system enables the collection of data at a scale and resolution that is orders of magnitude greater than any existing method, while dramatically reducing the cost of monitoring. The quality and sheer volume of data collected as a result will enable previously infeasible research in hydrology

Temporal Geophysical Signatures from Contaminant-Mass Remediation

We have previously documented changes in bulk electrical conductivity, self-potential (SP), and ground-penetrating-radar (GPR) reflections in a field setting caused by biogeochemical transformations of hydrocarbon-contaminated media. These transformations are associated with hydrocarbon biodegradation. The results of surface geophysical surveys acquired in 1996, 2003, and 2007 document changes in geophysical signatures associated with removing hydrocarbon mass in the contaminated zone. Initial investigations in 1996 showed that relative to background, the contaminated area was characterized by higher bulk electrical conductivity, positive SP anomaly, and attenuated GPR reflections. Repeated surveys in 2003 and 2007 over the contaminated area showed that in 2007, the bulk electrical conductivity had reverted to near-background conditions, the positive SP anomaly became more negative, and the zone of attenuated GPR reflections showed increased signal strength. Removal of hydrocarbon mass in the vadose zone over the plume by a soil vapor-extraction system installed in 2001 was primarily responsible for the changing geophysical responses. Although chemical data from groundwater showed a 3-m-thick conductive plume in 2007, the plume was not imaged by electrical resistivity. Forward modeling suggests that apparent bulk electrical conductivity of the saturated zone plume has to be three to five times higher than background values to be imaged by electrical resistivity. We suggest that removing hydrocarbon-contaminant-mass reduction by natural or engineered bioremediation can be imaged effectively by temporal geophysical surveys

Geophysical Signatures of Disseminated Iron Minerals: A Proxy for Understanding Subsurface Biophysicochemical Processes

Previous studies have linked biogeophysical signatures to the presence of iron minerals resulting from distinct biophysicochemical processes. Utilizing geophysical methods as a proxy of such biophysicochemical processes requires an understanding of the geophysical signature of the different iron minerals. Laboratory experiments were conducted to investigate the complex conductivity and magnetic susceptibility signatures of five iron minerals disseminated in saturated porous media under variable iron mineral content and grain size. Both pyrite and magnetite show high quadrature and inphase conductivities compared to hematite, goethite, and siderite, whereas magnetite was the highly magnetic mineral dominating the magnetic susceptibility measurements. The quadrature conductivity spectra of both pyrite and magnetite exhibit a well-defined characteristic relaxation peak below 10kHz, not observed with the other iron minerals. The quadrature conductivity and magnetic susceptibility of individual and a mixture of iron minerals are dominated and linearly proportional to the mass fraction of the highly conductive (pyrite and magnetite) and magnetic (magnetite) iron minerals, respectively. The quadrature conductivity magnitude increased with decreasing grain size diameter of magnetite and pyrite with a progressive shift of the characteristic relaxation peak toward higher frequencies. The quadrature conductivity response of a mixture of different grain sizes of iron minerals is shown to be additive, whereas magnetic susceptibility measurements were insensitive to the variation in grain size diameters (1-0.075 mm). The integration of complex conductivity and magnetic susceptibility measurements can therefore provide a complimentary tool for the successful investigation of in situ biophysicochemical processes resulting in biotransformation or secondary iron mineral precipitation

Induced-Polarization Measurements on Unconsolidated Sediments from a Site of Active Hydrocarbon Biodegradation

To investigate the potential role that indigenous microorganisms 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 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

Spectral induced polarization (SIP) response of biodegraded oil in porous media

Laboratory experiments were conducted to investigate the effect of different oil saturation (0.2–0.8), wetting conditions (water-wet and oil-wet), and the addition of asphaltene on the spectral induced polarization (SIP) response of biodegraded and fresh crude oil in sand columns. In the water-wet case, no significant differences were observed for both the fresh and biodegraded oil and both displayed an increase in the magnitude of the phase (φ) and decrease in the magnitudes of the real (σ′) and imaginary (σ′′) conductivity components with increasing oil saturation. In this instance the SIP response is most likely controlled by the conduction and polarization of the electric double layer at the mineral–water interface. However, when oil is the wetting phase there were considerable differences in the magnitude of the SIP parameters between the fresh and biodegraded oil. The magnitude of φ and σ′′ increased with increasing oil saturation, whereas σ′ decreased. The magnitude of σ′ and σ′′ for the biodegraded oil-wetted sands were relatively higher compared to fresh oil-wetted sands. In experiments with fresh and biodegraded oil-wet sand, the addition of 1 per cent asphaltene increased σ′ and σ′′ with the biodegraded oil showing the highest magnitude. Asphaltenes are the most dipolar fraction of crude oil and increase in concentration with increasing biodegradation. Asphaltene creates a surface charge due to the ionization and complexation reactions of functional groups at interfaces. Therefore, the enhancement in the conduction and polarization observed with the biodegraded oil-wetted sands may be due to the increase in polar components (e.g. asphaltene) from the biodegradation process and the interactions of the polar components with the surfaces of water and mineral grains. Further studies are required to investigate the effect of other components in biodegraded oil such as resins, trace metals, biogenic metallic minerals (e.g. magnetite) and organic acids on the SIP response of porous media.Peer reviewedGeolog

Thermal Perturbations beneath the Incipient Okavango Rift Zone, Northwest Botswana

We used aeromagnetic and gravity data to investigate the thermal structure beneath the incipient Okavango Rift Zone (ORZ) in northwestern Botswana in order to understand its role in strain localization during rift initiation. We used three-dimensional (3-D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the rift and surrounding basement. We also used two-dimensional (2-D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal shallow CPD values (8-15 km) and high heat flow (60-90 mW m-2) beneath a ∼60 km wide NE-trending zone coincident with major rift-related border faults and the boundary between Proterozoic orogenic belts. This is accompanied by thin crust ( \u3c 30 km) in the northeastern and southwestern parts of the ORZ. Within the Precambrian basement areas, the CPD values are deeper (16-30 km) and the heat flow estimates are lower (30-50 mW m-2), corresponding to thicker crust (∼40-50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric shear zones as conduits. These fluids weaken the crust, enhancing rift nucleation. Our interpretation is supported by 2-D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. If our interpretation is correct, it may result in a potential paradigm shift in which strain localization at continental rift initiation could be achieved through fluid-assisted lithospheric weakening without asthenospheric involvement

Microbial Growth and Biofilm Formation in Geologic Media Is Detected with Complex Conductivity Measurements

Complex conductivity measurements (0.1-1000 Hz) were obtained from biostimulated sand-packed columns to investigate the effect of microbial growth and biofilm formation on the electrical properties of porous media. Microbial growth was verified by direct microbial counts, pH measurements, and environmental scanning electron microscope imaging. Peaks in imaginary (interfacial) conductivity in the biostimulated columns were coincident with peaks in the microbial cell concentrations extracted from sands. However, the real conductivity component showed no discernible relationship to microbial cell concentration. We suggest that the observed dynamic changes in the imaginary conductivity (σ″) arise from the growth and attachment of microbial cells and biofilms to sand surfaces. We conclude that complex conductivity techniques, specifically imaginary conductivity measurements are a proxy indicator for microbial growth and biofilm formation in porous media. Our results have implications for microbial enhanced oil recovery, CO2 sequestration, bioremediation, and astrobiology studies

Microbial Nanowires: Is the Subsurface Hardwired ?

The Earth\u27s shallow subsurface results from integrated biological, geochemical, and physical processes. Methods are sought to remotely assess these interactive processes, especially those catalysed by micro-organisms. Using saturated sand columns and the metal reducing bacterium Shewanella oneidensis MR-1, we show that electrically conductive appendages called bacterial nanowires are directly associated with electrical potentials. No significant electrical potentials were detectable in columns inoculated with mutant strains that produced non-conductive appendages. Scanning electron microscopy imaging revealed a network of nanowires linking cells-cells and cells to mineral surfaces, hardwiring the entire length of the column. We hypothesize that the nanowires serve as conduits for transfer of electrons from bacteria in the anaerobic part of the column to bacteria at the surface that have access to oxygen, akin to a biogeobattery. These results advance understanding of the mechanisms of electron transport in subsurface environments and of how microorganisms cycle geologic material and share energy