NS51A-01
Geophysical Monitoring of Microbial Activity within a Wetland Soil
We performed Induced Polarization (IP) and Self Potential (SP) measurements to record the geoelectrical signatures of microbial activity within a wetland soil. The experiment was conducted in laboratory, utilizing an open flow column set up. Soil samples from Kearny Marsh (KM), a shallow water wetland, were collected and stored at 4o Celsius prior to the start of the experiment. Two columns were dry packed with a mix of KM soil and sterile Ottawa sand (50% by weight). One column was sterilized and used as a control while the other column retained the biologically active soil sample. Both columns were saturated with a minimal salts medium capable of supporting microbial life; after saturation, a steady flow rate of one pore volume per day was maintained throughout the experiment. Ambient temperature and pressure changes (at the inflow and outflow of each column) were continuously monitored throughout the experiment. Common geochemical parameters, such as Eh, pH, and fluid conductivity were measured at the inflow and outflow of each column at regular intervals. IP and SP responses were continuously recorded on both columns utilizing a series of electrodes along the column length; additionally for the SP measurements we used a reference electrode at the inflow tube. Strong SP anomalies were observed for all the locations along the active column. Black visible mineral precipitant also formed in the active column. The observed precipitation coincided with the times that SP anomalies developed at each electrode position. These responses are associated with microbial induced sulfide mineralization. We interpret the SP signal as the result of redox processes associated with this mineralization driven by gradients in ionic concentration and mobility within the column, similar to a galvanic cell mechanism. IP measurements show no correlation with these visual and SP responses. Destructive analysis of the samples followed the termination of the experiment. Scanning electron microscopy (SEM) and Energy Dispersive Spectrometry (EDS) were used to identify and quantify the presence and composition of the mineral precipitation in the control and active columns. Further geochemical measurements are currently being performed in order to confirm and more accurately quantify the mineralization and associated processes.
NS51A-02
Using Groundwater Point Velocity Probes and Ground Penetrating Radar to Investigate Microbially Mediated Changes in Flow Properties of a Contaminated Aquifer
In the practice of aquifer remediation, groundwater velocity is an essential parameter for determining contaminant
fate and transport, and calculating residence times for passive treatment technologies such as reactive barriers.
Over the past decade, there has developed an increased reliance on natural attenuation of various contaminants
in porous media, and much of the attenuation is associated with biological processes that might affect local
groundwater velocities. The point velocity probe (PVP), was recently developed to directly measure centimeter-
scale groundwater velocities. The PVP velocity estimates are based on a mini tracer-test completed around the
circumference of a small cylinder emplaced in the aquifer. The PVP functions without a well and therefore
requires no calibration. Also, velocity calculations are independent of Darcy's Law, thus eliminating the need for
hydraulic gradients that can usually only be measured over scales much larger than those of local microbial
effects. The development of the PVP instrument raises the possibility of measuring velocity changes associated
with biological activity in porous media.
Field and laboratory studies confirm the PVP consistently and accurately measures groundwater velocity in
porous media. An array of multilevel PVPs instrumented throughout a flow gate in the Borden aquifer coupled with
borehole radar and tomographic methods were consistent in identifying changes in spatial heterogeneities
apparently associated with the evolution of a petroleum hydrocarbon plume. The hydrocarbons were released
into the aquifer and then subject to stimulated biodegradation using oxygen release compound (ORC) in wells
up-gradient of the PVP array. Observed temporal changes in the heterogeneous properties of the Borden aquifer
were attributed to enhanced biological activity with changing groundwater velocities corresponding spatially to
areas of ORC application and greatest plume attenuation. To compliment PVP and radar field observations, a
large-scale laboratory experiment is underway investigating both biological growth and activity as mechanisms
for flow and dielectric property variations in the contaminated aquifer material.
http:www.people.ku.edu/~jfdevlin/Research.html
NS51A-03
Laboratory Measurements of Self-Potential (SP) During Biologically-Induced Precipitation of Calcite
Self-potential (or natural electrical field) data often provide complementary information for hydrological and environmental applications. Particularly, this method can be used to detect and quantify the variations of fluid flow or chemistry, as SP field results mainly from pressure and concentration gradients. Recent laboratory and field works have demonstrated that bacterial activity can also impact on this natural electrical field, even though the chemical and/or physical processes involved are still not well understood. It seems that direct transfer of electrons through biofilms triggers an electrical signal. Moreover, in an indirect way, the bacterial activity affects the fluid composition and the solute concentrations, as well as the properties of the surface of the mineral matrix, and thus changes the SP response. In order to provide some insights into the link between bacterial activity, chemistry and geophysics, we performed some laboratory experiments in sand-boxes. In particular, we studied the chemical and electrical response to the hydrolysis of urea by Bacillus pasteurii in presence of calcium. Schematically, bacteria were confined in the centre of the sand-box to avoid chemotactic migration. The sand was fully saturated with a nutrient solution containing urea (5 g/L), NaCl (8 g/L) and CaCl2-2(H2O) (2.8 g/L). The SP field was recorded continuously using small unpolarizable electrodes placed inside the sand. The chemical evolution of the solution at different distances from the centre of the box was daily analyzed. We report our preliminary results. The chemical evolution with time evidences at least two phases. First, a strong ammonium production, associated with the biodegradation of the urea, and a concomitant decrease of calcium ion content, due to the precipitation of calcite. Then, the biodegradation rate decreases or even goes to zero, while the by-products diffuse from the centre of the sand-box toward its borders. The SP variations are correlated with these different phases of biological activity.
NS51A-04
Using Nuclear Magnetic Resonance to Monitor Iron Mineralization Processes
Proton nuclear magnetic resonance (NMR) relaxation time measurements can be used to probe the molecular- scale physical and chemical environment of water in the pore-space of geological materials. In this study, we extend upon previous work to explore the use of NMR relaxation time measurements as a method for monitoring iron mineralization processes. Laboratory NMR measurements were used to monitor changes in the mineralogical form of iron oxide as it reacted with ferrous iron. Specifically, columns packed with ferrihydrite- coated quartz sand were reacted with anaerobic media containing ferrous iron under advective flow conditions at a pH of 7.5 and a flow rate of 5 pore volumes per hour. The experiment was conducted with two concentrations of ferrous iron: 2 mM and 0.02 mM. The NMR relaxation measurements were shown to be very sensitive to increases in the proportion of magnetite, a mixed ferrous-ferric oxide produced during ferrihydrite transformation in the sand columns. In both the 2 mM and 0.02 mM columns an overall decrease in the relaxation time was observed. In the 2 mM columns, intermediate reactions were indicated by an increase in the relaxation time following the introduction of ferrous iron to the system. The mineralogy was determined using extended x-ray adsorption fine structure (EXAFS) spectroscopy, and x-ray diffraction. Ferric and ferrous iron concentrations were determined spectrophotometrically using the ferrozine method. These results demonstrate the potential of NMR field instruments as a method for monitoring geochemical reactions.
NS51A-05
Are Microbial Nanowires Responsible for Geoelectrical Changes at Hydrocarbon Contaminated Sites?
Significant advances in near-surface geophysics and biogeophysics in particular, have clearly established a link between geoelectrical response and the growth and enzymatic activities of microbes in geologic media. Recent studies from hydrocarbon contaminated sites suggest that the activities of distinct microbial populations, specifically syntrophic, sulfate reducing, and dissimilatory iron reducing microbial populations are a contributing factor to elevated sediment conductivity. However, a fundamental mechanistic understanding of the processes and sources resulting in the measured electrical response remains uncertain. The recent discovery of bacterial nanowires and their electron transport capabilities suggest that if bacterial nanowires permeate the subsurface, they may in part be responsible for the anomalous conductivity response. In this study we investigated the microbial population structure, the presence of nanowires, and microbial-induced alterations of a hydrocarbon contaminated environment and relate them to the sediments' geoelectrical response. Our results show that microbial communities varied substantially along the vertical gradient and at depths where hydrocarbons saturated the sediments, ribosomal intergenic spacer analysis (RISA) revealed signatures of microbial communities adapted to hydrocarbon impact. In contrast, RISA profiles from a background location showed little community variations with depth. While all sites showed evidence of microbial activity, a scanning electron microscope (SEM) study of sediment from the contaminated location showed pervasive development of "nanowire-like structures" with morphologies consistent with nanowires from laboratory experiments. SEM analysis suggests extensive alteration of the sediments by microbial Activity. We conclude that, excess organic carbon (electron donor) but limited electron acceptors in these environments cause microorganisms to produce nanowires to shuttle the electrons as they seek for distant electron acceptors. Hence, electron flow via bacterial nanowires may contribute to the geoelectrical response.
NS51A-06
The role of bacterial nanowires on naturally-occurring electrical fields
Microbial activity is shown to significantly contribute to the naturally occurring electrical fields observed in geologic media. To investigate the source mechanism of this phenomenon we constructed models of the subsurface by inoculating saturated sand columns with the bacterium Shewanella oneidensis MR-1 and a mutant strain as control. S. oneidensis (and other microoragnisms) are known to develop a structured network of electron- conductive bacterial nanowires as a response to electron acceptor limitation conditions. We show that strong voltage gradients (SP) were concomitant with redox potential gradients in the MR-1 column but absent in the mutant columns. We hypothesize that deeply submerged (suffocating) cells gain access to oxygen through the nanowire network which extends to the groundwater - atmospheric air interface. The nanowires serve as conduits for transfer electrons from bacteria in the anaerobic part of the column to bacteria at the surface that have access to oxygen. We suggest that this process is directly linked with SP and redox gradients that develop across the column. We also observed that in the absence of conductive nanowires (or other conductors) redox potential gradients do not necessarily promote SP gradients as documented in our control columns. Our results suggest that microbial activity and nanowires greatly impact the electrical properties of porous materials and contribute to our understanding of the mechanisms that underlie geophysical methods for mapping microbial activity in near subsurface environments.