NS11B-0493
Polarization Force Microscopy of the Cell-Mineral Interface: Insights Into the Bioelectric Signature
The success of bioremediation strategies is dependent upon effective monitoring of microorganisms in the subsurface. Induced polarization (IP) may represent a cost-effective, complementary technique to existing borehole-based microbe detection schemes. Recent studies show a significant, yet poorly understood IP effect associated with the presence of bacteria in aqueous and porous media. This effect is believed to be rooted in the physicochemical surface interactions between cells and minerals which we probe using polarization and electric force microscopy. Dispersions of the local permittivity inferred from polarization force data that was collected over a hydrated mineral surface correspond to dispersions modeled for a bacterium. In each case, absolute permittivities and frequency cut-off values increase with surface potential and ion mobility, respectively. Potentially similar polarization mechanisms between the inorganic and organic condition are inferred. Further polarization force microscopy measurements of the mineral-microbe interface will provide molecular-level insight that complements column and field-scale IP observations. Anticipated is a more comprehensive mechanisitic description of the bioelectric IP response that facilitates application of IP to bioremediation.
NS11B-0494
Attenuated geophysical signatures associated with ongoing remediation efforts at Wurtsmith Air Force Base, Oscoda, Michigan
Previous geophysical investigations (1996, 1997, 2003, and 2004) conducted at the decommissioned Wurtsmith Air Force Base former Fire Training Cell (FT-02) showed a clearly defined high conductivity anomaly associated with hydrocarbon contaminants in the vadose zone and ground water near the source area. The source of the geophysical anomalies was attributed to biogeochemical modifications of the contaminated zone resulting from intrinsic bioremediation. During these previous surveys, ground penetrating radar (GPR) data showed a zone of attenuated GPR reflections extending from the vadose zone to below the water table. Self potential data (SP) data defined a positive anomaly coincident with the hydrochemically defined plume, while electrical resistivity data showed anomalously high conductivity within the zone of impact. In 2007, another integrated geophysical study of the site was conducted. GPR, SP, electrical resistivity, and induced polarization surveys were conducted with expectations of achieving similar results as the past surveys. However, preliminary assessment of the data shows a marked decrease in electrical conductivity and SP response over the plume. GPR data still showed the attenuated signals, but the zone of attenuation was only observed below the water table. We attribute the attenuation of the observed geophysical anomalies to ongoing soil vapor extraction initiated in 2003. Significant removal of the contaminant mass by the vapor extraction system has altered the subsurface biogeochemical conditions and these changes were documented by the 2007 geophysical and geochemical data. The results of this study show that the attenuation of the contaminant plume is detectable with geophysical methods.
NS11B-0495
NMR Studies of Biodegradation Reactions in Porous Media
Subsurface contamination caused by organic compounds is a widespread environmental problem. Biodegradation is the main process to reduce the mass of organic contaminants in soils and groundwater. Often the biodegradation is limited by the supply of electron acceptors at the location of the contaminants, and mixing and diffusion are therefore coupled to the degradation process. Nuclear magnetic resonance (NMR) allows non- invasive and non-destructive insight into diffusion processes and effects of microbial biomass on the pore scale, where the mixing and degradation processes actually are taking place. Studying key processes non-invasively with high temporal resolution will contribute to a better understanding of the effective biodegradation rates of organic contaminants in saturated porous systems. The application of 1H NMR relaxometry and PFG NMR diffusometry provide the possibilities to study diffusive transport at the mixing zones in porous media, where contaminants and electron acceptors show strong concentration gradients due to degradation reactions. Nuclear spin relaxation times of water reflect its interaction with the porous media as well as the biomass. To localize microbial biomass the mobility of the water molecules (relaxation times) can be classified corresponding to bound water in the biomass and bulk water. Oxygen is the most important electron acceptor that stimulate the activity and growth of aerobic microbes, and iron(III) a major one for anaerobes. In our work we applied low field (0.2 T) and high field (3 T) NMR measurements to analyze the diffusion in water and biomass and to monitor the concentration changes of electron acceptors. The experimental setup consists of small columns containing saturated porous media applying inversion recovery (IR) and pulsed field gradient (PFG) sequences. We show that both, oxygen as well as iron(III) affect the relaxation times by their paramagnetic properties, and can be determined by NMR relaxometry in environmentally relevant concentrations. So relaxation time measurements in presence of either oxygen or iron(III) can be used to detect consumption of these electron acceptors and thereby redox reactions and degradation rates.
NS11B-0496
A Laboratory Tank Experiment to Investigate the Effects of Microbial Growth on Water Flow and GPR Wave Propagation
A detailed laboratory tank experiment was conducted, as follow-up to preliminary findings from an earlier experiment, to investigate the effects of microbial growth on groundwater flow, and the sensitivity of GPR to measure changes in the aquifer resulting from that growth. The preliminary work involved the biostimulation of saturated sand in a Plexiglas® tank measuring 1.0 m high by 1.0 m long by 0.3 m wide. Bacteria were stimulated with a solution of tryptic soy broth and sodium acetate that served as a carbon and nutrient source. Twice weekly, GPR data were acquired in a transillumination survey mode through the sand, across the width of the tank. The experiment indicated that GPR wave velocities were highly variable in the bioactive zone compared to the non- stimulated zone in the tank. In particular, the effect of microbial growth appeared to produce a net decrease in GPR wave velocity. In order to assess the validity of these findings, the experiment was repeated with a higher sampling density and additional monitored parameters. As before, the experiment was conducted in a large flow-through sandbox reactor using groundwater acquired from a local, shallow aquifer, amended with a carbon source. Measured parameters included two-way GPR travel time and amplitude, pH, electrical conductivity, and temperature, all collected twice daily both upstream and downstream of the nutrient injection ports. This was maintained over the period that biological activity became established. Groundwater velocity (multilevel point velocity probes), and attached biomass were evaluated at the beginning and end of the experiment. Baseline conditions were documented over a period of about a month, before nutrient injections were begun. During this time, a dye tracer experiment was performed to visualize the flow paths within the porous medium prior to biostimulation. At the time of writing the experiment was underway with final results pending.
NS11B-0497
Field and Laboratory Investigations of Enhanced Biological Activity Influencing Groundwater Velocity and Electromagnetic Wave Propagation
Field observations of groundwater velocity using point velocity probes (PVPs) and aquifer dielectric properties using ground penetrating radar (GPR) were made in a bioremediating, gasoline contaminated aquifer to which dissolved oxygen was introduced using Oxygen Release Compound (ORC®). PVPs are sensitive to changes in groundwater flow; whereas GPR is sensitive to changes in the dielectric and geoelectric properties of the pore- fluid and aquifer material. Therefore, the two methods provided complimentary and independent evidence for pore-scale changes associated with enhanced biological activity. Following the addition of ORC, PVP and GPR measured velocities nearest the ORC wells changed in a consistent manner throughout the experiment. GPR velocities measured further down-gradient, outside the influence of the ORC, did not indicate the same trend. Results from geochemical and flow modeling, combined with gas and biomass measurements, show that PVP and GPR velocity changes were not artifacts of seasonally fluctuating hydraulic gradient or temperature. Rather, the changes appear to be caused by factors related to elevated levels biological activity in the aquifer. Preliminary geophysical laboratory testing in a large sandbox reactor previously identified consistent trends with field GPR data that mirrored the field observations. A new, more detailed, laboratory experiment using GPR and PVPs was initiated to 1) confirm with confidence the phenomena observed in the field and the preliminary lab tank experiment, and 2) to gain additional understanding of the mechanisms responsible for PVP and GPR responses. At the time of writing, the results of the detailed experiment are pending.
NS11B-0498
Electrical potentials associated with Microbial Activity in a Winogradsky Column
Abstract: Electrical Potential (EP) measurements using Ag-AgCl electrodes have been conducted in a 1-D column containing a complex of sulphate-reducing microbial communities. The Winogradksky column design, typically used to study the microbial diversity of photoautotrophic anaerobes, represents in the laboratory a model biogeochemical soil-water interface. Once the bacterial communities in the column begin to grow dissolved oxygen content decreases downwards; an H2S gradient, produced by Sulphate Reducing Bacteria (SRB) develops in the anaerobic sediment at the base of the column, increases. These two gradients, acting in opposite directions, then create a range of habitats for a variety of microorganisms. The results presented here show EP data monitored over a period of 10 months at several points in the column during the establishment and evolution of the microbial niches, along with certain dissolved gases (O2, CO2, and H2S). The interpretation of EP data and analysis of gases produced in the column due biogeochemical activities show that it is possible to monitor and identify several states of microbial activity that could be inferred as a series of microbially mediated electron acceptors. Thus, overall, the EP observations show that Ag-AgCl electrodes have potential to be used to monitor the activities of biofilm growth in complex environments. Although the EP response may not reflect necessarily the ‘true' geophysical response of microbial activity but a galvanic reaction mediated by microbes or caused by the by-products produced during biofilm growth, nevertheless EP still has its uses in the non-invasive monitoring of environmental and microbial systems coupled with other geophysical techniques.
NS11B-0499
Property Changes in Aqueous Solutions due to Surfactant Treatment of PCE: Implications to Geophysical Measurements
Select physicochemical properties of aqueous solutions composed of surfactants, dye, and perchloroethylene (PCE) were evaluated through a response surface quadratic design model of experiment. Nine surfactants, which are conventionally used in the remediation of PCE, were evaluated with varying concentrations of PCE and indicator dyes in aqueous solutions. Two hundred forty experiments were performed using PCE as a numerical factor (coded A) from 0 to 200 parts per million (ppm), dye type (coded B) as a 3-level categorical factor, and surfactant type (coded C) as a 10-level categorical factor. Five responses were measured: temperature (°C), pH, conductivity (μS/cm), dissolved oxygen (DO, mg/L), and density (g/mL). Diagnostics proved a normally distributed predictable response for all measured responses except pH. The Box-Cox plot for transforms recommended a power transform for the conductivity response with lambda (λ) = 0.50, and for the DO response, λ =2.2. The overall mean of the temperature response proved to be a better predictor than the linear model. The conductivity response is best fitted with a linear model using significant coded terms B and C. Both DO and density also showed a linear model with coded terms A, B, and C for DO; and terms A and C for density. Some of the surfactant treatments of PCE significantly alter the conductivity, DO, and density of the aqueous solution. However, the magnitude of the density response is so small that it does not exceed the instrument tolerance. Results for the conductivity and DO responses provide predictive models for the surfactant treatment of PCE and may be useful in determining the potential for geophysically monitoring surfactant enhanced aquifer remediation (SEAR) of PCE. As the aqueous physicochemical properties change due to surfactant remediation efforts, so will the properties of the subsurface pore water which are influential factors in geophysical measurements. Geoelectrical methods are potentially the best suited to measure SEAR alterations in the subsurface because the conductivity of the pore fluid has the largest relative change. This research has provided predictive models for alterations in the physicochemical properties of the pore fluid to SEAR of PCE. Future investigations should address the contribution of the solid matrix in the subsurface and the solid-fluid interaction during SEAR of PCE contamination. Notice: Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy. Mention of trade names or commercial products does not constitute endorsement or recommendation by EPA for use.
NS11B-0500
Electrical potential source mechanisms in microbial induced sulfate reducing environments
In order to compare self-potential (SP) signals resulting from possible ‘geobattery' effects with electrodic potential signals based on a known galvanic cell (GC) effect in the presence of sulfide, we designed a column experiment using dual sensor Ag-AgCl electrodes. Water from the Langan River (Belfast, UK), known to contain a sulfate reducing microbial community, was obtained. Two experimental columns were packed with fine-grained glass beads. One column continuously circulated (closed loop) with autoclaved river water as a control, while the other retained biologically active natural river water. Six Ag-AgCl electrodes equally spaced along one side of each column, and three Ag-AgCl self potential electrodes (where the metal is in electrolytic contact with the column via a sterilized 1M KCl agar gel), were placed on the other side of each column. Electrical potential signals were continuously recorded with both sensor types. Induced polarization, electrical resistivity, temperature and aqueous geochemistry measurements (pH, Eh, and conductivity) were taken once daily. Over the 20 day experiment duration, darkening of the circulating fluid, biofilm formation and a sulfurous smell were observed in the biologically active column whereas no such color change (or smell) was observed for the control column. In the active column electrodic potential readings approached 570 mV whereas stable and small electrodic potential values (~8 mV) were detected in the control column.. Self potential signals were consistently only 1-8 mV in both columns. The experiment shows although electrodic potentials (at the electrode) are diagnostic of microbial driven sulfate reduction there is no measurable self potential (geobattery) effect associated with this microbial process.