Biogeophysics II Posters
Presiding: L Slater, Rutgers University; E Atekwana, University of Missouri at Rolla
NS51B-01 0830h
The Influence of Bacterial Activity on the Speciation and Transport of Arsenic Across the Ground Water-Surface Water Transition Zone at a Contaminated Site
Field investigations have been conducted to understand the fate of arsenic in contaminated ground water during discharge into a small lake. The ground water plume contains elevated levels of arsenic and BTEX compounds derived from historical disposal of process wastes from up gradient industrial activities. A ground-water monitoring network was established in conjunction with the collection of surface water and sediment samples to establish the extent and characteristics of the zone of contaminant discharge into the small lake. Examination of the spatial patterns of specific conductance in ground water and surface water along with sediment characteristics have been used to develop a conceptual model of the biogeochemical processes controlling the speciation and transport of arsenic across the ground water-surface water transition zone. Patterns in specific conductance and sediment mineralogy point to the importance of bacterial iron and sulfate reduction processes. Spatial relationships between these indicator parameters and arsenic speciation in ground water at the point of plume discharge indicate that degradation of BTEX compounds and other hydrocarbons influences the ultimate fate of arsenic within the system. Notice: This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.
NS51B-02 0830h
Hydrogeophysical Monitoring of Water Uptake in Root Zones of Small Plants
We have monitored the water content in root zones in hydrogeophysical experiments and studied daily and seasonal variations of water uptake. Plants grew in plastic pots filled with fine sand. The surface of the pots was isolated to minimize evaporation, i.e., most water is consumed for transpiration. We installed geoelectric surface and subsurface profiles (electrode interval = 1.5 cm), and used 900/1500 MHz antennas to measure the travel times of radar waves reflected from a metallic plate at the base. Also a central and peripheral TDR and a tensiometer probe were installed. A continuous data acquisition was conducted to monitor the spatiotemporal water content of root zones and its variations. Our observations clearly reflect a decrease of pore water content with time and its abrupt increase directly after each irrigation cycle. TDR and tensiometer curves are parallel and mirror images of the resistivity curve. Observed soil water content in the day time was consistently lower than in the night time (no plant transpiration). Long-term observations of water uptake by roots show that the plants behavior is a function of the background moisture content. The maximum water uptake of optimum growth occurs at intermediate water content. Seasonal variations could be observed. The water uptake in May is obviously twice that of November. This can be explained by the fact that the available light (required for photosynthesis) was higher in May than in November. Also the effect of day light on the water uptake can be observed. The light sunny days show higher water uptake than the dark rainy days. The peripheral water content values that decrease with time are lower than that of the central root zone and show small night and day changes. This may imply that the central TDR probe measures the water content both within the wet root branches and the bounding soils, whereas the peripheral TDR reading represents the soil pore water only. Electrical models of the root zone show a systematic daily resistivity minimum in the morning that jumps continuously during the day time approaching maximum in the night time. We established empirical petrophysical relationships of water content to both resistivity and radar velocity and studied root effects in the 4-phase medium (sand, air, water, organic roots).
NS51B-03 0830h
Magnetotactic bacteria: when does the magnetic field help?
Magnetotactic bacteria are a diverse group of organisms that use the Earth's magnetic field to orient themselves. They are widespread in marine and freshwater sediments, particularly near the oxic-anoxic transition zone (OATZ). They are involved in the cycling of iron, magnesium, sulphur and carbon. Because they leave "magnetofossils" in the form of chains of magnetite particles with a narrow size range, they affect the magnetic properties of sediments and their presence has been detected in rocks as old as two billion years. Why do magnetotactic bacteria go to the trouble of synthesizing chains of magnetite particles? They use aerotaxis to stay near the optimal concentration of oxygen (at the OATZ), and under some circumstances the magnetic field helps them to locate this zone. However, the magnetic field does not help if it is parallel to the OATZ. A model is developed that compares the motion of magnetotactic bacteria in a magnetic field with the motion in zero field. The comparison depends on the type of aerotaxis. One type, "polar", uses only the magnitude of the oxygen concentration. This type is useless without an orienting mechanism, so it must have developed after magnetotaxis. The other type, "axial", uses both concentration and gradient to determine the probability per unit time of reversing the swimming direction. When a bacterium is going in the correct direction, it is much more likely to keep swimming in that direction. However, it takes about four seconds to accumulate enough information on the oxygen gradient to make a reliable decision. In this time rotational diffusion perturbs the swimming direction by an average of 60 degrees unless there is a restoring torque. Magnetotaxis counteracts rotational diffusion, allowing much longer runs in the correct direction. This effect is so important that magnetotaxis increases the efficiency of aerotaxis even when the field is nearly parallel to the OATZ.
http://www4.ncsu.edu/~ajnewell/magnetotaxis.html
NS51B-04 0830h
Investigating the Impact of Microbial Metabolic Byproducts on Electrical Measurements
The main goal of this research group is to test the hypothesis that microbial interaction with geologic media over short and long term can result in changes in physical properties which can be imaged using geophysical methodologies. Bacteria are able to alter mineral surface chemistry, affect water-rock interactions, and modify groundwater geochemistry with the potential to induce changes in in-situ physical properties of subsurface geologic media (e.g. porosity and permeability). Thus the challenge is decoupling these processes and quantifying their magnitudes and impact on the geophysical measurements. In order to better understand the relationship between biological processes and the electrical response of microbially-impacted geologic media, bench-scale laboratory column experiments were conducted to investigate the effect of microbial byproducts (i.e. organic acids) on electrical measurements. Organic acids are common intermediates of microbial mineralization of organic carbon in natural environments, and have the potential to impact electrical measurements (1) by directly contributing to the ionic strength of an aqueous solution, (2) indirectly through the dissolution of minerals which may increase the pore water conductivity of the solution, and (3) through the dissolution of minerals leading to secondary or enhanced porosity. Electrical measurements were collected for three organic acids (acetic, formic, propionic) at three different concentrations (10, 100, 1000 uM) and compared to electrical measurements on salts (sodium chloride, calcium chloride, aluminum chloride) of the same concentration. With the exception of formic acid, no significant difference was observed between the conductivity magnitude of the organic acids and salts at the same concentrations. This suggests that both the acids and salts may contribute similarly to the ionic strength of the solution. The magnitude of the conductivity of formic acid was observed to be up to four times that of other acids at the same concentration. We infer from these observations, that depending on the type of organic acid, different organic acids may contribute more to the fluid conductivity than other acids of the same concentration. Finally, the data from the current study was compared to fluid conductivity and organic acid concentration data from a hydrocarbon contaminated site in Michigan. Preliminary calculations suggest that the direct presence of organic acids in the groundwater at the site may contribute to more than half of the total fluid conductivity measured at the site.
NS51B-05 0830h
Proton Nuclear Magnetic Resonance: A Novel Approach for Monitoring In Situ Iron Mineralization Processes.
Transformation or dissolution of iron minerals resulting from (a)biotic reduction can have a pronounced impact on the fate and transport of nutrients and contaminants in terrestrial environments. Accordingly, non-invasive in situ measurements of such geochemical reactions are of significant interest. 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 present a novel method based on proton nuclear magnetic resonance (NMR) relaxation time measurements for in situ monitoring of iron mineralization processes. Laboratory NMR measurements were used to monitor changes in the chemical (oxidation state) and mineralogical form of iron oxides during abiotic reduction reactions by ferrous iron. Specifically, columns packed with ferrihydrite-coated quartz sand (1 wt % Fe) were reacted with anaerobic media containing ferrous iron under advective flow conditions at circum neutral pH. The abiotic reduction of ferrihydrite with aqueous ferrous ion has previously been shown to result in the intermediary precipitation of goethite and lepidocrosite followed by dissolution and re-precipitation of magnetite (Hansel et al., 2003). The NMR relaxation measurements, obtained over a 24 hour period, were shown to be very sensitive to changes in the mineralogical form of iron minerals. A 60 % decrease in the relaxation time was observed as ferrihydrite converted to magnetite which is in agreement with NMR control studies of ferrihydrite - magnetite mixtures. Intermediary mineral precipitation was indicated by an increase in the relaxation time following the introduction of ferrous iron to the system. These results demonstrate the potential of NMR field instruments as an in situ method for monitoring geochemical reactions.
NS51B-06 0830h
Electrical Measurements on Microbial Cells in Suspension and in Sand Columns
Recent studies show that microbial processes impact the electrical properties of earth materials. However the mechanisms generating the observed electrical signatures remain uncertain. The addition of microbial cells will increase the total surface area and surface charged density of a soil. Biofilms may coat soil surfaces and alter the electrical properties of the mineral-fluid interface. We attempted to detect the presence of microbial cells in suspensions and in sand columns with four electrode geophysical measurements. Cell cultures of Shewanella putrefaciens and Escherichia coli (up 5g/l dry weight) were suspended in NaNO3 electrolyte solutions of varying ionic strength (0.1 M - 0.001 M). Similar suspensions were later injected into Ottawa sand columns. Electrical measurements (impedance magnitude and phase shift) were made from 0.05 Hz - 10 kHz with all datasets calibrated to known solutions. The impact of the microbial cells on the suspension conductivity was detectable only at the very lowest electrolyte concentrations. Any effect on the phase shift was beyond the detection limit of the geophysical instrumentation in all experiments due to the dominance of the conduction term over the polarization. Our results suggest that the increased conduction and polarization observed with geophysical methods at microbial active hydrocarbon sites may not be directly related to the microbial population itself.
NS51B-07 0830h
Monitoring Microbial Chemotaxis and Sulfate-Reduction Using the Self-Potential Method
There is increasing interest in the use of the self-potential (SP) method for non-invasively characterizing subsurface redox conditions. As variations in the redox state are indicative of conditions favorable to specific types of microbial activity, the ability to monitor spatiotemporal changes in the redox state of aquifer sediments would be of great value in evaluating stimulated bioremediation. We have used the self-potential method to track the onset and location of microbial sulfate-reduction in saturated sediments at the laboratory scale during conditions of organic carbon amendment. Anomalies of greater than -400 mV were observed as sulfate-reduction coincided with the incomplete oxidation of lactate. The timing and location of the SP anomalies correlated with increases in the concentration of planktonic cells and decreases in sulfate. In the absence of chelating metals, the reduction of sulfate results in elevated concentrations of dissolved sulfide. The SP anomalies observed here are believed to result from electrochemical concentration gradients between regions of high and low dissolved sulfide. Temporal variations in the location of the SP anomaly corresponded to the location of active sulfate-reduction, which in turn was governed by microbial chemotaxis towards elevated lactate concentrations. Abiotic experiments in which sulfide concentration gradients were systematically varied showed a positive correlation between the magnitude of the measured SP anomaly and the difference in sulfide concentration. These results suggest the ability to measure the changes in the spatiotemporal location of sulfate-reduction during bioremediation and to perhaps quantify the ensuing sulfide concentration gradients.
NS51B-08 0830h
Investigating the structure and dynamics of microbial communities in zones of anomalous geophysical signatures and the effect of these communities on electrical properties
It has been recently recognized that microorganisms can impact both the electrolytic and interfacial electrical properties of subsurface geologic media and thereby influencing geoelectrical measurements. We hypothesize that geoelectrical methods in turn, can be used to delineate subsurface zones containing maximal microbial activity allowing for a better understanding of geomicrobiological processes and characterization of the microbial community. To investigate the structure and dynamics of microbial communities in zones of anomalous geophysical signatures, we used traditional culture-based microbiological and non-culture-based molecular methods. The employment of culture-based enrichment techniques at a hydrocarbon-polluted study site resulted in the isolation of multiple strains of the bacterial Rhodococcus species. Since it is known that only a fraction of the microbial soil community is amenable to enrichment cultures, we chose a non-culture-based approach as well. We constructed two clone libraries based on the 16S rRNA gene of bacteria, one from the hydrocarbon-contaminated study site and one from a non-contaminated background site. The comparison of the two clone libraries will reveal whether there is any significant difference in microbial community composition associated with areas of anomalous geoelectrical measurements. Moreover, to study the effects of microbial biofilm formation on the physical properties of sediments, we inoculated sterile sand with known bacterial cultures and monitored the biofilm formation over time using colorimetric dyes and microscopic methods. Increased biofilm formation was observed between 3 and 6 days after inoculation. Resulting changes in the porosity and surface area of the sands will be measured by induced polarization methods. This interdisciplinary project between geophysicists and microbiologists will enhance our understanding of the effects of microorganisms on geologic media and their influence on geoelectrical signatures of subsurface sediments.