NS41A-01
Altered Geophysical Response Reflects Changes in Microbial Community Structure in Petroleum Contaminated Sediments
The microbial diversity of a petroleum contaminated site was investigated in order to further elucidate biogeophysical relationships of the subsurface. Increases in the sediment bulk conductivity at hydrocarbon contaminated sites have been suggested to result from enhanced mineral weathering due to increased microbial activity within the free-phase contamination zone. Therefore, our main objective was to characterize the microbial community structure in zones of higher and lower conductivity. Culture-independent 16S rRNA gene libraries were constructed using sediment samples collected from the contaminated and background sites. Diversity indices indicated that the microbial community at the contaminated site contained fewer, more dominant populations, whereas the microbial community at the background site was much more diverse. This confirms the notion that environmental disturbances decrease the diversity of microbial communities. Pairwise comparisons of the microbial communities revealed that the communities from the contaminated versus background sites were significantly different. Phylogenetic analysis demonstrated that the largest proportion of the microbial community from within the petroleum contaminated zone was affiliated with members of the {\it Proteobacteria} division, while the largest proportion from the vadose zone and background sediments were affiliated with members of the {\it Acidobacteria} division. Specific microbial populations detected only within the petroleum contaminated zone were related to the dissimilatory iron reducing bacteria (DIRB), including {\it Desulfitobacterium} and {\it Rhodoferax ferrireducens}. {\it Rhodoferax ferrireducens} is known to reduce Fe(III) by direct contact, which could conceivably enhance mineral dissolution. Other significant populations were affiliated with the syntrophic bacteria {\it Syntrophus} and {\it Pelotomaculum}. Members of the {\it Syntrophus} genus are characteristically found in methanogenic environments, suggesting methanogenesis as the dominant terminal electron accepting process (TEAP) within the anaerobic petroleum contaminated zone. Moreover, a variety of methylo- and methanotrophic bacteria were found to be present around the plume. Thus, the microbial communities in and around the aged underground petroleum plume have become highly adapted to the available carbon source (petroleum hydrocarbons), and strongly influence the geophysical and geochemical surroundings through their catabolic activities. These results suggest that changes in microbial community structures parallel changes in the geophysical properties of contaminated sediments and provide strong evidence favoring the use of geoelectrical measurements for the monitoring of natural or engineered bioremediation processes.
NS41A-02
Direct and Indirect Contribution of Microbial Metabolic Byproducts to the Electrical Properties of Porous Media
Organic acids and biosurfactants are common intermediates of microbial mineralization of organic carbon in natural environments, and both have the potential to impact electrical measurements by directly contributing to the ionic strength of an aqueous solution. Organic acids impact electrical measurements indirectly by mineral dissolution which increases the pore water conductivity and/or secondary porosity. We present results of laboratory sand-packed column experiments designed to simulate the effect of common microbial metabolic byproducts on electrical measurements. We collected low frequency (0.1-1000 Hz) electrical measurements of silica sand-packed columns saturated with organic acids or biosurfactants of varying concentration. The electrolytic (bulk and fluid) conductivity increased with increasing concentrations of organic acids or biosurfactants, while the interfacial (imaginary) conductivity remained relatively steady. The contribution to ionic strength by dissolution by organic acids was also investigated by collecting low frequency electrical measurements on natural sand-packed columns saturated with organic acids for a period of 120 days. Background measurements were collected from columns saturated with tap water, and compared to the columns with organic acids. Both columns saturated with organic acid and tap water showed a steady increase in the imaginary, real, and fluid conductivities for the first 60 days. The pH increased rapidly for the first 10 days, after which it remained nearly constant. After ~60 days, the organic acid and tap water columns appeared to reach equilibrium in the electrical parameters, and remained relatively constant for the duration of the experiment. The similar trend in both acid and tap water treated columns suggests that increases in electrical parameters and pH are enhanced by more than the presence of organic acids. The results of this study show that although the direct presence of metabolic byproducts increased the real conductivity, the change in the imaginary conductivity was negligible. The dissolution experiment showed higher real and imaginary conductivity with time, consistent with increases in the ionic strength of the fluid and enhancement of polarization in the sands.
NS41A-03
Detecting Microbial Growth and Metabolism in Geologic Media with Complex Conductivity Measurements
Complex conductivity measurements between 0.1-1000 Hz were obtained from biostimulated sand-packed (coarse and mixed fine and medium grain) columns to investigate microbial growth, biofilm formation, and microbial metabolism on the electrical properties of porous media. Microbial growth and metabolism was verified by direct microbial counts, pH changes, and environmental scanning electron microscope imaging. Peaks in imaginary (interfacial) conductivity in the coarse grain columns occurred concurrently with peaks in the microbial cell concentrations. The magnitude of the imaginary conductivity response in the mixed fine and medium grain columns, however, was low compared to the coarse grain sand columns, consistent with lower microbial cell concentrations. It is possible that the pore size in the mixed fine and medium grain sand restricted bacteria cell division, inhibiting microbial growth, and thus the smaller magnitude imaginary conductivity response. The biostimulated columns for both grain sizes displayed similar trends and showed an increase in the real (electrolytic) conductivity and decrease in pH over time. Dynamic changes in the imaginary conductivity arises from the growth and attachment of microbial cells and biofilms to surfaces, whereas, changes in the real conductivity arises from the release of byproducts (ionic species) of microbial metabolism. We conclude that complex conductivity techniques are feasible sensors for detecting microbial growth (imaginary conductivity measurements) and metabolism (real conductivity measurements) with implications for bioremediation and astrobiology studies.
NS41A-04
Self potential observations during DNAPL dissolution
Dense non aqueous phase liquids (DNAPLs) are a major environmental problem and are considered to be long term heavy contaminant sources in the subsurface. Accurate monitoring of DNAPL breakdown is required to monitor remediation efforts. We aim to evaluate the efficiency of geophysical methods to monitor DNAPL remediation. Toward this goal we performed self potential (SP) measurements on laboratory columns packed with DNAPL contaminated sand undergoing (a) biodegradation, and (b) abiotic DNAPL dissolution. Geochemical monitoring showed higher concentration of dissolved DNAPL byproducts in the abiotic columns; the use of HgCl2 as a biocide probably increased the rates of DNAPL dissolution in the abiotic columns. The concentration of DNAPL byproducts is significantly lower in the biotic columns due to microbial activity since DNAPL degrading bacteria within the column consume the breakdown products. SP responses are significantly higher ($\sim$ 90 mV) in the abiotic columns; in the microbial active columns SP values remain steady with a value ~ 10 mV. High SP signals (up to 110 mV) are associated with DNAPL byproduct concentration gradients within the abiotic columns and exhibit a temporal behavior that mimics total organic carbon concentrations. Although microbial activity in organic rich contaminated areas has been associated with strong negative SP anomalies our results show that positive SP anomalies can also be generated in contaminated areas in the absence of any microbial activity. We discuss a possible SP source mechanism and the implications in geophysical monitoring of DNAPL remedial processes.
NS41A-05
Geophysical Detection of Biomineralization Within Selenium Contaminated Lake Sediment
Geophysical measurements during active selenite reduction were made on lake sediment from Belews Lake in North Carolina that was verified to contain indigenous cultures known to precipitate the metalloid selenium. The sediment was wet-packed into columns with a mixture of Ottawa sand (80% sand, 20% sediment) to facilitate flow through the sediment. Electrical geophysical measurements were continually monitored as a selenite medium flowed through the column. Electrical conductivity of the influent and outflow solutions was also monitored and outflow samples, taken twice a day, were used to determine the selenite concentrations reduced within the column. Within a few days of beginning the experiment, the sediment/sand mixture within the column exhibited widespread red discoloration, confirming the microbial reduction of selenite to elemental selenium within the column. The electrical formation factor exhibited a smooth increase in time that we attribute to the continual precipitation of selenium biominerals. The geophysical dataset provides a novel means to ascertain rates of biomineralization within the column; information that is difficult to extract from aqueous or solid phase chemistry. The geophysical data indicate that most of the microbial activity occurred within the first 5 days of the experiment.
NS41A-06
Biogeophysical Monitoring: Isolating the "Bio" From the "Geo"
Growing interest in the use of time-lapse geophysical methods for monitoring changes in the physical properties of the subsurface during stimulated bioremediation has led us to consider the feasibility of using such methods to discriminate between a host of coupled biogeochemical processes (e.g. gas generation, mineral alteration, etc.). To date, our work has shown great promise in utilizing geophysical techniques to monitor these complex phenomena. However, dealing with non-uniqueness issues and the ability to discriminate between biotic and abiotic processes presents a significant challenge. Here, we consider the wide range of biogeochemical transformations that can accompany remediation schemes based on organic carbon amendment, and the impact that each can have on time-varying geophysical signals. Given the great diversity in potential mineralogical and microbiological pathways following carbon amendment, great care must be taken in interpreting biogeophysical datasets and the responses observed therein. Through examples from both the lab and field, we will illustrate the difficulty in ascribing variations in the geophysical response to a single process. Specifically, changes in pore fluid composition and mineralogical makeup have been observed to yield time-varying geophysical responses that are not solely the result of microbial activity. The rush to ascribe any and all regions of anomalous geophysical response to biology must be tempered with the thoughtful consideration of multiple lines of evidence, including the complete suite of observed changes in geochemistry, mineralogy, and microbiology, and the integration of all available data and models via estimation frameworks that can handle non-uniqueness.
NS41A-07
The self-potential method as a non-intrusive redox sensor in organic-rich contaminant plumes
Self-potential signals correspond to passively measured electrical potential anomalies at the ground surface that provides, once filtered, the signature of polarization processes occurring in the ground. Two contributions have been recognized as providing these signals. They are the streaming potential (related to the flow of the ground water)and redox processes. We will show how the self-potential method can be used quantitatively as a non-intrusive redox sensor for the case of contaminant plumes and oil spills in the ground. In this case, we have evidenced a biogeobattery phenomenon in which the redox potential acts as a thermodynamic force while the presence of biofilms and the precipitation of metallic particles in areas of high gradients in the redox potential allows the transfer of electrons creating a net source of current in the system. Consequently, organic- rich contaminant plumes in the ground behaves as natural geobatteries creating their own electrical field that can be measured at the ground surface and inverted to retrieve the geometry of the source and to determine the in situ redox potential in a non-intrusive way.
NS41A-08
An Updated Model for the Anomalous Resistivity of LNAPL Plumes in Sandy Environments
Anomalously low resistivities have been observed at some sites contaminated by light non-aqueous phase liquid (LNAPL) since. The model that has been used to explain this phenomenon was published in 2000. This working hypothesis invokes both physical mixing and bacterial action to explain the low resistivities near the base of the vadose zone and the upper part of the aquifer. The hydrocarbon-degrading bacteria (of which there are numerous species found in soils) produce organic acids and carbonic acids. The acidic pore waters dissolve readily soluble ions from the native soil grains and grain coatings, to produce a leachate high in total dissolved solids. The free product LNAPL is initially a wetting phase, although not generally more than 50% extent, and seasonal water table fluctuations mix the hydrocarbons vertically through the upper water saturated zone and transition zone. This update introduces several new aspects of the conductive model. The first is that, in addition to the acids being produced by the oil-degrading bacteria, they also produce surfactants. Surfactants act similarly to detergents in detaching the oil phase from the solid substrate, and forming an emulsion of oil droplets within the water. This has helped to explain how continuous, high-TDS capillary paths can develop and pass vertically through what appears to be a substantial free product layer, thus providing easy passage for electrical current during electrical resistivity measurements. Further, it has also been shown that the addition of organic acids and biosurfactants to pore fluids can directly contribute to the conductivity of the pore fluids. A second development is that large-diameter column experiments were conducted for nearly two years (8 columns for 4 experiments). The columns had a vertical row of eletrodes for resistivity measurements, ports for extracting water samples with a syringe, and sample tubes for extracting soil samples. Water samples were used for chemical analysis and the soil samples for bacterial assays. Those columns to which diesel oil had been added (4) clearly showed the resistivity to decrease, the TDS of water samples to increase, and a radical change in bacterial populations favoring the oil degraders to occur. In the laboratory, most of the significant changes occurred in the time span of 4 to 6 months. The third development that may have a bearing on anomalous conductivity was the recent discovery that certain bacteria, (e.g., Geobacter sulfurreducens) have hairlike structures called pili that are excellent electrical conductors. These pili (nano-wires), longer than the bacterial body itself, are apparently used for electron transfer by the bacteria. In sufficient numbers, they could conceivably alter the electrical resistivity of a volume of earth. In summary, our understanding for the cause of anomalous conductive zones associated with areas impacted by LNAPL spills has been significantly improved by these recent developments.