Biogeophysics I
Presiding: D Werkema, U.S. Environmental Protection Agency; K Williams, Lawrence Berkeley National Laboratory
NS44A-01 15:30h
Electrical Imaging of Roots and Trunks
We applied geoelectric and GPR techniques to analyze problems of botanical structures and even processes, e.g., mapping root zones, internal structure of trunks, and water uptake by roots. The dielectric nature of root zones and trunks is generally a consequence of relatively high moisture content. The electric method, applied to root zones, can discriminate between old, thick, isolated roots (high resistivity) and the network of young, active, and hydraulically conductive zones (low resistivity). Both types of roots show low radar velocity and a strong attenuation caused by the dominant effect of moisture (high dielectric constant) on the electromagnetic wave propagation. Single root branches could be observed in radargrams by their reflection and diffraction parabolas. We have perfected the inversion method for perfect and imperfect cylindrical objects, such as trunks, and developed a new multielectrodes (needle or gel) ring array for fast applications on living trees and discs. Using synthetic models we tested the technique successfully and analyzed it as a function of total electrode number and configuration. Measurements at a trunk show a well established inverse relationship between the imaged resistivity and the moisture content determined from cores. The central resistivity maximum of healthy trees strongly decreases toward the rim. This agrees with the moisture decrease to the outside where active sap flow processes take place. Branching, growth anomalies (new or old shoots) and meteorological effects (sunshine and wind direction) lead to deviations of the concentric electric structure. The strongest anomalies are related to infections causing wet, rotting spots or cavities. The heartwood resistivity is highest in olive and oak trunks, intermediate in young fruit trees and lowest in cork oak trunks that are considered to be anomalously wet. Compared to acoustic tomography our electric technique shows a better resolution in imaging internal ring structures where moisture is the most dominating factor. We conclude that our imaging resistivity technique is applicable for investigating or controlling the botanical and physical conditions of endangered trees (health inspection) and capable to monitor dynamic processes of sap flow if adequate tracers are used.
NS44A-02 15:45h
3D Self-Potential Inversion for Monitoring DNAPL Contaminant Distributions
Self-potential (SP) data are collected over an area known to be contaminated with Dense Non-Aqueous Phase Liquids (DNAPLs) at the Savannah River Site in South Carolina. The field experiment consists of approximately 100 SP measurements on a surface grid and in four boreholes, and is repeated after one year. DNAPLs are known to undergo redox reactions during their degradation in the environment, which is often biologically mediated. Self-potential geophysics is employed in this study because of its sensitivity to the in-situ biochemical processes that degrade the contaminants. These reactions provide an electrochemical source that is manifested as an SP signature at the measurement locations remote from the contaminated areas. 3D inversion of the SP data is therefore needed to spatially locate the distribution of sources, which is related to contaminant presence. The inversion incorporates the 3D resistivity structure collected at the same site, and is better constrained in depth by using borehole data and regularization. Ground truth information taken after the first field experiment provides concentration data with depth for several DNAPL species in five boreholes. There is a good correlation between the ground truth data and SP source inversion, though this comparison is limited by several factors: the difference in resolution of the ground truth and inverted data, and the dependence of the redox processes on other constituents that were not measured during the ground truthing, such as oxygen content or microbial presence. Inversion of the second year's dataset provides information on the changes in the contaminant distribution, either due to natural degradation or ongoing remediation.
NS44A-03 INVITED 16:00h
Dielectric Response Of Complex Systems
The qualitative shape of the dielectric dispersion curves for living cells in suspensions or tissues was reported to exhibit prominent features: alpha, beta, gamma, delta regions. In this talk I will argue that the dielectric behavior at low frequencies is dominated by the effect of the diffusion of the free charges on the cell surfaces. The live cell suspensions are modeled as suspension of arbitrarily shaped, shelled and charged particles. The theoretical results are valid in the low range of frequencies (alpha region) as well as in the high range of frequencies (beta region). The dispersion curves are then measured experimentally by fast and nondestructives methods. By fitting the experimental data with the theoretical ones, I will argue that one can measure many cell parameters as: membrane potential, mobility of free charges or the volume fraction of the suspension.
NS44A-04 INVITED 16:15h
The microbial cell surface electric field: life in an ion cloud
Electrical charge on microbial cell surfaces arises from the ionization of proton-active functional groups attached to cell wall polymers. In Gram-positive cell walls, ionizable functional groups are associated with peptidoglycan and secondary polymers such as teichoic or teichuronic acids. Carboxyl functional groups attached to the unlinked peptide crosslinks of peptidoglycan and phosphoryl groups associated with the teichoic acids can deprotonate to form negatively charged surface sites. These anionic functional groups generate charge in the cell wall which results in the formation of an electric field that surrounds the entire cell. The cell surface electric field controls the concentration and spatial distribution of ions and counterions at the cell-water interface, and strongly affects microbe-fluid and microbe-mineral interactions. Recently, we have used potentiometric titration, infrared spectroscopy, electrophoretic mobility, metal sorption experiments to characterize the surface electrical potential properties of the various Gram-positive and Gram-negative bacterial species. Potentiometric titration experiments show that the deprotonation of acidic cell wall functional groups generate surface charge density values typically ranging from 1.1 to 2.2 mol sites/g of bacteria. Spectroscopic measurements have confirmed that the dominant proton-active sites in the cell wall are carboxyl functional groups. Electrophoretic mobility experiments show that the magnitude of the electrostatic surface potential increases with increasing pH, and decreases with increasing ionic strength. Metal sorption experiments conducted with Ca(II), Sr(II) and Ba(II) exhibit strong ionic strength dependence, suggesting that high concentrations of metal ions are electrostatically bound to bacterial cell walls via outer-sphere complexation. We demonstrate that the electrostatic potential effects on ion sorption at the cell-water interface can be quantified using the Donnan model.
NS44A-05 16:30h
The colloid behavior of nanoparticles in aqueous environments
Microbial metabolism-driven biomineralization is a productive source of nanoscale oxide and sulfide minerals in subsurface environments. In common with synthetic nanoparticles, biogenic nanoparticles can possess modified surface and interior structures, and confined electronic structure. The lifetime and geochemical behavior of nanoparticles in the environment are poorly understood, but it is anticipated that their fate and mobility are strongly affected by their colloid properties. Electron microscopy indicates nanoparticles to be gregarious, readily clustering to form nanoporous aggregates. Such aggregate formation reduces the nanoparticle surface area that is accessible to aqueous species, and permits growth via oriented aggregation. Nanoparticle transport properties are dependent on aggregate structure, which thus is a significant factor for the effectiveness of contaminant immobilization through bioremediation. We present synchrotron- and laboratory-based studies of the colloid behavior of synthetic nanoparticles under environmentally-relevant conditions and describe approaches for the quantitative description of aggregate structure.
NS44A-06 INVITED 16:45h
Bacterial-Induced Minerlization of Fe (Hydr)oxides and Subsequent Modification of Surface Reactivity
Due to their ubiquity and intrinsic reactivity, Fe(III) (hydr)oxides serve as an important sink for various metals and nutrients. Upon (a)biotic reductive dissolution, Fe(II) may repartition in the solid-phase and/or serve as a catalyst for Ostwald ripening of the (hydr)oxide substrate. Considering the variable reactivity of Fe(II, III, and II-III) phases coupled with the reducing capacity of Fe(II) species, secondary mineralization of Fe(III) (hydr)oxides will have a direct bearing on the fate and transport of numerous (in)organic constituents within the environment. Here we compare the operating secondary mineralization pathways of various Fe(III) (hydr)oxides (2-line ferrihydrite, goethite, hematite) following dissimilatory iron reduction within a minimal groundwater medium under advective flow. The conversion of Fe(III) (hydr)oxides occurs via a coupled biotic-abiotic reaction pathway such that bacterial-generated Fe(II) reacts abiotically with the residual (hydr)oxide surface. Upon Fe(II) reaction, the ferrihydrite surface is converted to goethite and/or magnetite being a function, for the most part, of bacterial- and flow-regulated Fe(II) concentrations. The operating mineralization pathway is a function of competitive reactions of Fe(II) with the ferrihydrite surface and stability of a lepidocrocite precursor. Conversely, the surfaces of goethite and hematite undergo atomic relaxation followed by electron delocalization within the bulk solid and/or minor organization to spinel-like domains. Surprisingly, while the surfaces of goethite and hematite become more disordered upon reductive dissolution and Fe(II) reaction, the reducing capacity and bioavailability decreases. Similarly, the extensive mineralization of ferrihydrite occludes the residual ferrihydrite leading to slower sustained bacterial reduction rates consistent with those of the more crystalline Fe(III) (hydr)oxides. Dissimilatory reduction of natural Fe(III) (hydr)oxides, however, results in the initial, preferential consumption of more recalcitrant Fe(III) phases (e.g. goethite) relative to ferrihydrite. Iron(II) reaction with natural Fe (hydr)oxide surfaces results solely in the internal ordering of ferrihydrite to hematite, most likely, a consequence of compromised surface reactivity by sorbed and/or coprecipitated ions. Thus, the operating mineralization pathways within soils and sediments undergoing Fe reduction will be a function of Fe(III) (hydr)oxide structural order and surface reactivity, which ultimately controls Fe(II) generation and ensuing surface conversion.