Subsurface Interactions Research Articles

Subsurface interactions of actinide species and microorganisms: Implications for the bioremediation of actinide-organic mixtures

By reviewing how microorganisms interact with actinides in subsurface environments, we assess how bioremediation controls the fate of actinides. Actinides often are co-contaminants with strong organic chelators, chlorinated solvents, and fuel hydrocarbons. Bioremediation can immobilize the actinides, biodegrade the co-contaminants, or both. Actinides at the IV oxidation state are the least soluble, and microorganisms accelerate precipitation by altering the actinide's oxidation state or its speciation. We describe how microorganisms directly oxidize or reduce actinides and how microbiological reactions that biodegrade strong organic chelators, alter the pH, and consume or produce precipitating anions strongly affect actinide speciation and, therefore, mobility. We explain why inhibition caused by chemical or radiolytic toxicities uniquely affects microbial reactions. Due to the complex interactions of the microbiological and chemical phenomema, mathematical modeling is an essential tool for research on and application of bioremedation involving co-contamination with actinides. We describe the development of mathematical models that link microbiological and geochemical reactions. Throughout, we identify the key research needs.

The impact of flash floods on microbial distribution and biogeochemistry in the parafluvial zone of a desert stream

Summary1. The impact of flash flooding on microbial distribution and biogeochemistry was investigated in the parafluvial zone (the part of the active channel lateral to the surface stream) of Sycamore Creek, a Sonoran Desert stream in central Arizona.2. It was hypothesized that subsurface bacteria were dependent on the import of algal‐derived organic matter from the surface stream, and it was therefore predicted that microbial numbers and rates of microbially mediated processes would be highest at locations of surface to subsurface hydrologic exchange and at times when algal biomass was high.3. Prior to a flash flood on 19 July 1994, chlorophyll a was high (≈ 400 mg m–2) in the surface stream and microbial numbers were highest at the stream–parafluvial interface and declined along parafluvial flowpaths, supporting the hypothesized algal–bacterial linkage. Immediately following the flash flood, chlorophyll a was low (≈ 7 mg m–2), and microbial numbers were reduced at the stream–parafluvial interface.4. Counter to expectations, parafluvial functioning (in terms of nitrate production and dissolved oxygen decline along flowpaths) re‐established immediately after the flood receded. Therefore, material other than algal exudates supported parafluvial metabolism immediately postflood, and terrestrially derived dissolved organic matter is the likely source.5. Algae in the surface stream recovered quickly following flooding, but recovery of parafluvial bacteria lagged somewhat behind. These results highlight the importance of surface–subsurface interaction to stream ecosystem functioning and show that the nature of these interactions changes substantially in successional time.

Effect of reaction kinetics on predicted concentration profiles during subsurface bioremediation

The lack of a complete mechanistic description of subsurface interactions between contaminants and microbes has limited the ability to predict the effectiveness of subsurface bioremediation processes. Important microbial processes which must be included in such mechanistic descriptions of bioremediation are microbial growth, death and transport, including microbial attachment to and detachment from the soil surfaces. Recent advances have provided a better understanding of the relationships between contaminant destruction rates and such microbial processes. In this manuscript, contaminant profiles predicted using more realistic descriptions are compared with those obtained using less complete, previously published mathematical models. The more comprehensive model described here includes mathematical descriptions for microbial growth and transport. Contaminant profiles predicted using this model are compared with those obtained when the contaminant destruction processes are described by assuming that (1) no reaction occurs, (2) the contaminant reacts instantaneously with any available oxygen, and (3) dual-substrate Monod kinetics without consideration of microbial growth and transport. This work demonstrates that if reaction kinetics are ignored, the size of the plume will be overpredicted at large times and if the reaction is assumed to occur instantaneously, the plume will be underpredicted at small times. Thus, if these processes are not included in the mathematical description of bioremediation, erroneous interpretations of site characterization data collected during this period would be likely.

Evaluating the performance of a deterministic daily rainfall–runoff model in a low-flow context

The characterization of a stream's low-flow regime is required for ecological purposes, water quality studies and various other water projects. If observed stream flow records are insufficient, low-flow characteristics may need to be estimated from simulated daily stream flow time-series. The model employed should conceptualize low-flow generation mechanisms and surface–subsurface interactions adequately. The ability of the model to simulate low-flow regimes may be assessed by means of various low-flow analysis techniques. This paper illustrates the approach using the example of the physically based, semi-distributed VTI daily rainfall–runoff model. The model has been applied to five perennial headwater catchments in South Africa, which are located in different parts of the country, represent different physiographical environments and are characterized by different baseflow responses. The model simulations are evaluated in terms of both conventional goodness-of-fit criteria and several low-flow measures such as recession characteristics, baseflow volumes, flow duration curves and continuous low-flow events below specified threshold discharges. For all the catchments considered the model has been found to perform successfully in terms of conventional fit statistics and flow duration curves. However, its ability to reproduce recession characteristics and continuous low-flow spells appears to be less satisfactory. This suggests that daily model simulations should be evaluated by low-flow criteria, which are frequently ignored in water resource assessment practices. © 1998 John Wiley & Sons, Ltd.

Pyrite composition and ore genesis in the Prince Lyell copper deposit, Mt Lyell mineral field, western Tasmania, Australia

The Prince Lyell copper-gold-silver deposit occurs in the late Cambrian Mt Read Volcanics, at Queenstown, Tasmania. Steeply plunging, broadly conformable lenses of disseminated and stringer pyrite-chalcopyrite mineralisation occur in quartz-sericite-chlorite rocks derived from intense alteration of predominantly felsic lavas and volcaniclastic rocks. Middle Devonian deformation has substantially modified primary sulphide textures. Although extensively fractured, pyrite grains in the ore have retained their original pre-deformation internal structure and chemistry which are revealed by etching and electron microprobe analysis. Earliest sulphide mineralisation produced oscillatory zoned, cobalt-rich pyrite (Pyrite I), coeval with chalcopyrite mineralisation. Cobalt-rich pyrite is commonly associated with Cambrian volcanic rocks in western Tasmania and suggests a volcanogenic origin for the ore fluids at Prince Lyell. Pyrite I was corroded by later hydrothermal fluids and reprecipitated as unzoned, trace element-poor pyrite (Pyrite II), commonly as overgrowths on Pyrite I cores. Minor amounts of a second cobalt-rich pyrite (Pyrite III) occurs with Pyrite II in composite pyrite overgrowths. Sulphur isotope ratios from all pyrite generations fall within a small range (3 to 11‰). In situ isotopic analyses showed no consistent δ 34S variation between the various pyrite generations, suggesting recycling of sulphur derived from a single Cambrian volcanogenic source. Hematite alteration, derived from oxidised fluids possibly from the adjacent hematitic Owen Conglomerate, occurs in the structural footwall volcanics and the Great Lyell fault zone. Hematite inclusions in Pyrite II and III indicate that these pyrite generations occurred after or during deposition of the conglomerate. It is postulated that Pyrite II and III were deposited during waning volcanism, contemporaneous with Owen Conglomerate sedimentation in the late Cambrian or early Ordovician. The Great Lyell fault would have acted as a growth fault margin between a terrestrial basin, filling rapidly from the east, and the volcanic terrane to the west. The scenario raises the possibility that the concentration of mineral deposits and hematitic alteration along the Great Lyell fault resulted from the subsurface interaction of reduced volcanogenic fluids and oxidised basin waters along the growth fault contact.

Scattering from a metallic object embedded near the randomly rough surface of a lossy dielectric

Two-dimensional electromagnetic scattering from a perfectly conducting target embedded near the randomly rough surface of an isotropic lossy dielectric is investigated. The randomly rough surface is illuminated by a finite width beam from an antenna in the free space above the surface, with off-normal incidence. Standard integral equation methods are applied and include all subsurface interactions between the object and rough surface. For a chosen embedded target, Monte Carlo simulations are performed for a selection of ensembles of rough surface types intended to be suggestive of natural ground. Far field scattering coefficient distributions and corresponding synthetic images suggest when the buried object should be discernible. Sensitivities are explored in terms of surface type, polarization of the incident field, depth and orientation of target, soil characteristics, incidence angle, and beamwidth. Many of the scattering features identified should also apply in 3D.

Spectroscopy of Plutonium-Organic Complexes

AbstractInformation on the spectroscopy of plutonium-organic complexes is needed to help establish the speciation of these complexes under environmentally relevant conditions. Laser photoacoustic spectroscopy (LPAS) and absorption spectrometry were used to characterize the Pu(IV)-citrate and Pu(IV)-nitrilotriacetic acid (NTA) complexes at concentrations of 1−3 – 10−7 M in aqueous solution. Good agreement was observed between the band shape of the LPAS and absorption spectra for the Pu(IV)-NTA complex. Agreement for the Pu(IV)-citrate complex was not quite as good. In both cases, a linear dependence of the LPAS signal on laser power and total concentration of the complexes was noted. This work is part of an ongoing research effort to study key subsurface interactions of plutonium-organic complexes.

Subsurface interactions of humic substances with copper(II) in saturated media. [Erratum to document cited in CA118(22):219123f

Subsurface interactions of humic substances with copper(II) in saturated media. [Erratum to document cited in CA118(22):219123f

Subsurface interactions of humic substances with copper(II) in saturated media

The interactions between natural organic matter (NOM) and Cu(II) during transport through saturated media were investigated with column experiments. Six sources of NOM represented different humic fractions. SiO 2 and α-Al 2 O 3 comprised the mineral surfaces. The experiments were conducted at pH 6.2 and 7.5. Total Cu(II) was measured by atomic absorption spectroscopy; NOM was measured as UV absorbance at 254 nm. Results showed variation in NOM breakthrough as a function of source. The presence of Cu(II) caused greater retardation of all NOM sources relative to the NOM-alone systems. The NOM effect on Cu(II) transport was mixed, depending on the NOM source. At pH 7.5, NOM greatly facilitated Cu(II) transport.

Approximating the scattering by a material-filled 2-D trough in an infinite plane using the impedance boundary condition

An application of the impedance boundary condition to 2-D trough scatterers is described in which the approximate field solutions are obtained at a much reduced computational cost compared with more exact numerical approaches. The geometries investigated have appreciable subsurface interactions that must be accounted for in the surface impedance. After a brief canonical study of the coated circular cylinder, the filled 2-D trough in a ground plane (which can be used as a model for an antenna-cavity mount) is analyzed and compared with a more exact numerical approach. The results of a parametric study of this geometry demonstrate that excellent far-field agreement is obtained when the material and structure satisfy only the requirements for the traditional impedance boundary condition. This model is also shown to provide a very good simulation for 2-D troughs with slanted sidewalls which produce inhomogeneous equivalent surface impedance representations. >