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Plume
Kinds of Plume Terms modified by Plume Selected AbstractsModelling the effect of buoyancy on the transport of anchovy (Engraulis capensis) eggs from spawning to nursery grounds in the southern Benguela: an IBM approachFISHERIES OCEANOGRAPHY, Issue 3 2003C. Parada Abstract An individual-based model (IBM) was used to investigate the effects of physical and biological variables on the transport via a jet current of anchovy (Engraulis capensis) eggs from spawning to the nursery grounds in the southern Benguela ecosystem. As transport of eggs and early larvae is considered to be one of the major factors impacting on anchovy recruitment success, this approach may be useful to understand further the recruitment variability in this economically and ecologically important species. By coupling the IBM to a 3D hydrodynamic model of the region called Plume, and by varying parameters such as the spatial and temporal location of spawning, particle buoyancy, and the depth range over which particles were released, we could assess the influences of these parameters on transport success. A sensitivity analysis using a General Linear Model identified the primary determinants of transport success in the various experimental simulations, and model outputs were examined and compared with patterns observed in field studies. Model outputs compared well with observed patterns of vertical and horizontal egg distribution. Particle buoyancy and area of particle release were the major single determinants of transport success, with an egg density of 1.025 g cm,3 maximizing average particle transport success and the western Agulhas Bank being the most successful spawning area. This IBM may be useful as a generic prototype for other upwelling ecosystems. [source] Stratigraphic and structural evolution of the Blue Nile Basin, Northwestern Ethiopian PlateauGEOLOGICAL JOURNAL, Issue 1 2009N. DS. Abstract The Blue Nile Basin, situated in the Northwestern Ethiopian Plateau, contains ,1400,m thick Mesozoic sedimentary section underlain by Neoproterozoic basement rocks and overlain by Early,Late Oligocene and Quaternary volcanic rocks. This study outlines the stratigraphic and structural evolution of the Blue Nile Basin based on field and remote sensing studies along the Gorge of the Nile. The Blue Nile Basin has evolved in three main phases: (1) pre-sedimentation phase, include pre-rift peneplanation of the Neoproterozoic basement rocks, possibly during Palaeozoic time; (2) sedimentation phase from Triassic to Early Cretaceous, including: (a) Triassic,Early Jurassic fluvial sedimentation (Lower Sandstone, ,300,m thick); (b) Early Jurassic marine transgression (glauconitic sandy mudstone, ,30,m thick); (c) Early,Middle Jurassic deepening of the basin (Lower Limestone, ,450,m thick); (d) desiccation of the basin and deposition of Early,Middle Jurassic gypsum; (e) Middle,Late Jurassic marine transgression (Upper Limestone, ,400,m thick); (f) Late Jurassic,Early Cretaceous basin-uplift and marine regression (alluvial/fluvial Upper Sandstone, ,280,m thick); (3) the post-sedimentation phase, including Early,Late Oligocene eruption of 500,2000,m thick Lower volcanic rocks, related to the Afar Mantle Plume and emplacement of ,300,m thick Quaternary Upper volcanic rocks. The Mesozoic to Cenozoic units were deposited during extension attributed to Triassic,Cretaceous NE,SW-directed extension related to the Mesozoic rifting of Gondwana. The Blue Nile Basin was formed as a NW-trending rift, within which much of the Mesozoic clastic and marine sediments were deposited. This was followed by Late Miocene NW,SE-directed extension related to the Main Ethiopian Rift that formed NE-trending faults, affecting Lower volcanic rocks and the upper part of the Mesozoic section. The region was subsequently affected by Quaternary E,W and NNE,SSW-directed extensions related to oblique opening of the Main Ethiopian Rift and development of E-trending transverse faults, as well as NE,SW-directed extension in southern Afar (related to northeastward separation of the Arabian Plate from the African Plate) and E,W-directed extensions in western Afar (related to the stepping of the Red Sea axis into Afar). These Quaternary stress regimes resulted in the development of N-, ESE- and NW-trending extensional structures within the Blue Nile Basin. Copyright © 2008 John Wiley & Sons, Ltd. [source] Irreversible Phosphorus Sorption in Septic System Plumes?GROUND WATER, Issue 1 2008W. D. Robertson The mobility of phosphorus (P) in septic system plumes remains a topic of debate because of the considerable reactivity of this constituent. In this study, a septic system plume in Ontario was monitored over a 16-year period with detail that clearly shows the advancing frontal portion of the P plume. This monitoring record provides insight into the extent of secondary P attenuation in the ground water zone beyond that available from previous studies. A P plume 16 m in length developed over the monitoring period with PO4 -P concentrations (3 to 6 mg/L) that approached the concentrations present under the tile bed. Simulations using an analytical model showed that when first-order solute decay was considered to account for the possibility of secondary P attenuation in the ground water zone, field values could only be matched when decay was absent or occurred at an exceedingly slow rate (half-life greater than 30 years). Thus, hypothesized secondary P attenuation mechanisms such as slow recystallization of sorbed P into insoluble metal phosphate minerals, diffusion into microsites, or kinetically slow direct precipitation of P minerals such as hydroxyapatite were inactive in the ground water zone at this site or occurred at rates that were too slow to be observed in the context of the current 16-year study. Desorption tests on sediment samples from below the tile bed indicated a PO4 distribution coefficient (Kd) of 4.8, which implies a P retardation factor of 25, similar to the field apparent value of 37 determined from model calibrations. This example of inactive secondary P attenuation in the ground water zone shows that phosphorus in some ground water plumes can remain mobile and conservative for decades. This has important implications for septic systems located in lakeshore environments when long-term usage scenarios are considered. [source] Particle-in-Cell Simulation of Stationary Plasma ThrusterCONTRIBUTIONS TO PLASMA PHYSICS, Issue 8-9 2007F. Taccogna Abstract A very good example for the application of PIC techniques for detailed studies of low-temperature plasmas is the Hall thrusters. Here, a variety of models with different complexities are needed to get better insight into the physics of these systems. Particular emphasis has been spent for the geometrical scaling, for the simulation of the plasma-wall interaction inside the acceleration channel and for ion-neutral collision into the plume emitted from the thruster. Results show the axial acceleration mechanism, the secondary electron emission instability, the azimuthal fluctuations into the channel and the ion backflow and electron trapping in the plume. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Laser plasma plume kinetic spectroscopy of the nitrogen and carbon speciesCONTRIBUTIONS TO PLASMA PHYSICS, Issue 7 2003Z. Zelinger Abstract The formation and decay of carbon and nitrogen atoms, CN radicals and C2 molecules were monitored using spatial- and time-resolved emission spectroscopy in a plasma plume formed during laser ablation of a graphite target in nitrogen atmosphere. A simple exponential model was used to explain the effect of the individual chemical reactions and plasma dynamics on the measured kinetic characteristics. The succession of emissions C , N , CN was observed in the time-resolved spectra, supporting the suggestion that the CN radical is formed mainly by the direct reaction C + N , CN or C2 + N2 , 2CN. The formation of CN radical was enhanced by the additional generation of atomic nitrogen through the RF discharge. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Distribution, phylogenetic diversity and physiological characteristics of epsilon- Proteobacteria in a deep-sea hydrothermal fieldENVIRONMENTAL MICROBIOLOGY, Issue 10 2005Satoshi Nakagawa Summary Epsilon- Proteobacteria is increasingly recognized as an ecologically significant group of bacteria, particularly in deep-sea hydrothermal environments. In this study, we studied the spatial distribution, diversity and physiological characteristics of the epsilon- Proteobacteria in various microbial habitats in the vicinity of a deep-sea hydrothermal vent occurring in the Iheya North field in the Mid-Okinawa Trough, by using culture-dependent and -independent approaches. The habitats studied were inside and outside hydrothermal plume, and annelid polychaete tubes. In addition, we deployed colonization devices near the vent emission. The polychaete tubes harboured physiologically and phylogenetically diverse microbial community. The in situ samplers were predominantly colonized by epsilon -Proteobacteria. Energy metabolism of epsilon- Proteobacteria isolates was highly versatile. Tree topology generated from the metabolic traits was significantly different (P = 0.000) from that of 16S rRNA tree, indicating current 16S rRNA gene-based analyses do not provide sufficient information to infer the physiological characteristics of epsilon- Proteobacteria. Nevertheless, culturability of epsilon- Proteobacteria in various microbial habitats differed among the phylogenetic subgroups. Members of Sulfurimonas were characterized by the robust culturability, and the other phylogenetic subgroups appeared to lose culturability in seawater, probably because of the sensitivity to oxygen. These results provide new insight into the ecophysiological characteristics of the deep-sea hydrothermal vent epsilon- Proteobacteria, which has never been assessed by comparative analysis of the 16S rRNA genes. [source] Eliminating a sulfuric acid mist plume from a wet caustic scrubber on a petroleum coke calcinerENVIRONMENTAL PROGRESS & SUSTAINABLE ENERGY, Issue 3 2001Charles A. Brown Reducing sulfuric acid mist emissions and eliminating the visible plume that remained after steam dissipation were the objectives of a project recently completed for a petroleum coke calciner. The coke calciner produces flue gas containing SO2 that is treated with a wet caustic scrubber. The scrubber is extremely efficient at removing most of the SO2, but some oxidizes to SO3, catalyzed by vanadium in the coke dust that collects in the waste heat boiler. Submicron droplets of H2SO4 form when the flue gas is quenched by the scrubber liquor, and makes its way through the scrubber. This paper describes the installation of, and performance test results for, a wet electrostatic precipitator (WESP), as well as SO2 scrubber modifications that were re q u i red to make room for the WESP. The successful project significantly reduced sulfuric acid mist emissions, eliminating the visible plume while maintaining a very low SO2 outlet concentration, even after removing one of three gas-liquid contactors. [source] Biomarker study of a municipal effluent dispersion plume in two species of freshwater musselsENVIRONMENTAL TOXICOLOGY, Issue 3 2002F. Gagné Abstract The toxicological effects of a primary-treated municipal effluent plume were investigated in two species of freshwater mussels, Elliptio complanata and Dreissena polymorpha, exposed for 62 days at sites upstream and downstream of an effluent outfall in the St. Lawrence River (Quebec, Canada). Levels of metallothioneins (MT), cytochrome P4501A1 activity, DNA damage, total lipids, relative levels of vitellins, and phagocytic activity (in E. complanata hemocytes) were determined after the exposure period. A parallel analysis measured heavy metals and coprostanol in mussel tissues. The results show that significant levels of coprostanol and some metals (specifically, Cu, Hg, Sb, Se, and Zn) had accumulated in mussels caged 5 km downstream of the effluent plume. Mixed-function oxidase activity, MT in gills, total lipids, DNA damage (in D. polymorpha only), and total hemolymph bacteria (in E. complanata only) had increased in these mussels, while levels of total cadmium (Cd), MT in digestive glands or whole soft tissues, phagocytic activity, and DNA damage in the digestive gland (in E. complanata only) were diminished. The exposure of mussels to surface waters contaminated by a municipal effluent led to many stress responses, depending on both the tissues and the species being examined. © 2002 Wiley Periodicals, Inc. Environ Toxicol 17: 149,159, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/tox.10046 [source] Immunocompetence of bivalve hemocytes as evaluated by a miniaturized phagocytosis assayENVIRONMENTAL TOXICOLOGY, Issue 3 2002C. Blaise Abstract Immune function in bivalves can be adversely affected by long-term exposure to environmental contaminants. Investigating alterations in immunity can therefore yield relevant information about the relationship between exposure to environmental contaminants and susceptibility to infectious diseases. We have developed a rapid, cost-effective, and miniaturized immunocompetence assay to evaluate the phagocytic activity, viability, and concentration of hemocytes in freshwater and marine bivalves. Preliminary experiments were performed to optimize various aspects of the assay including 1) the time required for adherence of hemocytes to polystyrene microplate wells, 2) the time required for internalization of fluorescent bacteria, 3) the ratio of hemocytes to fluorescent bacteria in relation to phagocytosis, 4) hemolymph plasma requirements, and 5) the elimination of fluorescence from (noninternalized) bacteria adhering to the external surface of hemocytes. The results of these experiments showed the optimal adherence time for hemocytes in microplate wells to be 1 h, that phagocytosis required at least 2 h of contact with fluorescently labeled E. coli cells, that the number of fluorescent E. coli cells had a positive effect on phagocytic activity, that at least 2.5 million cells/mL were required to measure a significant intake, and that a linear increase in uptake of bacteria (R = 0.91; p < 0.01) could be obtained with concentrations of up to 1.3 × 106 hemocytes/mL. Afterward, the assay was used in two field studies to identify sites having the potential to affect the immunocompetence of bivalves. The first study was conducted on Mya arenaria clams collected at selected contaminated sites in the Saguenay River (Quebec, Canada), and the second examined Elliptio complanata freshwater bivalves that had been exposed to a municipal effluent plume in the St. Lawrence River (Quebec, Canada). In the Saguenay River field study a significant increase in phagocytosis was observed at sites closest to polluted areas. Phagocytotic activity varied over time and was highest during the warmest months (June, July, and August), closely paralleling the spawning period of Mya arenaria clams. In contrast, a drop in phagocytic activity was observed in Elliptio complanata mussels exposed to surface water 4 km downstream of a major municipal effluent plume, with a concomitant increase in the number of hemocytes in the hemolymph. It appears that both immunosuppressive and immunostimulative effects are likely to occur in the field and that responses will be influenced by the type and intensity of contaminants at play. © 2002 Wiley Periodicals, Inc. Environ Toxicol 17: 160,169, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/tox.10047 [source] Using spatial models and kriging techniques to optimize long-term ground-water monitoring networks: a case studyENVIRONMETRICS, Issue 5-6 2002Kirk Cameron Abstract In a pilot project, a spatial and temporal algorithm (geostatistical temporal,spatial or GTS) was developed for optimizing long-term monitoring (LTM) networks. Data from two monitored ground-water plumes were used to test the algorithm. The primary objective was to determine the degree to which sampling, laboratory analysis, and/or well construction resources could be pared without losing key statistical information concerning the plumes. Optimization of an LTM network requires an accurate assessment of both ground-water quality over time and trends or other changes in individual monitoring wells. Changes in interpolated concentration maps over time indicate whether ground-water quality has improved or declined. GTS separately identifies temporal and spatial redundancies. Temporal redundancy may be reduced by lengthening the time between sample collection. Spatial redundancy may be reduced by removing wells from the network which do not significantly impact assessment of ground-water quality. Part of the temporal algorithm in GTS involves computation of a composite temporal variogram to determine the least redundant overall sampling interval. Under this measure of autocorrelation between sampling events, the lag time at which the variogram reaches a sill is the sampling interval at which same-well measurements lack correlation and are therefore non-redundant. The spatial algorithm assumes that well locations are redundant if nearby wells offer nearly the same statistical information about the underlying plume. A well was considered redundant if its removal did not significantly change: (i) an interpolated map of the plume; (ii) the local kriging variances in that section of the plume; and (iii) the average global kriging variance. To identify well redundancy, local kriging weights were accumulated into global weights and used to gauge each well's relative contribution to the interpolated plume map. By temporarily removing that subset of wells with the lowest global kriging weights and re-mapping the plume, it was possible to determine how many wells could be removed without losing critical information. Test results from the Massachusetts Military Reserve (MMR) indicated that substantial savings in sampling, analysis and operational costs could be realized by utilizing GTS. Annual budgetary savings that would accrue were estimated at between 35 per cent and 5 per cent for both LTM networks under study.Copyright © 2002 John Wiley & Sons, Ltd. [source] Laser-induced breakdown spectroscopy for soil diagnosticsEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 2 2001J. Bublitz Summary Laser-optical measurements and fibre optics are potentially attractive tools for applications in soil science because of their great sensitivity and selectivity and their capabilities for on-line and in situ analysis. We have investigated laser-induced breakdown spectroscopy (LIBS) for the quantitative detection of metal ions on the surface of natural soil samples from two sites (Hohenschulen and Oderbruch, Germany). The LIBS technique allows the spatially resolved investigation of adsorption and desorption effects of ions in soil. A frequency doubled (532 nm) and Q-switched Nd:YAG laser with a pulse duration of 8 ns is focused on the soil surface and induces a plasma. Typical power densities are 150 mJ mm,2. The plasma emission is recorded in time and spectrally resolved by a gateable optical multichannel analyser (OMA). A delay time of about 500 ns between laser pulse and OMA gate was used to resolve single atomic and ionic spectral lines from the intense and spectrally broad light that is emitted by the plasma itself. The dependency of the LIBS signal of a single spectral line on the amount of water in the sample is investigated in detail. The results indicate that quenching of water in the plasma plume reduces the line intensities, while the interaction with aquatic colloids increases the intensity. The two processes compete with each other, and a non-linear correlation between measured line intensities and the amount of water in the sample is obtained. This is verified by a simple computer simulation and has to be taken into account for the quantitative interpretation of LIBS signals, e.g. when absolute concentrations are estimated. In the present investigation natural calcium concentrations <,2 ,g kg,1 were measured with the LIBS technique in the samples for the two test sites. In addition, measurements were made with dry and water-saturated BaCl2 mixed soil samples, and no significant difference in the detection limit for barium was obtained. [source] Growth and movement patterns of early juvenile European anchovy (Engraulis encrasicolus L.) in the Bay of Biscay based on otolith microstructure and chemistryFISHERIES OCEANOGRAPHY, Issue 3 2010NAROA ALDANONDO Abstract Various hypotheses have been put forward to explain the mechanisms in the Bay of Biscay that result in a good recruitment of European anchovy. Anchovy larvae from the spawning area in the Gironde River plume are advected towards off-shelf waters, where juveniles are commonly observed. Otolith microstructural and chemical analysis were combined to assess the importance of this off-shelf transport and to determine the relative contribution of these areas for anchovy survival. Chemical analysis of otoliths showed that anchovy juveniles in the Bay of Biscay can be divided into two groups: a group that drifts towards off-shelf waters early in their life and returns later, and a group that remains in the low salinity waters of the coastal area. The first group presents significantly faster growth rates (0.88 mm day,1) than those remaining in the coastal waters (0.32 mm day,1). This may be due to off-shelf waters being warmer in spring/summer, and to the fact that the lower food concentration is compensated for by higher prey visibility. Furthermore, the group of juveniles that drifted off the spawning area and had faster growth rates represents 99% of the juvenile population. These findings support the hypothesis that anchovy in the Bay of Biscay may use off-shelf waters as a spatio-temporal loophole, suggesting that transport off the shelf may be favourable for recruitment. [source] Real-time quadrupole mass spectrometer analysis of gas in borehole fluid samples acquired using the U-tube sampling methodologyGEOFLUIDS (ELECTRONIC), Issue 3 2006B. M. FREIFELD Abstract Sampling of fluids in deep boreholes is challenging because of the necessity of minimizing external contamination and maintaining sample integrity during recovery. The U-tube sampling methodology was developed to collect large volume, multiphase samples at in situ pressures. As a permanent or semi-permanent installation, the U-tube can be used for rapidly acquiring multiple samples or it may be installed for long-term monitoring applications. The U-tube was first deployed in Liberty County, TX to monitor crosswell CO2 injection as part of the Frio CO2 sequestration experiment. Analysis of gases (dissolved or separate phase) was performed in the field using a quadrupole mass spectrometer, which served as the basis for determining the arrival of the CO2 plume. The presence of oxygen and argon in elevated concentrations, along with reduced methane concentration, indicates sample alteration caused by the introduction of surface fluids during borehole completion. Despite producing the well to eliminate non-native fluids, measurements demonstrate that contamination persists until the immiscible CO2 injection swept formation fluid into the observation wellbore. [source] Controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics: differences and similaritiesGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2009Evgueni Burov SUMMARY Mantle plume activity and lithospheric folding by far-field stresses exerted from plate boundaries are two important end-members as mechanisms for continental intraplate deformation. The topographic expression of mantle plume impingement on continental lithosphere and lithospheric folding has some striking similarities. Observations from a number of areas in Europe's intraplate lithosphere demonstrate that these mechanisms commonly interact in space and time. We present the results of thermomechanical modelling addressing the role of factors such as the presence of a hot upper mantle, the spatial dimensions of the plume and the time constants involved in the temporal succession of plume activity and lithospheric folding by stress accumulation in intraplate continental lithosphere. The results demonstrate that both the processes, plume,lithosphere interactions and folding may interact resulting either in strong amplification, attenuation or modification of their surface expression. These inferences are compatible with a number of key observations on the nature and the temporal succession of topography evolution in the Alpine foreland, the Pannonian Basin, the Scandinavian continental margin and the Iberian Peninsula. [source] Upwelling plumes, superswells and true polar wanderGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004Marianne Greff-Lefftz SUMMARY The geological evolution of the rotational axis of the Earth is most likely controlled by internal mass redistribution within the mantle. Palaeomagnetic observations suggest that it is episodic in nature, with periods of quasi-standstill alternating with periods of faster wander. Here, we investigate two models for the influence of mantle plumes that vary at different spatial wavelengths on the time variations of the rotational axis (true polar wander; TPW). In the first model, we represent an upwelling plume as a sphere whose radius varies as a function of the flux of material in the conduit and that traverses the mantle at the Stokes velocity. Such a plume produces very little wander of the rotational axis. We then study the effects of two superswells that mimic the ones observed with seismic tomography and conclude that a doming regime within the mantle involves significant polar wander. Some of the features of this TPW that are directly linked to the periodicity of doming are reminiscent of observed phases of slow and fast TPW, with similar peak velocities. [source] Cretaceous,Tertiary geodynamics: a North Atlantic exerciseGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2001Trond H. Torsvik Summary New reconstructions are presented for the Cretaceous,Early Tertiary North Atlantic using a combination of palaeomagnetic, hotspot and magnetic anomaly data. We utilize these reconstructions in an analysis of previously described misfits between the North Atlantic Plate elements at successive intervals during this time period. We are able to achieve reasonable overlap between the hotspot and palaeomagnetic reconstructions between 40 and 95 Ma and thus are able to support the idea that the Indo,Atlantic hotspots are relatively stationary. Small, but systematic discrepancies for this time interval can readily be modelled with a long-term, octopole non-dipole field contribution (G3 = g30/g10 = 0.08). However, hotspot and palaeomagnetic reconstructions for the Early Cretaceous North Atlantic show substantial differences that cannot be explained by constant, non-dipole fields and we favour an explanation for these discrepancies in terms of true polar wander (TPW) triggered by mantle instabilities between 125 and 95 Ma; this constitutes the only identifiable event of significant TPW since the Early Cretaceous. Taken in the context of available geochronological and geological data and seismic tomography from the region, the 95,40 Ma reconstructions and their time-consequent geological products are interpreted in terms of specific conditions of mantle-crust coupling and global plate motions/tectonic activity. Highlights from these reconstructions show uniform NE movement of the coupled North American, Greenland and Eurasian plates from 95 to 80 Ma; a marked cusp in the paths for all three elements at 80 Ma where the three plates simultaneously change direction and follow a uniform NW-directed motion until c. 20 Ma when Eurasia diverges NE, away from the still-NW-moving Greenland and North American elements. Positioning of the Iceland plume beneath the spreading-ridge at 20 Ma may have increased upwelling below the ridge, increased the ridge-push, and caused a NE shift in the absolute direction of Eurasia. [source] Seismic evidence for a mantle plume oceanwards of the Kamchatka,Aleutian trench junctionGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2001A. Gorbatov Summary A non-linear iterative P- wave traveltime tomography has revealed a mantle plume originating at a depth of nearly 1000 km, rising across the 600 km discontinuity, and deflecting subhorizontally in the uppermost mantle presumably by shear flow due to the overlying moving plate. Data from the Geophysical Survey of Russia (1955,1997) were inverted jointly with the catalogues of International Seismological Centre and USGS National Earthquake Information Centre (1964, 1998). The result shows a 300,500 km-wide cylindrical low-velocity anomaly (, , 2 per cent) that extends from a depth of greater than 900 km to shallower than 200 km. The anomaly is almost vertical at depths up to ,400 km and rises obliquely to the north up to ,200 km under the ocean floor near the northern end of Emperor seamounts. Above ,300 km depth a subsidiary anomaly extends subhorizontally to the NW in fair agreement with the direction of movement of the Pacific Plate. The overlying seafloor is characterized by anomalously high heat flow, which may be attributed to the thermal effect of the mantle plume. [source] Analyses of the stress field in southeastern France from earthquake focal mechanismsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2001Emmanuel Baroux Summary Owing to the apparent deformation field heterogeneity, the stress regimes around the Provence block, from the fronts of the Massif Central and Alpine range up to the Ligurian Sea, have not been well defined. To improve the understanding of the SE France stress field, we determine new earthquake focal mechanisms and compute the present-day stress states by inversion of the 89 available focal mechanisms around the Provence domain, including 17 new ones calculated in the current study. This study provides evidence of six distinct deformation domains around the Provence block, with different tectonic regimes. On a regional scale, we identify three zones characterized by significantly different stress regimes: a western one affected by an extensional stress (normal faulting) regime; a southeastern one characterized by a compressional stress (reverse to strike-slip faulting) regime with NNW- to WNW-trending ,1; and a northeastern one, namely the Digne nappe front, marked by a NE-trending compression. Note that the Digne nappe back domain is controlled by an extensional regime that is deforming the western Alpine core. This extensional regime could be a response to buoyancy forces related to the Alpine high topography. The stress regimes in the southeast of the Argentera Massif and around the Durance fault are consistent with a coherent NNW-trending ,1, implying a left-lateral component of the active reverse oblique slip of the Moyenne Durance Fault. In the Rhone Valley, an E-trending extension characterizes the tectonic regime, implying a normal component of the present-day N,^mes fault displacement. This study provides evidence for short-scale variation of the stress states, which arises from abrupt changes in the boundary force influences on upper crustal fragments (blocks). These spatial stress changes around the Provence block result from the coeval influence of forces applied at both its extremities, namely in the northeast the Alpine front push, and in the southeast the northward African plate drift. In addition to these boundary forces, the mantle plume under the Massif Central influences the western block boundary. [source] Crustal structure of central and northern Iceland from analysis of teleseismic receiver functionsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000Fiona A. Darbyshire We present results from a teleseismic receiver function study of central and northern Iceland, carried out during the period 1995,1998. Data from eight broad-band seismometers installed in the SIL network operated by the Icelandic Meteorological Office were used for analysis. Receiver functions for each station were generated from events for a wide range of backazimuths and a combination of inversion and forward modelling was used to infer the crustal structure below each station. The models generated show a considerable variation in the nature and thickness of the crust across Iceland. The thinnest crust (20,21 km) is found in the northern half of the Northern Volcanic Zone approximately 120 km north of the centre of the Iceland mantle plume. Thicker crust (24,30 km) is found elsewhere in northern and central Iceland and the thickest crust (37 km) is found close to the plume centre. Velocity,depth profiles show a distinct division of the crust into two main sections, an upper high-velocity-gradient section of thickness 2,8 km and a lower crustal section with small or zero overall velocity gradient. The thickness of the upper crust correlates with the tectonic structure of Iceland; the upper crust is thickest on the flanks of the northern and central volcanic rift zones and thinnest close to active or extinct central volcanoes. Below the Krafla central volcano in northeastern Iceland the receiver function models show a prominent low-velocity zone at 10,15 km depth with minimum shear wave velocities of 2.0,2.5 km s,1. We suggest that this feature results from the presence of partially molten sills in the lower crust. Less prominent low-velocity zones found in other regions of Iceland may arise from locally high temperatures in the crust or from acidic intrusive bodies at depth. A combination of the receiver function results and seismic refraction results constrains the crustal thickness across a large part of Iceland. Melting by passive decompression of the hot mantle below the rift zone in northern Iceland forms a crust of thickness ,20 km. In contrast, the larger crustal thickness below central Iceland probably arises from enhanced melt production due to active upwelling in the plume core. [source] A radiomagnetotelluric survey on an oil-contaminated area near the Brazi Refinery, RomaniaGEOPHYSICAL PROSPECTING, Issue 3 2005B. Tezkan ABSTRACT Scalar radiomagnetotelluric measurements were carried out on a contaminated test area close to the Brazi Refinery in Romania in order to detect and to monitor a 1 m thick oil layer expected at 5 m depth. Radio transmitters broadcasting in a frequency range from 10 kHz to 300 kHz were selected to observe the apparent resistivity and the phase data associated with the E- and B-polarizations. They were located parallel and perpendicular to the assumed strike direction of the contamination plume. The data were interpreted by a 2D inversion technique from which the conductivity structure of the area was derived. The 2D inversion models of all profiles on the contaminated area show a poor-conductivity zone above the groundwater table which could be associated with the oil contamination. A first attempt was also made to monitor the contaminated layer: the radiomagnetotelluric measurements were repeated on the same profiles a year later, but this time in a dry period, not in a rainy one. The 2D inversion results of the measurements in the dry period indicate that the high-resistivity layer moved closer to the surface. Additional reference measurements were then carried out on a non-contaminated area situated at a distance from the refinery, in the opposite direction to the flow of the groundwater. These reference measurements were used for the derivation of the unperturbed geology and they were also compared with the measurements of the contaminated test area. There is a significant difference in the frequency dependences of the apparent resistivities of the reference and contaminated areas, which could indicate a contamination at shallow depth. The 2D inversion results show the increase of resistivity at a depth of about 5 m beneath the contaminated area where the oil contamination is expected according to the information from the boreholes. [source] Irreversible Phosphorus Sorption in Septic System Plumes?GROUND WATER, Issue 1 2008W. D. Robertson The mobility of phosphorus (P) in septic system plumes remains a topic of debate because of the considerable reactivity of this constituent. In this study, a septic system plume in Ontario was monitored over a 16-year period with detail that clearly shows the advancing frontal portion of the P plume. This monitoring record provides insight into the extent of secondary P attenuation in the ground water zone beyond that available from previous studies. A P plume 16 m in length developed over the monitoring period with PO4 -P concentrations (3 to 6 mg/L) that approached the concentrations present under the tile bed. Simulations using an analytical model showed that when first-order solute decay was considered to account for the possibility of secondary P attenuation in the ground water zone, field values could only be matched when decay was absent or occurred at an exceedingly slow rate (half-life greater than 30 years). Thus, hypothesized secondary P attenuation mechanisms such as slow recystallization of sorbed P into insoluble metal phosphate minerals, diffusion into microsites, or kinetically slow direct precipitation of P minerals such as hydroxyapatite were inactive in the ground water zone at this site or occurred at rates that were too slow to be observed in the context of the current 16-year study. Desorption tests on sediment samples from below the tile bed indicated a PO4 distribution coefficient (Kd) of 4.8, which implies a P retardation factor of 25, similar to the field apparent value of 37 determined from model calibrations. This example of inactive secondary P attenuation in the ground water zone shows that phosphorus in some ground water plumes can remain mobile and conservative for decades. This has important implications for septic systems located in lakeshore environments when long-term usage scenarios are considered. [source] Determination of Transverse Dispersion Coefficients from Reactive Plume LengthsGROUND WATER, Issue 2 2006Olaf A. Cirpka With most existing methods, transverse dispersion coefficients are difficult to determine. We present a new, simple, and robust approach based on steady-state transport of a reacting agent, introduced over a certain height into the porous medium of interest. The agent reacts with compounds in the ambient water. In our application, we use an alkaline solution injected into acidic ambient water. Threshold values of pH are visualized by adding standard pH indicators. Since aqueous-phase acid-base reactions can be considered practically instantaneous and the only process leading to mixing of the reactants is transverse dispersion, the length of the plume is controlled by the ratio of transverse dispersion to advection. We use existing closed-form expressions for multidimensional steady-state transport of conservative compounds in order to evaluate the concentration distributions of the reacting compounds. Based on these results, we derive an easy-to-use expression for the length of the reactive plume; it is proportional to the injection height squared, times the velocity, and inversely proportional to the transverse dispersion coefficient. Solving this expression for the transverse dispersion coefficient, we can estimate its value from the length of the alkaline plume. We apply the method to two experimental setups of different dimension. The computed transverse dispersion coefficients are rather small. We conclude that at slow but realistic ground water velocities, the contribution of effective molecular diffusion to transverse dispersion cannot be neglected. This results in plume lengths that increase with increasing velocity. [source] Ground Water Discharge and Nitrate Flux to the Gulf of MexicoGROUND WATER, Issue 3 2004Carolyn B. Dowling Ground water samples (37 to 186 m depth) from Baldwin County, Alabama, are used to define the hydrogeology of Gulf coastal aquifers and calculate the subsurface discharge of nutrients to the Gulf of Mexico. The ground water flow and nitrate flux have been determined by linking ground water concentrations to 3H/3He and 4He age dates. The middle aquifer (A2) is an active flow system characterized by postnuclear tritium levels, moderate vertical velocities, and high nitrate concentrations. Ground water discharge could be an unaccounted source for nutrients in the coastal oceans. The aquifers annually discharge 1.1 ± 0.01 × 108 moles of nitrate to the Gulf of Mexico, or 50% and 0.8% of the annual contributions from the Mobile-Alabama River System and the Mississippi River System, respectively. In southern Baldwin County, south of Loxley, increasing reliance on ground water in the deeper A3 aquifer requires accurate estimates of safe ground water withdrawal. This aquifer, partially confined by Pliocene clay above and Pensacola Clay below, is tritium dead and contains elevated 4He concentrations with no nitrate and estimated ground water ages from 100 to 7000 years. The isotopic composition and concentration of natural gas diffusing from the Pensacola Clay into the A3 aquifer aids in defining the deep ground water discharge. The highest 4He and CH4 concentrations are found only in the deepest sample (Gulf State Park), indicating that ground water flow into the Gulf of Mexico suppresses the natural gas plume. Using the shape of the CH4 -He plume and the accumulation of 4He rate (2.2 ± 0.8 ,cc/kg/1000 years), we estimate the natural submarine discharge and the replenishment rate for the A3 aquifer. [source] Natural Attenuation Reactions at a Uranium Mill Tailings Site, Western U.S.A.GROUND WATER, Issue 1 2002Chen Zhu This paper presents a modeling analysis of the geochemical evolution of a contaminated sandy aquifer at a uranium mill tailings site in the western United States. The tailings pond contains fluids having a pH of 1.5 to 3.5 and high levels of As, Be, Cd, Cr, Pb, Mo, Ni, Se, 226Ra, 228Ra, 230Th, 238U, and 234U. Seepage of tailings fluids into the aquifer has formed a low-pH ground water plume. The reclamation plan is to install a low-permeability cover on the tailings pond to stop the seepage and allow the plume to be attenuated by reactions with the aquifer matrix and flushed by uncontaminated upgradient ground water. To evaluate this reclamation scenario, ground water and sediment core samples were analyzed along one flowpath. Speciation-solubility and mass-transfer modeling revealed two sets of chemical reactions for acid seepage and flushing, respectively. The current concentrations and distribution of ground water constituents can be interpreted as being controlled by stepwise pH-buffer reactions with calcite, amorphous aluminum hydroxide, and amorphous iron hydroxides. These buffer reactions divide the aquifer into zones of near-constant pH, separated by interface zones. For the flushing stage, it is predicted that reactions with surface-bound species will dominate the reaction paths, and more pore volumes are required to neutralize the plume than predicted by models that do not consider surface reactions. Direct mineralogical and surface analysis is needed to substantiate this assertion. [source] Automatic Delineation of Capture Zones for Pump and Treat Systems: A Case Study in Piedmont, ItalyGROUND WATER MONITORING & REMEDIATION, Issue 2 2010Tiziana Tosco The design of a pump and treat (P&T) system for the hydraulic control of a contaminated plume in a confined aquifer is presented here. Being the system designed for the emergency containment of a nonaqueous phase liquid plume, the evaluation of the system's short-term efficiency was considered an important issue. For this reason, both time-related and ultimate capture zones were defined. They were traced using the automatic protection area (APA) model, a capture-zone delineation tool based on a hybrid forward-backward particle tracking algorithm, that provides an automatic post-processing encirclement of capture zones. Two simple indexes are here proposed for the evaluation of the performance of the hydraulic barrier, that is, the efficacy and efficiency indexes, calculated from the capture areas provided by APA. The discharge rates of the wells were dimensioned applying the APA algorithm, maximizing efficacy and efficiency of the barrier. Results proved both visually (via plotting of capture zones) and numerically (via calculation of the indexes) that the P&T system can provide a complete capture of the contaminated area and minimizes the volume of extracted water. Consequently, the APA algorithm was proved to be a useful tool in capture zone delineation. As a future perspective, it could be coupled with the real-time measurement of pumping rates and water levels and be implemented as a part of a tuning tool for the management of the hydraulic barrier. [source] Monitored Natural Attenuation of Manufactured Gas Plant Tar Mono- and Polycyclic Aromatic Hydrocarbons in Ground Water: A 14-Year Field StudyGROUND WATER MONITORING & REMEDIATION, Issue 3 2009Edward F. Neuhauser Site 24 was the subject of a 14-year (5110-day) study of a ground water plume created by the disposal of manufactured gas plant (MGP) tar into a shallow sandy aquifer approximately 25 years prior to the study. The ground water plume in 1988 extended from a well-defined source area to a distance of approximately 400 m down gradient. A system of monitoring wells was installed along six transects that ran perpendicular to the longitudinal axis of the plume centerline. The MGP tar source was removed from the site in 1991 and a 14-year ground water monitored natural attenuation (MNA) study commenced. The program measured the dissolved mono- and polycyclic aromatic hydrocarbons (MAHs and PAHs) periodically over time, which decreased significantly over the 14-year period. Naphthalene decreased to less than 99% of the original dissolved mass, with mass degradation rates of 0.30 per year (half-life 2.3 years). Bulk attenuation rate constants for plume centerline concentrations over time ranged from 0.33 ± 0.09 per year (half-life 2.3 ± 0.8 years) for toluene and 0.45 ± 0.06 per year (half-life 1.6 ± 0.2 years) for naphthalene. The hydrogeologic setting at Site 24, having a sandy aquifer, shallow water table, clay confining layer, and aerobic conditions, was ideal for demonstrating MNA. However, these results demonstrate that MNA is a viable remedial strategy for ground water at sites impacted by MAHs and PAHs after the original source is removed, stabilized, or contained. [source] Nitrate Removal Rates in a 15-Year-Old Permeable Reactive Barrier Treating Septic System NitrateGROUND WATER MONITORING & REMEDIATION, Issue 3 2008W.D. Robertson Permeable reactive barriers (PRBs) have gained popularity in recent years as a low-cost method for ground water remediation. However, their cost advantage usually requires that these barriers remain maintenance free for a number of years after installation. In this study, sediment cores were retrieved from a pilot-scale PRB consisting of a sand and wood particle (sawdust) mixture that has been in continuous operation for 15 years treating nitrate from a septic system plume in southern Ontario (Long Point site). Reaction rates for the 15-year-old media were measured in dynamic flow column tests and were compared to rates measured in year 1 using the same reactive mixture. Nitrate removal rates in the 15-year-old media varied, as expected, with temperature in the range of 0.22 to 1.1 mg N/L/d at 6 °C to 10 °C to 3.5 to 6.0 mg N/L/d at 20 °C to 22 °C. The latter rates remained within about 50% of the year 1 rates (10.2 ± 2.7 mg N/L/d at 22 °C). Near the end of the year 15 column test, media particles >0.5 mm in diameter, containing most of the wood particles, were removed from the reactive media by sieving. Nitrate removal subsequently declined by about 80%, indicating that the wood particles were the principal energy source for denitrification. This example shows that some denitrifying PRBs can remain maintenance free and be adequately reactive for decades. [source] Permanganate Treatment of an Emplaced DNAPL SourceGROUND WATER MONITORING & REMEDIATION, Issue 4 2007Neil R. Thomson In situ chemical oxidation (ISCO) using permanganate is one of the few promising technologies that have recently appeared with the capability of aggressively removing mass from nonaqueous phase liquid (NAPL) source zones. While NAPL mass in regions of the treatment zone where delivery is dominated by advection can be removed rather quickly, the rate of mass removal from stagnant zones is diffusion controlled. This gives rise to partial mass removal and a concomitant reduction in the NAPL mass, downgradient ground water concentrations, and the dissolution rate associated with the source zone. Therefore, monitoring the performance of a permanganate ISCO treatment system is important to maintain the desired efficiency and to establish a treatment end point. In this paper, we illustrate the use of various monitoring approaches to assess the performance of a pilot-scale investigation that involved treatment of a multicomponent NAPL residual source zone with permanganate using a ground water recirculation system for 485 d. Ongoing treatment performance was assessed using permanganate and chloride concentration data obtained from extraction wells, 98 piezometers located approximately 1 m downgradient from the source, and ground water profiling. At the completion of treatment, 23 intact soil cores were extracted from the source zone and used to determine the remaining NAPL mass and manganese deposition. Based on the data collected, more than 99% of the initial NAPL mass was removed during treatment; however, remnant NAPL was sufficient to generate a small but measurable dissolved phase trichloroethene (TCE) and perchloroethene (PCE) plume. As a result of treatment, the ambient-gradient discharge rates were reduced by 99% for TCE and 89% for PCE relative to baseline conditions. The lack of complete source zone oxidation was presumed to be the result of dissolution fingers, which channeled the permanganate solution through the source zone preventing direct contact with the NAPL and giving rise to diffusion-limited mass removal. [source] Subsurface Imaging of an Abandoned Solid Waste Landfill Site in Norman, OklahomaGROUND WATER MONITORING & REMEDIATION, Issue 2 2006Joseph T. Zume Leachate plume emanating from an old unlined municipal landfill site near the city of Norman, Oklahoma, is discharging into the underlying alluvial aquifer. Subsurface imaging techniques, electrical resistivity tomography and electrical conductivity (EC) logging, were used on the site to detect and map the position of the leachate plume. Anomalous EC zones, delineated with the two methods, correlated with the occurrence of the plume detected by water chemistry analyses from multilevel monitoring wells. Specific conductance, a potential indicator of leachate contamination, ranged from 1861 to 7710 ,S/cm in contaminated zones and from 465 to 2180 ,S/cm in uncontaminated ground water. Results are in agreement with those from earlier studies that the leachate plume emerges from the landfill along preferential pathways. Additionally, there are indications that the leading edge of the plume has migrated, at least, 200 m away from the landfill in the direction of ground water flow. [source] Pharmaceuticals and Other Organic Waste Water Contaminants Within a Leachate Plume Downgradient of a Municipal LandfillGROUND WATER MONITORING & REMEDIATION, Issue 2 2004Kimberlee K. Barnes Ground water samples collected from the Norman Landfill research site in central Oklahoma were analyzed as part of the U.S. Geological Survey (USGS) Toxic Substances Hydrology Program's national reconnaissance of pharmaceuticals and other organic waste water contaminants (OWCs) in ground water. Five sites, four of which are located downgradient of the landfill, were sampled in 2000 and analyzed for 76 OWCs using four research methods developed by the USGS. OWCs were detected in water samples from all of the sites sampled, with 22 of the 76 OWCs being detected at least once. Cholesterol (a plant and animal steroid), was detected at all five sites and was the only compound detected in a well upgradient of the landfill. N,N-diethyltoluamide (DEBT used in insect repellent) and tri(2-chloroethyl) phosphate (fire-retardant) were detected in water samples from all four sites located within the landfill-derived leachate plume. The sites closest to the landfill had more detections and greater concentrations of each of the detected compounds than sites located farther away. Detection of multiple OWCs occurred in the four sites located within the leachate plume, with a minimum of four and a maximum of 17 OWCs detected. Because the landfill was established in the 1920s and closed in 1985, many compounds detected in the leachate plume were likely disposed of decades ago. These results indicate the potential for long-term persistence and transport of some OWCs in ground water. [source] | |||||||||||||||||||||||||||||||||||||||||||