High-metallicity, photoionized gas in intergalactic large-scale filaments     

Bastien Aracil  Todd M. Tripp †  David V. Bowen  Jason X. Prochaska  Hsiao-Wen Chen  Brenda L. Frye 《Monthly notices of the Royal Astronomical Society》2006,367(1):139-155

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Oxygen chemical diffusion in three basaltic liquids at elevated temperatures and pressures     

Todd Dunn 《Geochimica et cosmochimica acta》1983,47(11):1923-1930

The diffusivity of oxygen has been measured in three basaltic liquids from 1280 to 1450°C and 4 to 21 kilobars using a solid media piston-cylinder apparatus. The measurements were done by monitoring the reduction of ferric iron in previously oxidized spheres of basalt melt. The compositions studied were olivine nephelinite, alkali basalt, and 1921 Kilauea tholeiite.The isobaric temperature dependence of oxygen diffusivity is adequately described by Arrhenius relationships for the three liquids studied. Arrhenius activation energies were determined at 12 kilobars for olivine nephelinite (62± 6 kcal/mole) and tholeiite (51 ± 4 kcal/mole) and at 4, 12, and 20 kilobars for alkali basalt (70 ± 7, 86 ± 6, and 71 ± 14 kcal/mole, respectively). The Arrhenius parameters for the three compositions define a compensation law which is indistinguishable from those for oxygen diffusion in simple silicate melts (DUNN, 1982) and for divalent cation diffusion in basaltic melts (Hofmann, 1980). These results suggest that the principal species contributing to the total diffusivity of oxygen is the oxide anion (O^2?).The isothermal pressure dependence of oxygen diffusion is complex and quite different from that observed for cationic diffusion in silicate melts. All three compositions show a sharp decrease in oxygen diffusivity at approximately the same pressure as the change in the liquidus phase from olivine to pyroxene, but otherwise the pressure dependence can be described by Arrhenius type equations. The equations yield negative activation volumes for the olivine nehpelinite and the alkali basalt. The activation volumes determined for the tholeiite are near zero at low pressure and positive at high pressure. A negative activation volume represents a decrease in the average size of the principal diffusing species.The results of this study are consistent with a melt model which includes both continuous changes in the relative proportions of the various anionic species in the melt with pressure and the occurrence of anionic disproportionation reactions within narrow pressure ranges.  相似文献   

Oxygen diffusion in three silicate melts along the join diopside-anorthite     

Todd Dunn 《Geochimica et cosmochimica acta》1982,46(11):2293-2299

The self-diffusion of oxygen has been measured for three silicate melts along the join diopsideanorthite. The experiments were done by isotope exchange between an “infinite” reservoir of oxygen gas and spheres of melt. The oxygen self-diffusion coefficients for the three melts are given as: C-1(diopside): D = 1.64 × 10¹ exp(?(63.2 ± 20)(kcal/mole)/RT) cm²/sec C-2(Di₅₈An₄₂): D = 1.35 × 10^?1 exp(?(46.8 ± 9)(kcal/mole)/RT) cm²/sec C-3(Di₄₀An₆₀): D = 1.29 × 10^?2 exp(?(44.2 ± 6)(kcal/mole)/RT) cm²/secThe self-diffusion coefficients do not agree with the Eyring equation unless mean ionic jump distances (λ) considerably larger than the diameter of oxygen anion are assumed. However, the sense of variation of the actual diffusivities is as the Eyring equation predicts.Consideration of the results of this study and the bulk of previous work shows that oxygen appears to conform to the compensation law for cationic diffusion in silicate melts and glasses. The range of oxygen diffusivities was also found to encompass the field of divalent cation diffusivities in silicate melts.Those results imply that the diffusion of oxygen in silicate melts may involve a contribution from a cation-like diffusion mechanism (discrete O^2? anions) as well as contributions from the diffusion of larger structural units.  相似文献   

A low-cost method of reclaiming strip-mined land in Iowa to agriculture     

Lon D. Drake  G. Todd Ririe 《Environmental Geology》1981,3(5):267-279

Strippable coal in Iowa is overlain by sulfidebearing black shales capped with glacial till and loess. Weathering of these shales produces acid levels toxic to most plants, which necessitates rapid burial of the spoils. We have designed and tested a loess terrace method for returning Iowa strip mines to crop land as mining progresses. During the 1970s, corn growth and yields were monitored on various thicknesses of bess over leveled acid spoils in Mahaska County, Iowa. We evaluated the costs of reclaiming mined land to acceptable levels of productivity. When saturated loess was emplaced, the resulting compaction seriously reduced corn yields during the initial years of reclamation. This problem was substantially reduced at an adjacent site by emplacement during a dry season. After compaction had been partially alleviated by growth of sweet-clover, chisle plowing, freeze-thaw, and increase in organic matter, yields were clearly proportional to loess thickness. During years of normal rainfall, yields of approximately 100 bushels per acre were produced from about 3 1/2 feet of loess cover. Four feet of loess cover produced yields equivalent to the county average in 1978 (114 bushels/acre) and 1979 (119 bushels/acre). Although the underlying spoils were toxic (pH 3–4), upward migration of acids into the loess was minor, even during drought years. The cost of loess terrace reclamation was evaluated for 3 to 5 feet of loess cover. Assuming an average strippable coal seam thickness of 3 1/2 feet, the reclamation cost would have averaged 6.8% of the FOB price of coal during the 1970s. If the coal were trucked 50 miles to an electric utility, reclamation costs would have averaged 4.9% of the delivered price. Loess terrace reclamation would have increased the price of residential electricity by about 1%.  相似文献   

Identifying and Removing Barometric Pressure Effects in Confined and Unconfined Aquifers   总被引：5，自引：0，他引：5  

Todd C. Rasmussen  Leslie A. Crawford 《Ground water》1997,35(3):502-511

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Wellbore Skin Effect in Slug-Test Data Analysis for Low-Permeability Geologic Materials     

Y. Jeffrey Yang  Todd M. Gates 《Ground water》1997,35(6):931-937

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Control of BTEX Migration Using a Biologically Enhanced Permeable Barrier   总被引：2，自引：0，他引：2  

Robert C. Borden  Russell Todd Goin  Chih-Ming Kao 《Ground Water Monitoring & Remediation》1997,17(1):70-80

A permeable barrier system. consisting of a line of closely spaced wclls. was installed perpendicular to ground water flow to control the migration of a dissolved hydrocarhon plume. The wells were charged wiih concrete briquets that release oxygen and nitrate at a controlled rate. enhancing aerobic bio-degradation in the downgradient aquifer.
Laboratory batch reactor experiments were conducted to identify concrete mixtures that slowly released oxygcn over an extended time period. Concretes prepared with urea hydrogen peroxide were unsatisfactory, while concretes prepared with calcium peroxide and a proprietary formalation of magnesium peroxide (ORC®) gradually released oxygen at a steadily declining rate. The 21 percent MgO₂ conerete cylinders and briquets released oxygen at measurable rates for up to 300 days, while the 14 percent CaO₂ briquets were exhausted by 100 days.
A full-scale permeable barrier system using ORC was constructed at a gasoline-spill site. During the first 242 days of operation. total BTFX decreased from 17 to 3.4 mg/L. and dissolved oxygen increased from 0.4 to 1.8 mg/L. during transport through the barrier. Over time, BTEX treatment efficiencies declined. indicating the barrier system had becomc less effective in releasing oxygen and nutrients to the highly contaminated portion of the aquifer. Point dilution tests and sediment analyses performed at the conclusion of the project indicated that ihc aquifer in the vicinity of the remediation wells had been clogged by precipitation with iron minerals. This clogging is believed to result from high pH from the concrete and oxygen released by ihc ORC. Oxygen-releasing permeable barriers and other aerobic bioremediation processes should be used with caution in aquifers with high levels of dissolved iron.  相似文献   

Harmful algal blooms in the Chesapeake and Coastal Bays of Maryland, USA: Comparison of 1997, 1998, and 1999 events   总被引：2，自引：0，他引：2  

Patricia M. Glibert  Robert Magnien  Michael W. Lomas  Jeffrey Alexander  Chunlei Tan  Erin Haramoto  Mark Trice  Todd M. Kana 《Estuaries and Coasts》2001,24(6):875-883

Harmful algal blooms in the Chesapeake Bay and coastal bays of Maryland, USA, are not a new phenomenon, but may be increasing in frequency and diversity. Outbreaks ofPfiesteria piscicida (Dinophyceae) were observed during 1997 in several Chesapeake Bay tributaries, while in 1998,Pfiesteria-related events were not found but massive blooms ofProrocentrum minimum (Dinophyceae) occurred. In 1999,Aureococcus anophagefferens (Pelagophyceae) developed in the coastal bays in early summer in sufficient densities to cause a brown tide. In 1997, toxicPfiesteria was responsible for fish kills at relatively low cell densities. In 1998 and 1999, the blooms ofP. minimum andA. anophagefferens were not toxic, but reached sufficiently high densities to have ecological consequences. These years differed in the amount and timing of rainfall events and resulting nutrient loading from the largely agricultural watershed. Nutrient loading to the eastern tributaries of Chesapeake Bay has been increasing over the past decade. Much of this nutrient delivery is in organic form. The sites of thePfiesteria outbreaks ranked among those with the highest organic loading of all sites monitored bay-wide. The availability of dissolved organic carbon and phosphorus were also higher at sites experiencingA. anophagefferens blooms than at those without blooms. The ability to supplement photosynthesis with grazing or organic substrates and to use a diversity of organic nutrients may play a role in the development and maintenance of these species. ForP. minimum andA. anophagefferens, urea is used preferentially over nitrate.Pfiesteria is a grazer, but also has the ability to take up nutrients directly. The timing of nutrient delivery may also be of critical importance in determining the success of certain species.  相似文献   

Implementation of a Locally Conservative Numerical Subgrid Upscaling Scheme for Two-Phase Darcy Flow   总被引：1，自引：0，他引：1  

Todd Arbogast 《Computational Geosciences》2002,6(3-4):453-481

We present a locally mass conservative scheme for the approximation of two-phase flow in a porous medium that allows us to obtain detailed fine scale solutions on relatively coarse meshes. The permeability is assumed to be resolvable on a fine numerical grid, but limits on computational power require that computations be performed on a coarse grid. We define a two-scale mixed finite element space and resulting method, and describe in detail the solution algorithm. It involves a coarse scale operator coupled to a subgrid scale operator localized in space to each coarse grid element. An influence function (numerical Greens function) technique allows us to solve these subgrid scale problems independently of the coarse grid approximation. The coarse grid problem is modified to take into account the subgrid scale solution and solved as a large linear system of equations posed over a coarse grid. Finally, the coarse scale solution is corrected on the subgrid scale, providing a fine grid representation of the solution. Numerical examples are presented, which show that near-well behavior and even extremely heterogeneous permeability barriers and streaks are upscaled well by the technique.  相似文献   

Approximation of Biodegradation Rate Constants for Monoaromatic Hydrocarbons (BTEX) in Ground Water   总被引：4，自引：0，他引：4  

Todd H. Wiedemeier  Matthew A. Swanson  John T. Wilson  Donald H. Kampbell  Ross N. Miller  Jerry E. Hansen 《Ground Water Monitoring & Remediation》1996,16(3):186-194

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