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261.
Dale A. Lang Trevor P. Bastow Ben G.K. Van Aarssen Ben Warton Greg B. Davis Colin D. Johnston 《Ground Water Monitoring & Remediation》2009,29(4):85-93
Hydrocarbon and nonhydrocarbon components dissolving in water from fresh diesel and field samples of highly weathered diesel (spilled up to 50 years ago) from two sites were investigated. The fresh and weathered diesels were equilibrated with water using a slow-stirring method, and the product and equilibrated aqueous water samples analyzed by a range of analytical procedures. The water phase equilibrated with weathered diesels had higher total dissolved organics concentrations (96 and 8.6 mg/L at the two sites) compared to the water phase equilibrated with fresh diesel (average of 3.4 mg/L). Compound class characterization of dissolved organics in water from the weathered diesel showed that polar components were a significant compound class (98% and 42% at the two sites) and appeared largely as an unresolved complex mixture (UCM) in the total ion chromatograms (TICs). Identification of 1-adamantanol in the polar fraction of both weathered diesel samples (3.6 and 0.3 μg/L at the two sites) suggested that at least some of the associated polar components are from a petroleum source. The analysis of total petroleum hydrocarbons (TPH) is aimed at measuring only dissolved carbon and hydrogen-containing compounds, and dissolved polar compounds present as a UCM are often assumed to be from natural organic matter (NOM) and removed. This may result in a gross underestimation of the total soluble organic material in water associated with weathered diesels. In addition, the risk posed by these fuel-derived polar compounds is unknown. 相似文献
262.
Greg Balco Jason Briner Robert C. Finkel John A. Rayburn John C. Ridge Joerg M. Schaefer 《Quaternary Geochronology》2009,4(2):93-107
The major uncertainty in relating cosmogenic-nuclide exposure ages to ages measured by other dating methods comes from extrapolating nuclide production rates measured at globally scattered calibration sites to the sites of unknown age that are to be dated. This uncertainty can be reduced by locating production rate calibration sites that are similar in location and age to the sites to be dated. We use this strategy to reconcile exposure age and radiocarbon deglaciation chronologies for northeastern North America by compiling 10Be production rate calibration measurements from independently dated late-glacial and early Holocene ice-marginal landforms in this region. 10Be production rates measured at these sites are 6–12% lower than predicted by the commonly accepted global 10Be calibration data set used with any published production rate scaling scheme. In addition, the regional calibration data set shows significantly less internal scatter than the global calibration data set. Thus, this calibration data set can be used to improve both the precision and accuracy of exposure dating of regional late-glacial events. For example, if the global calibration data set is used to calculate exposure ages, the exposure-age deglaciation chronology for central New England is inconsistent with the deglaciation chronology inferred from radiocarbon dating and varve stratigraphy. We show that using the regional data set instead makes the exposure age and radiocarbon chronologies consistent. This increases confidence in correlating exposure ages of ice-marginal landforms in northeastern North America with glacial and climate events dated by other means. 相似文献
263.
Incorporation of conceptual and parametric uncertainty into radionuclide flux estimates from a fractured granite rock mass 总被引:1,自引:1,他引:0
Donald M. Reeves Karl F. Pohlmann Greg M. Pohll Ming Ye Jenny B. Chapman 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(6):899-915
Detailed numerical flow and radionuclide simulations are used to predict the flux of radionuclides from three underground nuclear tests located in the Climax granite stock on the Nevada Test Site. The numerical modeling approach consists of both a regional-scale and local-scale flow model. The regional-scale model incorporates conceptual model uncertainty through the inclusion of five models of hydrostratigraphy and five models describing recharge processes for a total of 25 hydrostratigraphic–recharge combinations. Uncertainty from each of the 25 models is propagated to the local-scale model through constant head boundary conditions that transfer hydraulic gradients and flow patterns from each of the model alternatives in the vicinity of the Climax stock, a fluid flux calibration target, and model weights that describe the plausibility of each conceptual model. The local-scale model utilizes an upscaled discrete fracture network methodology where fluid flow and radionuclides are restricted to an interconnected network of fracture zones mapped onto a continuum grid. Standard Monte Carlo techniques are used to generate 200 random fracture zone networks for each of the 25 conceptual models for a total of 5,000 local-scale flow and transport realizations. Parameters of the fracture zone networks are based on statistical analysis of site-specific fracture data, with the exclusion of fracture density, which was calibrated to match the amount of fluid flux simulated through the Climax stock by the regional-scale models. Radionuclide transport is simulated according to a random walk particle method that tracks particle trajectories through the fracture continuum flow fields according to advection, dispersion and diffusional mass exchange between fractures and matrix. The breakthrough of a conservative radionuclide with a long half-life is used to evaluate the influence of conceptual and parametric uncertainty on radionuclide mass flux estimates. The fluid flux calibration target was found to correlate with fracture density, and particle breakthroughs were generally found to increase with increases in fracture density. Boundary conditions extrapolated from the regional-scale model exerted a secondary influence on radionuclide breakthrough for models with equal fracture density. The incorporation of weights into radionuclide flux estimates resulted in both noise about the original (unweighted) mass flux curves and decreases in the variance and expected value of radionuclide mass flux. 相似文献
264.
Brent M. Goehring Mark D. Kurz Greg Balco Joerg M. Schaefer Joseph Licciardi Nathaniel Lifton 《Quaternary Geochronology》2010,5(4):410-418
3He is among the most commonly measured terrestrial cosmogenic nuclides, but an incomplete understanding of the 3He production rate has limited robust interpretation of cosmogenic 3He concentrations. We use new measurements of cosmogenic 3He in olivine from a well-dated lava flow at Tabernacle Hill, Utah, USA, to calibrate the local 3He production rate. The new 3He measurements (n = 8) show excellent internal consistency and yield a sea level high latitude (SLHL) production rate of 123 ± 4 at g?1 yr?1 following the Lal (1991)/Stone (2000) scaling model [Lal, D., 1991. Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models. Earth and Planetary Science Letters, 104, 424–439.; Stone, J.O., 2000. Air pressure and cosmogenic isotope production. Journal of Geophysical Research, 105, 23753–23759.]. We incorporate the new measurements from Tabernacle Hill in a compilation of all published production rate determinations, characterizing the mean global SLHL production rates (e.g. 120 ± 9.4 at g?1 yr?1 with Lal (1991)/Stone (2000)). The internal consistency of the global 3He production rate dataset is as good as the other commonly used cosmogenic nuclides. Additionally, 3He production rates in olivine and pyroxene agree within experimental error. The 3He production rates are implemented in an age and erosion rate calculator, forming a new module of the CRONUS-Earth web-based calculator, a simple platform for cosmogenic nuclide data interpretation [Balco, G., Stone, J., Lifton, N.A., and Dunai, T.J., 2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from 10Be and 26Al measurements. Quaternary Geochronology, 3, 174–195.]. The 3He calculator is available online at http://www.cronuscalculators.nmt.edu/. 相似文献
265.
Mainstream conservation has been long dominated globally by the protected area paradigm. This approach has been widely challenged in recent years, and new conservation initiatives have emerged. The situation is mirrored in New Zealand, where ongoing biodiversity loss has prompted reappraisal. Within this context, we highlight the plight of kiwi, focusing on Northland and efforts there to (re)make space for this iconic bird which is at risk of extinction. While the state has primarily responded by fortifying ‘islands’ on public conservation land, Far North communities are working in a variety of ways and localities, both within and beyond ‘the fortress’, to secure a future for the ‘people's bird’. 相似文献
266.
Steady convective exchange flows down slopes 总被引:2,自引:0,他引:2
Jeff J. Sturman Carolyn E. Oldham Greg N. Ivey 《Aquatic Sciences - Research Across Boundaries》1999,61(3):260-278
267.
Soheila Taebi Ryan J. Lowe Charitha B. Pattiaratchi Greg N. Ivey Graham Symonds 《Ocean Dynamics》2012,62(4):585-602
The circulation driven by wave breaking, tides and winds within a fringing coral reef system (Ningaloo Reef) in Western Australia
was investigated using the ocean circulation model ROMS two-way coupled to the wave model SWAN. Currents within the system
were dominantly forced by wave breaking, with flow driven over the shallow reefs and towards the lagoon, which returned to
the ocean through channels in the reef. Hindcast model simulations were compared against an extensive field dataset, revealing
that the coupled wave–circulation model could accurately predict the waves and currents throughout this morphologically complex
reef–lagoon system. A detailed momentum budget analysis showed that, over the reef, a dominant cross-shore balance was established
between radiation stress gradients and a pressure (mean water level) gradient (similar to a beach). Within the lagoon, alongshore
currents were primarily balanced by alongshore gradients in wave setup, which drove flow towards (and ultimately out) the
channels. The importance of these wave-driven currents to Ningaloo Reef was quantified over a full seasonal cycle, during
periods when wave and wind conditions significantly differed. These results showed that wave breaking still overwhelmingly
dominated the circulation and flushing of Ningaloo Reef throughout the year, with winds playing an insignificant role. 相似文献
268.
269.
The evolution of gully headcut morphology: a case study using terrestrial laser scanning and hydrological monitoring 总被引:4,自引:0,他引:4
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We present observations and analysis of gully headcut erosion, which differ from previous headcut studies in both spatial and temporal detail. Using ten terrestrial laser scanning (TLS) surveys conducted over a period of 3 years, we mapped headcut erosion with centimeter‐scale detail on a sub‐annual basis. Erosional change is observed through point cloud differencing, which expands on previous studies of headcut retreat rate by revealing the evolution of the headcut morphology. Headcut retreat observations are combined with hydrological measurements to explore the controlling factors of erosional retreat. We find that (i) mass failure due to wetting, (ii) saturation weakening of shale bedrock in plunge pools, and (iii) direct wash over the headcut face all appear to contribute to headcut retreat; however, mass failure via wetting appears to be the dominant process. Soil moisture was monitored near the study headcut at 0.4 m depth, and time‐lapse photos show that soil wetting tends to be concentrated along the headcut apex after rainfall and snowmelt runoff events. We find that moisture concentration at the headcut apex leads to more rapid erosion at that location than along the headcut sidewalls, resulting in a semi‐ellipsoidal plan view morphology that is maintained as the headcut migrates up‐valley. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
270.