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1.
Late summer hypoxia (<3 ppm oxygen) in western Long Island Sound (WLIS) is a persistent environmental and management issue whose controlling processes are poorly understood. Measured rates of sediment and water-column oxygen consumption in the bottom water suggest that a condition of no oxygen should be attained on the time scale of 13–30 d. Observations, however, indicate the onset of hypoxia is of the order 150 d. Therefore, horizontal and/or vertical transport of oxygen into the area of hypoxia must play an important role. Hypoxia decreases benthic activity and the sediment flux of222Rn. The resulting horizontal gradient in bottom water222Rn was measured and used to estimate the effective horizontal transport rate (>5–50 m2 s?1), which is considerably slower than previous estimates. Scale analysis of the hypoxia process indicates that horizontal transport rates alone can explain the slow progression of hypoxia in XLIS but that vertical processes may also be capable of delaying the onset of hypoxia especially under conditions of weak stratification or weak intermediate layer oxygen consumption. This scale analysis indicates a delicately balanced process that is sensitive to both climatologically-driven variability in the rates of horizontal and vertical transport as well as the biologically-driven rates of oxygen consumption. An improved ability to predict and/or control hypoxia must be based on a better understanding of temporal and spacial variations in circulation, mixing, and stratification as well as the biological processes in the water column and the sediments. 相似文献
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We present a generic, semi-automated algorithm for generating non-uniform coarse grids for modeling subsurface flow. The method is applicable to arbitrary grids and does not impose smoothness constraints on the coarse grid. One therefore avoids conventional smoothing procedures that are commonly used to ensure that the grids obtained with standard coarsening procedures are not too rough. The coarsening algorithm is very simple and essentially involves only two parameters that specify the level of coarsening. Consequently the algorithm allows the user to specify the simulation grid dynamically to fit available computer resources, and, e.g., use the original geomodel as input for flow simulations. This is of great importance since coarse grid-generation is normally the most time-consuming part of an upscaling phase, and therefore the main obstacle that has prevented simulation workflows with user-defined resolution. We apply the coarsening algorithm to a series of two-phase flow problems on both structured (Cartesian) and unstructured grids. The numerical results demonstrate that one consistently obtains significantly more accurate results using the proposed non-uniform coarsening strategy than with corresponding uniform coarse grids with roughly the same number of cells. 相似文献
4.
Biomass Burning in Southern Africa: Individual Particle Characterization of Atmospheric Aerosols and Savanna Fire Samples 总被引：2，自引：0，他引：2
Xiande Liu Piet Van Espen Freddy Adams Jan Cafmeyer Willy Maenhaut 《Journal of Atmospheric Chemistry》2000,36(2):135-155
Ambient atmospheric aerosols and savanna fireparticulate emission samples from southern Africa werecharacterised in terms of particle classes and theirnumber abundance by electron probe X-ray microanalysis(EPXMA). About ten particle classes were identifiedfor each sample. The major classes werealuminosilicates and sea salts for ambient coarse(2–10 m equivalent aerodynamic diameter (EAD))samples, and K-S and S-only particles for ambient fine(<2 m EAD) samples. The K-S particles are oneof the major products of biomass burning. The EPXMAresults were found to be consistent with the resultsfrom bulk analyses on a sample by sample basis. Forsavanna fire fine samples, quantitative EPXMA revealedthat many particles had a composition of simple saltssuch as KCl. Some particles had a deviatingcomposition in the sense that more ionic species wereinvolved in sustaining the balance between cations andanions, and they were composite or mixed salts.Because of extensive processing during the atmospherictransport, the composition of the K-S particles in theambient samples was different from K2SO4,and such particles were enriched with S. The finepyrogenic KCl particles and the fine sea-saltparticles were much depleted in chlorine. 相似文献
5.
Controls governing the production of 4He and 222Rn in the solid phase as well as parameters and processes contributing to their transport into the pore fluid are discussed. 222Rn activity is used to quantify the uranium sources for 4He and the result is a simplified pore-fluid age equation which is virtually independent of the porosity, the water/rock ratio, the rock density and the uranium concentration, but does require a Th/U estimate. The crucial parameter is the ratio of the release factors for the two species, /GLRn//GLHe, which is discussed in terms of three possible release mechanisms: (1) recoil; (2) recoil followed by diffusion; and (3) weathering release of accumulated 4He. It was found that /GLRn//GLHe can vary over several orders of magnitude, but can be expressed in terms of the effective grain size r, and the diffusive half-length for 222Rn decay, re.4He measurements are used to “date” gases from known gas fields and the agreement with the assumed source-rock age is good. Application of 4He/ 222Rn measurements to continental freshwater springs indicates that the weathering release of accumulated 4He dominates the input and results in a large overestimate of groundwater age. Measurement in the Lardarello geothermal field indicates that the 4He/222Rn method can indicate relative transport direction. Other possible applications in various geochemical fields are suggested. 相似文献
6.
DNA tracers with information capacity and high detection sensitivity tested in groundwater studies 总被引：1，自引：0，他引：1
In order to investigate the usefulness of unique synthetic DNA tracers in groundwater, a field experiment was conducted in
Norway. DNA tracers and a sodium-chloride tracer were injected into an aquifer. The transport of DNA molecules was interpreted
by comparing with the plume of chloride ions under forced-gradient steady-state flow conditions. Spatial concentration moments
described the migration of conservative tracers. Mobility and migration of DNA in groundwater demonstrate that DNA tracers
can be detected by using the polymerase chain reaction (PCR) and DNA sequence analysis. The results indicate that DNA tracers
can be valuable tools as tracers in groundwater investigations.
Received, June 1997 / Revised, July 1998, December 1998 / Accepted, January 1999 相似文献
7.
Three-dimensional (3D) seismic data acquired for hydrocarbon exploration reveal that gas accumulations are common within the
2–3 km thick Plio-Pleistocene stratigraphic column of the south-western Barents Sea continental margin. The 3D seismic data
have relatively low-frequency content (<40 Hz) but, due to dense spatial sampling, long source-receiver offsets, 3D migration
and advanced interpretation techniques, they provide surprisingly detailed images of inferred gas accumulations and the sedimentary
environments in which they occur. The presence of gas is inferred from seismic reflection segments with anomalously high amplitude
and reversed phase, compared with the seafloor reflection, so-called bright spots. Fluid migration is inferred from vertical
zones of acoustic masking and acoustic pipes. The 3D seismic volume allows a spatial analysis of amplitude anomalies inferred to reflect the presence of gas and fluids. At several locations, seismic attribute
maps reveal detailed images of flat spots, inferred to represent gas–water interfaces. The data indicate a focused fluid migration
system, where sub-vertical faults and zones of highly fractured sediments are conduits for the migration of gas-bearing fluids
in Plio-Pleistocene sediments. Gas is interpreted to appear in high-porosity fan-shaped sediment lobes, channel and delta
deposits, glacigenic debris flows and sediment blocks, probably sealed by low-permeability, clayey till and/or (glacio)marine
sediments. Gas and fluid flow are here attributed mainly to rapid Plio-Pleistocene sedimentation that loaded large amounts
of sedimentary material over lower-density, fine-grained Eocene oozes. This probably caused pore-fluid dewatering of the high-fluid
content oozes through a network of polygonal faults. The study area is suggested to have experienced cycles of fluid expulsion
and hydrocarbon migration associated with glacial–interglacial cycles. 相似文献
8.
Kriging with Inequality Constraints 总被引：1，自引：0，他引：1
A Gaussian random field with an unknown linear trend for the mean is considered. Methods for obtaining the distribution of the trend coefficients given exact data and inequality constraints are established. Moreover, the conditional distribution for the random field at any location is calculated so that predictions using e.g. the expectation, the mode, or the median can be evaluated and prediction error estimates using quantiles or variance can be obtained. Conditional simulation techniques are also provided. 相似文献
9.
Helium isotopes in geothermal systems: Iceland,The Geysers,Raft River and Steamboat Springs 总被引：1，自引：0，他引：1
Helium isotope ratios have been measured in geothermal fluids from Iceland, The Geysers, Raft River, Steamboat Springs and Hawaii. These ratios have been interpreted in terms of the processes which supply He in distinct isotopic ratios (i.e. magmatic He, ~10 Ra; atmospheric He, Ra; and crustal He, ~0.1 Ra) and in terms of the processes which can alter the isotopic ratio (hydrologic mixing, U-Th series alpha production and weathering release of crustal He, magma aging and tritiugenic addition of 3He). Using this interpretational scheme, Iceland is found to be an area of hot-spot magmatic He implying an active volcanic source although the data are suggestive of high-temperature weathering release of crustal He incorporated in the geothermal fluids. By comparison to fumarolic gases from Hawaii and Juan De Fuca and Cayman Trench basaltic glass samples, The Geysers contains MOR type magmatic He again implying an active volcanic source possibly a “leaky” transform related to the San Andreas Fault System. Raft River contains only crustal He indicating no active volcanic sources. Steamboat Springs He isotope ratios are distinctly less than typical plate margin volcanics but must still have a magmatic source. A preliminary assessment of the cause for this low ratio is made assuming an “aging” magma source. 相似文献
10.
The rate of accumulation of 4He in the groundwaters of the J-aquifer of the Great Artesian Basin, Australia has been determined using 14C and hydrologic ages. For groundwaters less than 50 Kyr in age, the 4He accumulation rate is 4.6 × 10?12 HeAU (where ) in close agreement with in-situ production rate of 4He (3.95 × 10?12 HeAU) based on U and Th concentrations of 1.7 and 6.1, respectively, of the sandstone. For groundwaters older than 100 Kyr, the rate of 4He accumulation is 2.91 × 10?10 HeAU based on hydrologic ages; or 74 × the rate of in-situ production. The rate of 4He “production” due to weathering of the aquifer rock is calculated to be ~10?16 HeAU, indicating that the weathering input of 4He is insignificant. If the groundwater of the GAB can be considered as a trap for the total crustal production of 4He, the rate of 4He accumulation under a steady-state flux is calculated to be 3.02 × 10?10 HeAU, in agreement with the measured accumulation rate. It is concluded that over long times the 4He accumulation rate in groundwater aquifers may be controlled by the whole crust flux of 4He. 相似文献