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1.
Laboratory simulation of geogas microbubble flow 总被引：3，自引：0，他引：3
Preliminary laboratory tests provided first data on the behavior of gas microbubbles through porous media in the framework of the geogas theory. Under experimented conditions with laboratory equipment arranged for pressure controlled gas-tracer injection and sampling, gas microbubbles moved up to ten times faster than singlephase flow in dry media under the same injection pressure. Microbubbles were determined to be very sensitive to changes in injection pressure and their terminal velocity seems to be described with good approximation by the Stokes formula. The capability of microbubbles to pick up and transport upward for short distances solid ultra-small particles (metallic and radionuclide compounds) has been proved. Results are consistent with a time-dependent process linked to the transport properties of microbubbles (e.g., flotation), such as that reported by some authors. 相似文献
2.
现代侵蚀作用核素示踪研究新进展 总被引：11，自引：1，他引：10
利用放射性核素示踪环境地球化学过程是国际地球科学的前缘课题。介绍了利用宇宙线成因的短寿命散落核素7Be示踪土壤季节性侵蚀及其与湖泊沉积耦合关系的最新研究进展;证实了利用核爆炸散落核素137Cs示踪累计性土壤侵蚀和沉积计年的可靠性;阐述了226Ra和228Ra在土壤中比活度的形态变化分异具有很好的侵蚀－堆积示踪价值;揭示了在不同生物化学条件下,210Pb的行为特征和影响因素,并将210Pbex示踪碳酸盐岩区域土壤侵蚀速率结果与湖泊沉积速率相耦合,证明了利用210Pb示踪土壤侵蚀的可行性。 相似文献
3.
Stochastic analysis of the effect of heterogeneity and fractures on radionuclide transport in a low-permeability clay layer 总被引：1，自引：0，他引：1
Deep low-permeability clay layers are considered as safe environments for disposal of high-level radioactive waste. In Belgium,
the Boom Clay is a candidate host rock for deep geological disposal. In this study, we analyze the effects of fractures and
spatially variable hydraulic conductivity on radionuclide migration through the clay. Fracture geometry and properties are
simulated with Monte Carlo simulation. The heterogeneity of hydraulic conductivity is simulated by direct sequential co-simulation
using measurements of hydraulic conductivity and four types of secondary variables. The hydraulic conductivity and fracture
simulations are used as input for a transport model. Radionuclide fluxes computed with this heterogeneous model are compared
with fluxes obtained with a homogeneous model. The output fluxes of the heterogeneous model differ at most 8% from the homogeneous
model. The main safety function of the Boom Clay is thus not affected by the fractures and the spatial variability of hydraulic
conductivity. 相似文献
4.
Stochastic analysis of the effect of spatial variability of diffusion parameters on radionuclide transport in a low permeability clay layer 总被引：1，自引：0，他引：1
Most studies that incorporate subsurface heterogeneity in groundwater flow and transport models only analyze and simulate the spatial variability of hydraulic conductivity. Heterogeneity of the other flow and transport parameters are usually neglected. This approach is often justified, but there are, however, cases in which disregarding the heterogeneity of the other flow and transport parameters can be questionable. In low permeability media, for instance, diffusion is often the dominant transport mechanism. It therefore seems logical to incorporate the spatial variability of the diffusion parameters in the transport model. This study therefore analyses and simulates the spatial variability of the effective diffusion coefficient and the diffusion accessible porosity with geostatistical techniques and incorporates their heterogeneity in the transport model of a low permeability formation. The formation studied was Boom clay (Belgium), a candidate host rock for the deep geological disposal of high-level radioactive waste. The calculated output radionuclide fluxes of this model are compared with the fluxes calculated with a homogeneous model and a model with a heterogeneous hydraulic conductivity distribution. This analysis shows that the heterogeneity of the diffusion parameters has a much larger effect on the calculated output radionuclide fluxes than the heterogeneity of hydraulic conductivity in the low permeability medium under study. 相似文献
5.
Atomic weapons testing at the Nevada Test Site has introduced many tracers for quantifying subsurface hydrologic transport processes in arid climates. In 1975, groundwater adjacent to the Cambric test, conducted beneath Frenchman Flat 10 years earlier, was pumped steadily for 16 years to elicit information on the migration of residual radioactivity through the saturated zone. Radionuclides in the pumping well effluent, including tritium, 14C, 36Cl, and 85Kr, were extensively monitored prior to its discharge in an unlined ditch, where approximately a third of the flow infiltrated over a distance of 1 km. Radionuclide infiltration through a 220-m thick vadose zone created a second, and rather unique long-term field experiment. Effluent data have been utilized in conjunction with geologic data, new radionuclide measurements, isotopic age-dating estimates, and vadose zone flow and transport models to better understand the movement of radionuclides between the ditch, the water table and a nearby groundwater monitoring well. Detection of tritium in the monitoring well occurred approximately 16 years after its initial discharge into the ditch. Modeling and tritium age dating have suggested 3–5 years of this 16-year transit time occurred solely in the vadose zone. They also suggest considerable recirculation of the pumping well discharge back into the original pumping well. Notably, there have been no observations of 14C or 85Kr in the monitoring well, suggesting their preferential retention or volatilization during transit to the water table. 相似文献
6.
Predicting the long-term safety and performance of a repository for intermediate- to high-level radioactive wastes requires the evaluation of various scenarios which may influence the integrity of the repository system. One such scenario, radionuclide transport, is described, and some of the geochemical processes which serve to enhance or retard transport are illustrated with examples selected from natural analogue or natural system studies. These studies, as distinct from laboratory simulations, help considerably in understanding how near- and far-field radionuclide transport mechanisms work over geological timescales (thousands to hundreds of thousands of years), more in line with the anticipated lifespan of a repository.
Processes addressed include: stability of the spent fuel UO2 matrix; bentonite backfill as a barrier to radionuclide diffusion; retention of radionuclides by absorption on fracture minerals; transport and diffusion of radionuclides controlled by interconnected bedrock porosity; geochemical influence and behaviour of repository construction materials on radionuclide transport. 相似文献
7.
8.
Palaeoecological tools for improving the management of coastal ecosystems: a case study from Lake King (Gippsland Lakes) Australia 总被引：2，自引：0，他引：2
Krystyna M. Saunders Dominic A. Hodgson Jennifer Harrison Andrew McMinn 《Journal of Paleolimnology》2008,40(1):33-47
Since European settlement began over 200 years ago, many southeast Australian coastal lakes and lagoons have experienced substantial
human impacts, including nutrient enrichment. Present day management and conservation efforts are often hampered by a lack
of data on pre-impact conditions. We used a palaeoecological approach at Lake King, Gippsland Lakes, southeast Australia in
order to determine its pre-impact condition and to establish the nature and direction of subsequent environmental changes,
including responses to the construction of a permanent entrance to the sea in 1889. A 120 cm sediment core was analysed for
diatoms, chlorophyll a, total carbon, nitrogen and sulphur, and dated using 210Pb. Past phosphate and salinity concentrations were reconstructed using diatom-phosphate and diatom-salinity transfer functions
developed from a calibration set based on 53 sites from 14 southeast Australian coastal lakes and lagoons. Results show changes
in the diatom assemblage that record a shift from a brackish-water to marine diatom flora since construction of the permanent
entrance. Phosphate concentrations increased at the same time and experienced major peaks in the 1940s and 1950s to >100 μg/l.
Chlorophyll a concentrations were generally below 24 μg/l/gTOC in the core, but there has been a clear increase since the 1980s, peaking
at 120 μg/l/gTOC, likely associated with a recorded increase in the frequency of nuisance algal blooms. These results indicate
that the Lake King environment is now very different to that present during early European settlement. We conclude that by
identifying the nature and direction of environmental change, palaeoecological studies can contribute towards developing realistic
and well-informed management, conservation and restoration strategies in Australian coastal ecosystems. 相似文献
9.
Fractures have been recently identified in potential host rock for high level nuclear waste disposal, like indurated argilite formations. These fractures appear as potential rapid pathways for radionuclides transport and hydrodynamic properties of the transport inside these systems must thus be characterized. Miscible non-sorbing and sorbing tracers displacements were performed on a 2-D model derived from a real fracture geometry observed in the Tournemire argilite formation with a lattice-gas cellular automaton (LGA). LGA was shown to easily handle the complex geometry of such a fracture. The numerical breakthrough curves obtained were inverted with the 1-D CDE and MIM transport models. Two main conclusions were drawn: (i) at the length scale of the study, the non-sorbing tracer transport in our fracture geometry was more accurately interpreted in terms of the MIM model rather than in terms of the classical CDE model; (ii) in order to correctly model the sorbing tracers migration, the hydrodynamic dispersion coefficient value was found to increase with the increase of the retardation factor. A semi-empirical relation based on the Taylor–Aris theory was then used to describe this dependency. 相似文献
10.
An experimental study examined the 96-h net influx from seawater of the anthropogenic radionuclides (241)Am, (60)Co and (134)Cs through the egg-case of the spotted dogfish Scyliorhinus canicula. Net influx directly through the wall of the egg-case was greatest for (134)Cs, then (241)Am and lastly (60)Co. Within the egg-case wall itself the measured concentration factors (CFs) and their gradients in the external, median and internal layers showed that for both (241)Am and (60)Co they were >10(3) in the external layer and declined by an order of magnitude in the interior layer. In contrast (134)Cs had a CF of only about three in the external layer which declined by a factor of 2 towards the two more internal layers of the egg-case. The egg-case apertures, that open within the prehatching stage of embryological development, significantly (P<0.05) increased the net influx of (241)Am and (60)Co to the interior of the egg-case, although their water concentrations were still lower than those in the labelled seawater bath. In contrast, the aperture did not increase the net influx of (134)Cs whose water concentrations equilibrated with those in seawater. Together these results indicate that the egg-case wall is very permeable to (134)Cs, representing little barrier to its movement, and hence consistent with the lack of importance of the aperture in determining its internal water concentrations in the egg-case. In contrast, (241)Am and (60)Co show much higher rates of accumulation by the egg-case, consistent with the measured reduced permeability of its wall, and therefore giving greater prominence to its aperture in the net transfer of these two radionuclides to the egg case's interior. The presence of the embryo within its egg-case did not significantly (P>0.05) affect the rates of influx of radioisotopes, with the exception of an interactive effect for (60)Co with the egg-case aperture (P<0.05). The CF of only (241)Am in the embryo itself relative to the external seawater concentration was significantly (P<0.05) enhanced by the presence of the aperture. 相似文献