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211.
Nguyen Trung Minh L. L. Kashkarov Cu Sy Thang Doan Dinh Hung Nguyen Duc Chuy Nguyen Trung Kien Nguyen Thi Kim Thuong Nguyen Thi Thu Nguyen Van Pho 《Geochemistry International》2014,52(11):925-938
The concentration and distribution of U were measured in zircon samples separated from the streambed sediments collected at different sites along the rivers of northern Vietnam using semiconductor alpha spectrometry and alpha- and fission-track autoradiography. Based on the absolute U content and distribution, we distinguished two groups of zircon grains of different origin. Group 1 zircons have relatively low and uniform U contents ranging from 280 to 440 μg/g. Group 2 zircons are characterized by heterogeneous U distribution and U contents ranging from 490 to 2040μg/g. The determination of absolute alpha activity values and alpha spectrometric characteristics for a number of group 2 zircon grains indicated the presence of relatively short-lived alpha-emitters. These radionuclides are probably 210Po, 232U, 236Pu, 241Am, and 243Am, and all of them can originate exclusively from anthropogenic sources. A comparison of the observed alpha activity values for group 2 zircons with those of group 1 zircons and zircons separated from the suspended colloids in river water samples provided conclusive evidence for traces of anthropogenic radioactive contamination in the Lao Cai, Yen Bai, Tuyen Quang, Thac Ba, Viet Tri, and Hanoi streams flowing in the Song Hong (Red) and Song Da (Black) river basins of northern Vietnam and sourced in the southern regions of the neighboring state. 相似文献
212.
The southwestern South China Sea represents an area of continental crust frozen immediately before the onset of seafloor spreading. Here we compile a grid of multichannel seismic reflection data to characterize the continent-ocean transition just prior to full break-up. We identify a major continental block separated from the shelf margin by a basin of hyperextended crust. Oligocene-Early Miocene rifting was followed by mild compression and inversion prior to 16 Ma, probably linked to collision between the Dangerous Grounds, a continental block to the east of the study area, and Borneo. The timing of inversion supports models of seafloor spreading continuing until around 16 Ma, rather than becoming inactive at 20 Ma. The off-shelf banks experienced uplift prior to 16 Ma in an area, which had previously been a depocenter. The off-shelf banks continued to extend after this time when the rest of the region is in a phase of thermal subsidence. Post-rift magmatism is seen in the form of scattered seamounts (~5–10 km across) within or on the edge of the deeper basins, and are dated as Late Miocene and Pliocene. They are not clearly linked to any phase of tectonic activity. Further inversion of the off-shelf banks occurred in the Pliocene resulting in a major unconformity despite the lack of brittle faulting of that age. We speculate that this is part of a wider pattern of scattered magmatism throughout the South China Sea at this time. Prograding clinoforms are seen to build out from the shelf edge in the south of the study area during the Pliocene, after 5.3 Ma, and then more towards the north and east during the Pleistocene. At the same time a trough south of the off-shelf banks is filled with >1.35 km of mostly Pleistocene sediment. While we expect the bulk of the sediment to come from the Mekong River, we also suggest additional sediment supply from Borneo and the Malay Peninsula via the Molengraaff River and its predecessors. 相似文献
213.
The groundwater flow pattern of the western part of the Guarani Aquifer System (GAS), Brazil, is characterized by three regional recharge areas in the north, and a potentiometric divide in the south, which trends north–south approximately. Groundwater flow is radial from these regional recharge areas toward the center of Paraná Sedimentary Basin and toward the western outcrop areas at the border of the Pantanal Matogrossense, because of the potentiometric divide. The isotopic composition of GAS groundwater leads to understanding the paleoclimatic conditions in the regional recharge areas. The δ18O and δ2H isotopic ratios of GAS groundwaters vary, respectively, from –9.1 to –4.8‰ V-SMOW and –58.4 to –21.7‰ V-SMOW. In the recharge zones, enriched δ18O values are observed, while in the confined zone lighter δ18O values are observed. These suggest that climatic conditions were 10°C cooler than the present during the recharge of these waters. The δ13C ratios in groundwater of GAS, in the study area, vary from –19.5 to –6.5‰ VPDB, increasing along the regional flow lines toward the confined zone. This variation is related to dissolution of carbonate cement in the sandstones. 相似文献
214.
几种消暖云(雾)催化剂性能的实验研究 总被引:6,自引:2,他引:4
本文选取了十一种吸湿性物质,对它们的吸湿特性进行了实验研究,对其中一些吸湿性能较好的催化剂开展了云室消雾试验,结果表明,这些物质的吸湿能力及其随相对湿度变化的分布有较大的区别,其饱和状态下的平衡增长率也有较大差异,水泥类物质中,矿渣水泥及普通425^#K水泥的吸湿特性与氯化钠类似,氯化钠、氯化铵及矿潭水泥的云室消雾效果较好,但其作用机理可能有所不同。 相似文献
215.
216.
Tsai Christina W. Hung Serena Y. Wu Tsung-Han 《Stochastic Environmental Research and Risk Assessment (SERRA)》2020,34(2):397-413
Stochastic Environmental Research and Risk Assessment - A sediment particle in a flow not only follows the flow direction but also randomly diffuses through the surrounding fluid, because of the... 相似文献
217.
Roger D. Gonçalves Elias H. Teramoto Bruno Z. Engelbrecht Miguel A. Alfaro Soto Hung K. Chang Martinus Th. van Genuchten 《Ground water》2020,58(3):432-440
This study presents an extension of the concept of “quasi-saturation” to a quasi-saturated layer, defined as the uppermost dynamic portion of the saturated zone subject to water table fluctuations. Entrapped air here may cause substantial reductions in the hydraulic conductivity (K) and fillable pore water. Air entrapment is caused by a rising water table, usually as a result of groundwater recharge. The most significant effects of entrapped air are recharge overestimation based on methods that use specific yield (Sy), such as the water table fluctuation method (WTF), and reductions in K values. These effects impact estimation of fluid flow velocities and contaminant migration rates in groundwater. In order to quantify actual groundwater recharge rates and the effects of entrapped air, numerical simulations with the FEFLOW (Version 7.0) groundwater flow model were carried out using a quasi-saturated layer for a pilot area in Rio Claro, Brazil. The calculated recharge rate represented 16% of the average precipitation over an 8-year period, approximately half of estimates using the WTF method. Air entrapment amounted to a fillable porosity of 0.07, significant lower that the value of 0.17 obtained experimentally for Sy. Numerical results showed that the entrapped air volume in the quasi-saturated layer can be very significant (0.58 of the air fraction) and hence can significantly affect estimates of groundwater recharge and groundwater flow rates near the water table. 相似文献
218.
219.
In this study, laboratory rainfall simulation in an extensive area was used to study the infiltration, and interception and storage from surface runoff in points with different stone cover percentages (0, 10, 20 and 30%) and slopes (5°, 10° and 20°). The experimental results of this study showed that the interrelationships among the slope, stone cover percentage, groundwater level, surface runoff amount, and interception and storage of the ponds were varied and irregular. No systematic patterns were detected for the change in the groundwater level, surface runoff amount, and interception and storage of the ponds with different stone cover percentages at different slopes and no threshold values were apparent. For a 5° slope, if the stone cover percentage was increased, the amount of surface runoff was reduced, the infiltration and the groundwater level experienced no significant change, and the interception and storage of the ponds increased. For a 10° slope, if the stone cover percentage was increased, the amount of surface runoff increased, the infiltration decreased, the groundwater level experienced no significant change or decreased slightly at certain points, and the interception and storage of the ponds increased. For a 20° slope, if the stone cover percentage was increased, the amount of runoff increased, the infiltration decreased, the groundwater level experienced no significant change or decreased slightly at certain points, and the interception and storage of the ponds increased. With or without stone cover, when the hydraulic conductivity of the top material is close to that of fine sand or laterite, an increase in the slope gradient decreased the amount of surface runoff and increased the storage amount of the ponds. As for the stone distribution, an interlaced style showed better performance in the interception and storage of ponds than that of a regular style. There was no significant change in the groundwater level. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
220.
Ho Hung Chak Wai Ka Ming He Minhao Chan Ta-Chien Deng Chengbin Wong Man Sing 《Natural Hazards》2020,103(1):623-637
Natural Hazards - In this study, we applied the Weather Research and Forecasting model to project 2050 urban and rural temperature. We applied a time-stratified analysis to compare it with... 相似文献