全文获取类型
收费全文 | 252篇 |
免费 | 21篇 |
国内免费 | 6篇 |
专业分类
化学 | 69篇 |
力学 | 56篇 |
综合类 | 13篇 |
数学 | 59篇 |
物理学 | 82篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 8篇 |
2021年 | 9篇 |
2020年 | 10篇 |
2019年 | 5篇 |
2018年 | 8篇 |
2017年 | 12篇 |
2016年 | 12篇 |
2015年 | 1篇 |
2014年 | 19篇 |
2013年 | 56篇 |
2012年 | 12篇 |
2011年 | 3篇 |
2010年 | 16篇 |
2009年 | 15篇 |
2008年 | 7篇 |
2007年 | 8篇 |
2006年 | 6篇 |
2005年 | 8篇 |
2004年 | 5篇 |
2003年 | 8篇 |
2002年 | 4篇 |
2001年 | 8篇 |
2000年 | 1篇 |
1999年 | 5篇 |
1998年 | 11篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 1篇 |
1994年 | 4篇 |
1993年 | 1篇 |
1989年 | 3篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1984年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有279条查询结果,搜索用时 15 毫秒
141.
Abstract An example is presented for the application of groundwater age dating by tritium and chlorofluorocarbon (CFC) measurements to assess water protection zones. The investigation area is located in a rural area close to Osnabrück (Lower Saxony, Germany). Samples from 13 drinking water supply wells were collected in November 1995. By using an exponential model, groundwater residence times were calculated for the wells. The tritium and CFC data showed comparable residence times with slightly higher values derived from the CFCs (especially F113). Most of the wells showed residence times around or below 10 years indicating a rather high vulnerability with respect to anthropogenic contaminations. 相似文献
142.
Taher Sharshar H. Ebrahim Hassan Hassan A. Arida Abdulkadir Aydarous Salih A. Bazaid Mamdouh A. Ahmed 《Isotopes in environmental and health studies》2013,49(1):132-151
The levels of radioactivity and heavy metals in soil, plant and groundwater samples collected from the area of the new campus of Taif University, Saudi Arabia, and its neighbouring areas have been determined. High-resolution gamma-ray spectroscopy was used for radioactivity measurements, and inductively coupled plasma atomic emission spectroscopy was used to determine the concentration of heavy metals. The means of 226Ra, 228Ra and 40K concentrations in water samples collected from four wells were found to be 0.13±0.03, 0.05±0.03 and 1.3±0.5 Bq l?1, respectively. The means of 238U, 226Ra, 228Ra (232Th for soil samples) and 40K concentrations in wild plant and soil samples were found to be 3.7±4.1, 8.8±11.6, 3.8±2.9 and 1025±685, and 8.6±3.4, 12.8±3.4, 16.6±7.1 and 618±82 Bq kg?1 dry weight (DW), respectively. The 137Cs of artificial origin was also detected in soil samples with a mean concentration of 3.8±2.2 Bq kg?1 DW. Evaluating the results, it can be concluded that the concentrations of 238U, 226Ra, 232Th and 40K in soil samples fall within the world average. Furthermore, 19 trace and major elements in groundwater samples and 22 elements in soil and plant samples were determined. The sampling locations of soil can be classified into three groups (relatively high, medium and low polluted) according to their calculated metal pollution index using the contents of trace and major elements. A cluster analysis of the contents of radioactivity and trace element contents in soil samples shows the presence of two main distinct clusters of sampling locations. 相似文献
143.
Matthias Beyer Marcel Gaj Josefina Tulimeveva Hamutoko Paul Koeniger Heike Wanke Thomas Himmelsbach 《Isotopes in environmental and health studies》2013,49(4):533-552
The stable water isotope deuterium (2H) was applied as an artificial tracer (2H2O) in order to estimate groundwater recharge through the unsaturated zone and describe soil water movement in a semi-arid region of northern central Namibia. A particular focus of this study was to assess the spatiotemporal persistence of the tracer when applied in the field on a small scale under extreme climatic conditions and to propose a method to obtain estimates of recharge in data-scarce regions. At two natural sites that differ in vegetation cover, soil and geology, 500?ml of a 70?% 2H2O solution was irrigated onto water saturated plots. The displacement of the 2H peak was analyzed 1 and 10 days after an artificial rain event of 20 mm as well as after the rainy season. Results show that it is possible to apply the peak displacement method for the estimation of groundwater recharge rates in semi-arid environments via deuterium labelling. Potential recharge for the rainy season 2013/2014 was calculated as 45 mm a?1 at 5.6 m depth and 40 mm a?1 at 0.9 m depth at the two studied sites, respectively. Under saturated conditions, the artificial rain events moved 2.1 and 0.5 m downwards, respectively. The tracer at the deep sand site (site 1) was found after the rainy season at 5.6 m depth, corresponding to a displacement of 3.2 m. This equals in an average travel velocity of 2.8 cm d?1 during the rainy season at the first site. At the second location, the tracer peak was discovered at 0.9 m depth; displacement was found to be only 0.4 m equalling an average movement of 0.2 cm d?1 through the unsaturated zone due to an underlying calcrete formation. Tracer recovery after one rainy season was found to be as low as 3.6?% at site 1 and 1.9?% at site 2. With an in situ measuring technique, a three-dimensional distribution of 2H after the rainy season could be measured and visualized. This study comprises the first application of the peak displacement method using a deuterium labelling technique for the estimation of groundwater recharge in semi-arid regions. Deuterium proved to be a suitable tracer for studies within the soil–vegetation–atmosphere interface. The results of this study are relevant for the design of labelling experiments in the unsaturated zone of dry areas using 2H2O as a tracer and obtaining estimations of groundwater recharge on a local scale. The presented methodology is particularly beneficial in data-scarce environments, where recharge pathways and mechanisms are poorly understood. 相似文献
144.
W. Merbach D. Hölzel G. Schalitz J. Pickert H. J. Jacob C. Latus 《Isotopes in environmental and health studies》2013,49(1-2):53-59
Abstract Lysimeter experiments (soil: sandy treposol, from the region “Havelländisches Luch”, Brandenburg, Germany) with application of 15N labelled fertilizer (80 kg N per ha as 15NH4 15NO3, 10 at.-%15N exc.; for simulating mineralization in the early autumn period) were carried out to determine to what extent the amount of mineral- N was temporary conserved by winter catch crops, taken up subsequently in the vegetation periods by following crops, taken by subsequently in the vegetation periods by following crops, or percolated in the leaching water, respectively. The results were as follows: 1) Until winter or spring respectively, the catch crop uptake rates of applied mineral-N were 32% for phacelia (Phacelia tanacetifolia BENTH.), 25% for winter rape (Brassica napus L. cv. ‘AKELA’), and 16% for white mustard (Sinapis alba L.). 2) In the year after, following maize incorporated from 2.1 to 4.5% of the fertilizerborne N. The following plant community of fallow took up from 0.2 to 0.5% N originating from the fertilizer-N. 3) In comparison with the catch crops, N-leaching losses under fallow conditions were highest and equivalent to 17% of the applied fertilizer-N amount. In contrast to 3% of white mustard, phacelia and winter rape reduced N-leaching losses to 0.2 and 0.3% of the applied fertilizer-N amount. 4) In spring of the first year after the beginning of investigations, N-leaching losses were highest under fallow conditions and white mustard cultivation. Thus, the amounts of nitrate losses would exceed the EU limit for drinking water. 5) Three years after the investigations had been started, 10% (white mustard) and 20% (fallow) of the applied fertilizer-N was still found in th lysimeter soil. 相似文献
145.
C. Skark P. Obermann 《International journal of environmental analytical chemistry》2013,93(1-4):163-171
Abstract Pesticide transport was determined in a quarternary coarse grained aquifer which was characterized by low organic carbon content. Water spiked with the pesticides atrazine, chlortoluron, isoproturon, simazine and terbuthylazine and chloride salt as conservative tracer, was injected in the aquifer. Mean residence times (MRT) of pesticides were generally 1.5 to 3 times higher than the MRT of the tracer. Sediment retention capacity calculated from experimental data yielded generally small values below 250 ng/kg sediment. Retardation characteristics were in the order of magnitude expected from literature data on organic carbon/water partition coefficients. Transport of pesticides was due to convection and dispersion whereas adsorption and degradation was low. 相似文献
146.
More generalized groundwater model with space‐time caputo Fabrizio fractional differentiation 下载免费PDF全文
Jean‐Daniel Djida Abdon Atangana 《Numerical Methods for Partial Differential Equations》2017,33(5):1616-1627
We prove existence and uniqueness of the flow of water within a confined aquifer with fractional diffusion in space and fractional time derivative in the sense of Caputo‐Fabrizio using the classical contraction Banach theorem. We also propose the numerical approximation of the model using the Crank–Nicolson numerical scheme. To check the effectiveness of the model, stability analysis of the numerical scheme for the new model is presented.© 2017 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 33: 1616–1627, 2017 相似文献
147.
Mohammed Hssaisoune Baudelaire N’da Mouna Malki Houria Abahous Alan E. Fryar 《Isotopes in environmental and health studies》2017,53(3):298-312
Groundwater depletion and changes in isotopic and chemical contents constitute the main indicators of overexploitation, recharge, and flow paths in the Souss–Massa aquifer. These indicators highlight processes concerning sustainability of water resources in the aquifer (e.g. surface/groundwater interaction, recharge processes, and marine intrusion). The spatial variation of stable and radioactive isotopic contents indicates a mixing of modern and old water within the system. Recent recharge was observed mainly along the Souss River (the major surface-water drainage in the study area) and in the irrigated areas. Mapping of chemical and isotopic variation shows that the area is affected by abstraction, irrigation water return, and the evolution of modern recharge in time and space. The processes, distribution, and timing of groundwater flow are influenced by short- and long-term effects; long-term recharge is dependent on climatic conditions. This study can be used to make informed decisions about water-resource allocation and alternative management practices. 相似文献
148.
149.
Alise Babre Andis Kalvāns Konrāds Popovs Inga Retiķe Aija Dēliņa Rein Vaikmäe 《Isotopes in environmental and health studies》2016,52(6):706-725
A new data set of δ2H and δ18O in the groundwater from the central part of the Baltic Artesian Basin is presented. The hydrogeological section is subdivided into stagnation, slow exchange and active exchange zones. Na–Ca–Cl brine found at the deepest part – the stagnation zone – is characterized by δ18O values above ?5?‰ and δ2H values approaching ?40?‰ with respect to Vienna Standard Mean Ocean Water. The slow exchange zone where waters of mostly intermediate salinity reside is characterized by δ18O values around ?11.7?‰ and δ2H values around ?85.3?‰. Mean δ18O and δ2H values of the fresh groundwater in the active water exchange zone are ?11.1 and ?79.9?‰, respectively. Characteristically, the groundwater in the active and slow exchange zone is isotopically more depleted compared with the precipitation values observed, and the depletion increases with depth down to the level where strongly enriched brines are encountered. 相似文献
150.
Ioannis Matiatos Apostolos Alexopoulos 《Isotopes in environmental and health studies》2013,49(4):512-529
The present study examines the isotopic and hydrochemical composition of 18 inland spring waters and 3 coastal karstic spring waters, covering the period between October 2005 and March 2008. The stable isotopes (18O, 2H) processing has revealed the absence of significant evaporation phenomena and that the origin of fresh water samples is meteoric. Using 18O values in rainfall waters, an average line of isotopic depletion with altitude has been constructed, extracting a rate of?0.45‰/100 m as typical for the study area. Furthermore, the mean altitude of recharge of the springs has been estimated by plotting the groundwater sampling points on a δ18O versus altitude diagram. Hydrochemistry results have shown that the dissolution of carbonate, flysch and ophiolitic formations defines the hydrochemical characteristics of groundwater. Moreover, seawater intrusion in the coastal area is significantly high, causing the water in the three karstic springs to be brackish. 相似文献