Na and HTO diffusion in compacted bentonite: Effect of surface chemistry and related texture     

T. Melkior  E.C. Gaucher  C. Brouard  S. Yahiaoui  D. Thoby  Ch. Clinard  E. Ferrage  D. Guyonnet  C. Tournassat  D. Coelho 《Journal of Hydrology》2009,370(1-4):9-20

In underground repository concepts for radioactive waste, bentonite is studied as a reference swelling material to be used as an engineered barrier. Under the changing geochemical conditions prevailing within the barrier (saturation with the fluid coming from the host formation, diffusion of various chemical plumes caused by the degradation of some constituents of the barrier-system, etc.), the surface chemistry of the clay particles could evolve. This work aims to characterize the effects of these changes on (i) the microstructure of compacted bentonite samples and (ii) the diffusion properties of HTO and Na in these samples.For this purpose, bentonite sets were equilibrated with different solutions: NaCl, CaCl₂, CsCl solutions as well as an artificial clayey porewater solution. The microstructure of the different samples was characterized by HRTEM and XRD, in a water saturated state. In parallel, effective diffusion coefficients of both HTO and ²²Na were measured for the different samples. The density of the bentonite in the diffusion tests and in the HRTEM observations was set at 1.6 Mg m⁻³.From the microstructural observations and the results of diffusion tests, it is deduced that one key parameter is the occurrence of a gel phase in the material, which is found to depend strongly on the bentonite set: the gel phase dominates in Na-bentonite, while it is lacking in Cs-bentonite. The HTO diffusion coefficients are found to be lower in the samples with high gel phase content. Sodium diffusion does not follow the same trend: when compared with HTO, Na diffuses faster when the gel phase content is high. The latter result could indicate that the “accelerated diffusion mechanism” of cations, already mentioned in the literature, is enhanced in clayey materials that contain a gel phase.  相似文献   

Spatial variability and temporal stability of throughfall in a eucalyptus plantation in the hilly lowlands of southeastern Brazil   总被引：2，自引：0，他引：2  

Anderson Mululo Sato  André de Souza Avelar  Ana Luiza Coelho Netto 《水文研究》2011,25(12):1910-1923

Throughfall has been widely studied in forests but there is a scarcity of studies that focus on the spatial variability and temporal stability of throughfall in eucalyptus plantations. We examined throughfall in a daily basis in a 2·5‐year eucalyptus plantation in southeastern Brazil using three sample arrangements: (1) close to tree trunks (CT) and in the central point between trunks (BT), (2) four‐radial layout centred in tree trunk and (3) eight‐radial layout. Throughfall was spatially non‐uniform and varied according to the spatial monitoring arrangement: accumulated throughfall/precipitation ratio of 146% (CT) and 85% (BT) in arrangement 1, mean throughfall of 88% in arrangement 2, 84% (hilltop) and 85% (side slope) in arrangement 3. The highest throughfall values, spatial variability and persistence of dry and wet conditions were found close to eucalyptus trunks. Often accumulated throughfall close to trunks exceeded rainfall, especially for long‐duration rainfall > 5 mm. The ‘funnel effect’ was consistently observed in all three arrangements and we speculate that the high throughfall concentration and temporal stability are related to canopy structures of eucalyptus. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Enhancement of double integration procedure through spectral subtraction     

B. Coelho  P. Hölscher  F. Barends 《Soil Dynamics and Earthquake Engineering》2011

The presence of noise during dynamic measurements is one of the main sources of errors during integration procedures. To overcome this problem a methodology involving spectral subtraction of the noise amplitude from the signal is presented where noise characteristics are specified during ‘silence’ periods. This technique has been used for speech enhancement, and in this paper it is used to improve the integration procedure. The performance of the method is evaluated by means of both synthetic and real applications and by comparison with standard integration procedures. The proposed method has proven to lead to improved results, being a convenient way to address the integration problem, when noise is a major source of errors in the signal.  相似文献   

Climate change adaptation strategies for smallholder farmers in the Brazilian Sertão     

Jennifer Burney  Daniele Cesano  Jarrod Russell  Emilio Lèvre La Rovere  Thais Corral  Nereide Segala Coelho  Laise Santos 《Climatic change》2014,126(1-2):45-59

Climate models agree that semi-arid regions around the world are likely to experience increased rainfall variability and longer droughts in the coming decades. In regions dependent on agriculture, such changes threaten to aggravate existing food insecurity and economic underdevelopment, and to push migration to urban areas. In the Brazilian semi-arid region, the Sertão, farmers’ vulnerability to climate—past, present, and future—stems from several factors, including low yielding production practices and reliance on scarce and seasonally variable water resources. Using interpolated local climate data, we show that, since 1962, in the Bacia do Jacuípe—one of the poorest regions in the Sertão of Bahía state—average temperatures have increased?~?2 °C and rainfall has decreased?~?350 mm. Over the same time period, average milk productivity—the main rural economic activity in the county—has fallen while in Brazil and in Bahía as a whole milk productivity has increased dramatically. This paper teases apart the drivers of climate vulnerability of the Bacia do Jacuípe in relation to the rest of Bahía. We then present the results of a suite of pilot projects by Adapta Sertão, a coalition of organizations working to improve the adaptive capacity of farmers living in the semi-arid region. By testing a number of different technologies and arrangements at the farm level, Adapta Sertão has shown that interventions focused on balanced animal diets and efficient irrigation systems can help reduce (but not eliminate) the dependence of production systems from climate. They are thus viable adaptation strategies that should be tested at a larger scale, with implications for semi-arid regions worldwide.  相似文献   

Fatty acid profiles indicate the habitat of mud snails Hydrobia ulvae within the same estuary: Mudflats vs. seagrass meadows     

Helena Coelho  Teresa Lopes da Silva  Alberto Reis  Henrique Queiroga  João Serôdio  Ricardo Calado 《Estuarine, Coastal and Shelf Science》2011

Mud snails Hydrobia ulvae occupy different habitats in complex estuarine ecosystems. In order to determine if fatty acid profiles displayed by mud snails can be used to identify the habitat that they occupy within the same estuary, fatty acids of H. ulvae from one mudflat and one seagrass meadow in the Ria de Aveiro (Portugal) were analyzed and compared to those displayed by microphytobenthos (MPB), the green leaves (epiphyte-free) of Zostera noltii, as well as those exhibited by the epiphytic community colonizing this seagrass. MPB and epiphytic diatom-dominated samples displayed characteristic fatty acids, such as 16:1n-7 and 20:5n-3, while 18:2n-6 and 18:3n-3 were the dominant fatty acids in the green leaves of Z. noltii. Significant differences between the fatty acid profiles of H. ulvae specimens sampled in the mudflat and the seagrass meadow could be identified, with those from the mudflat displaying higher levels of fatty acids known to be characteristic of MPB. This result points towards the well known existence of grazing activity on MPB by mud snails. The fatty acid profiles displayed by H. ulvae inhabiting the seagrass meadows show no evidence of direct bioaccumulation of the two most abundant polyunsaturated fatty acids of Z. noltii (18:2n-6 and 18:3n-3) in the mud snails, which probably indicates that either these compounds can be metabolized to produce energy, used as precursors for the synthesis of essential fatty acids, or that the snails do not consume seagrass leaves at all. Moreover, the fatty acid profiles of mud snails inhabiting the seagrass meadows revealed the existence of substantial inputs from microalgae, suggesting that the epiphytic community colonizing the leaves of Z. noltii displays an important role on the diet of these organisms. This assumption is supported by the high levels of 20:5n-3 and 22:6n-3 recorded in mud snails sampled from seagrass meadows. In conclusion, fatty acid analyses of H. ulvae can be successfully used to identify the habitat occupied by these organisms within the same estuary (e.g. mudflats and seagrass meadows) and reveal the existence of contrasting dietary regimes.  相似文献   

Can Nassarius reticulatus be used as a bioindicator for Hg contamination? Results from a longitudinal study of the Portuguese coastline     

Coelho JP  Pimenta J  Gomes R  Barroso CM  Pereira ME  Pardal MA  Duarte A 《Marine pollution bulletin》2006,52(6):674-680

The focus of this work is to explore the use of the netted whelk, Nassarius reticulatus (L.), as an indicator of mercury (Hg) contamination, by assessing the concentration of Hg in the sediments and in the whelk along the entire Portuguese coast. Total Hg concentrations ranged from below the detection limit (0.01 ng absolute mercury) up to 0.87 mg kg(-1) dry weight (dwt) in sediments and between 0.06 and 1.02 mg kg(-1) (dwt) for organisms, with no significant differences between males and females. Although organic mercury was not detected in the sediments, it represented, on average, 52% of the total Hg in the whelk tissues, and as high as 88% in some cases, suggesting mercury accumulation from dietary intake. Significant negative correlations were found between the total Hg concentrations in the sediments and the log(10) of Hg concentrations in whelk tissues males (r=-0.64; P<0.01) and females (r=-0.52; P<0.01) indicating that the species is a poor indicator of Hg contamination. Nevertheless, since the highest concentrations of organic mercury in the whelk tissues were found in the least contaminated areas, this species must be highly relevant in the trophic web, namely on the possible biomagnification of mercury. The high dietary mercury accumulation from feeding on carrion and the low bioavailability of mercury to whelks in estuarine sediments may be the basis of the mercury accumulation pattern in N. reticulatus.  相似文献   

Life inhabits rocks: clues to rock erosion from electron microscopy of pisolite at a UNESCO heritage site in Brazil     

Kazue Tazaki  Ryuji Asada  Zara Gerhardt Lindenmayer  Tatsuya Shirotori  Juliana Missiaggia Vargas  Carlos Henrique Nowatzki  Osmar Wöhl Coelho 《International Journal of Earth Sciences》2009,98(1):227-238

Rock erosion is attracting increasing attention from scientists worldwide. The area encompassing the Saint John Baptist Church, Saint John Village, XVII century ruins in Rio Grande do Sul at the UNESCO World Heritage Site is considered a Brazilian treasure. However, the risk of damage to this site from rock erosion has recently increased tremendously. Generally, the rocky construction such as fence, wall and tomb stone, seems strong but is actually extremely sensitive to erosion caused by lichens, fungi, molds and bacteria. Because of biological erosion and massive exposure, the fresh rock is dominated by clays and microorganisms. Water-adsorbing clays and microorganisms influence the mechanisms of the rock erosion. In this study, the formation of bio-clay-minerals in porous structure of pisolite was demonstrated using electron microscopy. Bacterial clay mineralization can deform the rock structure and even produce organic materials. Biological activity could easily corrode rocky constructions around the Saint John Baptist Church site. The rocks are pisolitic laterites possibly formed in Tertiary over the Kretaceous Parana flood Basalts. Samples inhabited by lichens and fungi were collected from a collapsed wall in the ancient church. The zonal reddish-brown pisolites are 4 mm in diameter in a matrix of clays associated with porous and empty spaces. Elemental distribution maps from X-ray fluorescence microscopy show iron-rich spherules of pisolite, whereas the matrix is composed of Al, Si, Mn, and Sr; thus producing goethite and kaolinite. Transmission electron microscopic observation showed that various types of bacteria inhabit the spherule and are associated with clay minerals and graphite. STEM elemental analysis confirmed the bio-clay-mineralization with Al, Si, S, and Fe, around bacterial cells. The results presented here will improve our understanding of nm-scale bio-mineralization and bio-erosion in lateritic rocks. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

Exploring the impacts of the tropical Pacific SST on the precipitation patterns over South America during ENSO periods   总被引：2，自引：0，他引：2  

C. A. S. Coelho  C. B. Uvo  T. Ambrizzi 《Theoretical and Applied Climatology》2002,71(3-4):185-197

Summary Previous studies on precipitation over South America that strongly support the existence of links between precipitation and SST anomalies in the Pacific Ocean have identified specific regions where the ENSO signal is particularly stronger. Northeast of Brazil and some parts of southern South America are examples of these regions. However, the same attention was not taken to identify which regions in the Central and East Pacific ocean are better correlated with the South America precipitation during extreme ENSO events, and also which are the transition regions of the precipitation signal over South America during these events. Coincident periods of ENSO events for both SST over the tropical Pacific ocean and monthly precipitation sums from many observational stations over South America were selected and analyzed. Two statistical methods were used for the data analysis: Singular Value Decomposition (SVD) and Simple Linear Correlation (SLC). The SVD results for warmer events in the Pacific corroborate previous ones and also clearly identified a transition region between the drier conditions in the Northeast of Brazil and the wetter conditions in the Southeast/South of Brazil. Transition regions were also determined over Peru and central Amazon. The SLC results indicated that the SST anomalies in the tropical east Pacific ocean has the strongest influence in the South American precipitation during El Ni?o events. During La Ni?a events the central area of the Pacific, around 180°, has shown a more significant influence. Received August 10, 2000 Revised August 22, 2001  相似文献   

Subsurface hydrology of layered colluvium mantles in unchannelled valleys—south-Eastern Brazil     

Nelson F. Fernandes  Ana L. Coelho Netto  Willy A. Lacerda 《地球表面变化过程与地形》1994,19(7):609-626

The effects of subsurface hydrology on the evolution of erosive processes inside layered colluvium mantles in unchannelled valleys are investigated. Tensiometer nests are installed in different morphological conditions with their specific depths controlled by the subsurface structure of the colluvium mantle. Daily readings (during one year) and physical and mechanical analyses of the different colluvium layers are carried out. In spite of a highly discontinuous subsurface structure, present topography is the major factor controlling subsurface flow in this landscape. Paleotopographic features, usually resulting from the lateral mobilization of the previous topographic hollow axis, can locally control the preferential expansion of the saturated zone towards areas other than the present hollow axis. During rainy periods, pressure head at the base of the hollow axis increases towards a maximum value which is not increased by subsequent precipitation inputs. New water input is dissipated by direct precipitation onto the saturated zone and exfiltration by seepage at the gully head walls, precluding the development of the excess pore pressures required to trigger failure by landsliding.  相似文献   

Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones (Meuse/Haute–Marne,France)     

M. Descostes  V. Blin  F. Bazer-Bachi  P. Meier  B. Grenut  J. Radwan  M.L. Schlegel  S. Buschaert  D. Coelho  E. Tevissen 《Applied Geochemistry》2008

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