The ionization of the shell of Novae: A time-dependent model

The authors describe a simple model of an ionization-front propagating in a homogeneous expanding Nova shell. They have estimated the ejected shell mass, the shell ionization time and the Ly-c output from the remnant for several Novae, and present some correlations between these quantities and other Nova parameters.     

The masses of binary X-ray sources

From optical and X-ray data available for the binary X-ray sources, the masses of the components were determined univocally in three systems. A discussion about the masses in other three systems is also presented. Besides Cyg X-1, SMC X-1 is another black-hole candidate.     

Characterization and evaluation of a tropical peat for the removal of Cr(VI) from solution

Tropical peat soils present higher ash content than those generated at temperate climate areas. Therefore, this study evaluated the characteristics of a Brazilian organic soil (OS), commercialized as peat, as well as its capacity in removing Cr(VI) from contaminated waters. The OS is composed of 35.5 wt% of organic matter and 56 wt% of inorganic fraction (ash), which is formed by minerals and phytoliths rich in silica (29.2 wt%) and alumina (23.6 wt%). The Cr(VI) removal tests were carried out in batch and column systems using OS and solutions of Cr(VI) prepared with distilled water and groundwater. Batch tests revealed that the organic substances in the OS caused the reduction of Cr(VI) to Cr(III), with an efficiency depending on solution pH. At pH 5.0 the Cr(VI) removal was 0.45 mg g^?1 in 24 h; whereas at pH 2.0, this removal increased to 1.10 mg g^?1. Since this redox reaction is very slow, the removal of Cr(VI) at pH 5.0 increased to around 2 mg g^?1 after 5 days. The removal of Cr(VI) was more effective in the column tests than in the batch test due to the greater solid/solution ratio, and their half-lives were 4.4 and 26.2 h, respectively. Chemical analysis indicated that Cr(VI) was reduced by the humic substances of OS, followed by the precipitation and/or adsorption of Cr(III) into the organic and inorganic components, as anatase. The presence of Cr(III) increased the stability of anatase structure, avoiding its transformation into rutile, even after being heated at 800 °C/2 h.     

Tebuthiuron leaching in three Brazilian soils as affected by soil pH

Underground contamination water by herbicides depends on the interactions between their molecules with physical and chemical soil characteristics and climatic conditions. Studies with columns can estimate the leaching potential of herbicides in soils. This study aimed to determine the effect of soil pH on tebuthiuron leaching, and capacity of bioindicators to detect tebuthiuron residues in three Brazilian soils. Cucumber plants (Cucumis sativus) were more negatively affected when grown in soils with lower amounts of organic matter and clay, and in these soils, the tebuthiuron levels reached greater depths in the column. There was a positive correlation between tebuthiuron concentration and cucumber intoxication, and a negative correlation between tebuthiuron concentration and dry matter cucumber in all soils. The tebuthiuron leached up to 50 cm depth even in soils with higher organic matter and clay content. The increasing of soil pH can affect the leaching of nonionic herbicides, and liming practice may elevate the environmental contamination risk by tebuthiuron. The bioindicator method using Cucumis sativus is viable and can be recommended to detect tebuthiuron concentrations above 0.2 mg kg^?1.     

Assessment of future trends on groundwater quality in a nitrate vulnerable zone (Esposende–Vila do Conde sector,NW Portugal): towards a combined conceptual and mass transport modelling

Hydrogeology Journal - Nitrate contamination of groundwater is an important public health issue worldwide. For environmental and public health reasons, water should not contain more than...     

The effects of river discharge and seasonal winds on the shelf off southeastern South America

The Río de la Plata waters form a low salinity tongue that affects the circulation, stratification and the distributions of nutrients and biological species over a wide extent of the adjacent continental shelf. The plume of coastal waters presents a seasonal meridional displacement reaching lower latitudes (28°S) during austral winter and 32°S during summer. Historical data suggests that the wind causes the alongshore shift, with southwesterly (SW) winds forcing the plume to lower latitudes in winter while summer dominant northeasterly (NE) winds force its southward retreat. To establish the connection between wind and outflow variations on the distribution of the coastal waters, we conducted two quasi-synoptic surveys in the region of Plata influence on the continental shelf and slope of southeastern South America, between Mar del Plata, Argentina and the northern coast of Santa Catarina, Brazil. We observed that: (A) SW winds dominating in winter force the northward spreading of the plume to low latitudes even during low river discharge periods; (B) NE winds displace the plume southward and spread the low salinity waters offshore over the entire width of the continental shelf east of the Plata estuary. The southward retreat of the plume in summer leads to a volume decrease of low salinity waters over the shelf. This volume is compensated by an increase of Tropical waters, which dominate the northern shelf. The subsurface transition between Subantarctic and Subtropical Shelf Waters, the Subtropical Shelf Front, and the subsurface water mass distribution, however, present minor seasonal variations. Along shore winds also influence the dynamics and water mass variations along the continental shelf area. In areas under the influence of river discharge, Subtropical Shelf Waters are kept away from the coastal region. When low salinity waters retreat southward, NE winds induce a coastal upwelling system near Santa Marta Cape. In summer, solar radiation promotes the establishment of a strong thermocline that increases buoyancy and further enhances the offshore displacement of low salinity waters under the action of NE winds.     

Modeled Chaco low-level jets and related precipitation patterns during the 1997–1998 warm season

Summary Chaco jet events (CJEs) are a subset of South American low-level jet events to the east of the Andes, characterized by enhanced poleward penetration and by a strong impact on precipitation over southeastern South America. The present study uses the Eta model short range weather forecasts produced operationally in the Brazilian Center for Weather Forecasts and Climate Studies (Centro de Previs?o de Tempo e Estudos Climáticos, CPTEC) to characterize the CJEs and the related precipitation during the 1997–1998 warm season. An enhanced diurnal cycle in precipitation with respect to that found during the warm season mean can be recognized during CJEs in Eta/CPTEC model output, with preference for a nocturnal maximum over southern Brazil, Uruguay, and the central part of northern Argentina, and a daytime maximum near high topography (northwestern Argentina, the Brazilian Planalto). The analysis of thermodynamic and dynamic forcing appearing during CJEs, helps to explain the modeled precipitation cycle: the nocturnal maximum is mostly explained by enhanced low-level convergence at night, while the diurnal one is mainly a response to radiative warming. Boundary-layer convergence, and convective instability, present within the CJEs environment, work together to provide both dynamic forcing and potential for convection. The simulated precipitation cycle is complemented with surface observations of “current weather” that corroborate the main oscillations found in simulated precipitation.     

Efficient regionalization techniques for socio‐economic geographical units using minimum spanning trees

Regionalization is a classification procedure applied to spatial objects with an areal representation, which groups them into homogeneous contiguous regions. This paper presents an efficient method for regionalization. The first step creates a connectivity graph that captures the neighbourhood relationship between the spatial objects. The cost of each edge in the graph is inversely proportional to the similarity between the regions it joins. We summarize the neighbourhood structure by a minimum spanning tree (MST), which is a connected tree with no circuits. We partition the MST by successive removal of edges that link dissimilar regions. The result is the division of the spatial objects into connected regions that have maximum internal homogeneity. Since the MST partitioning problem is NP‐hard, we propose a heuristic to speed up the tree partitioning significantly. Our results show that our proposed method combines performance and quality, and it is a good alternative to other regionalization methods found in the literature.     

Methane Production and Isotopic Fingerprinting in Ethanol Fuel Contaminated Sites

Biodegradation of organic compounds in groundwater can be a significant source of methane in contaminated sites. Methane might accumulate in indoor spaces posing a hazard. The increasing use of ethanol as a gasoline additive is a concern with respect to methane production since it is easily biodegraded and has a high oxygen demand, favoring the development of anaerobic conditions. This study evaluated the use of stable carbon isotopes to distinguish the methane origin between gasoline and ethanol biodegradation, and assessed the occurrence of methane in ethanol fuel contaminated sites. Two microcosm tests were performed under anaerobic conditions: one test using ethanol and the other using toluene as the sole carbon source. The isotopic tool was then applied to seven field sites known to be impacted by ethanol fuels. In the microcosm tests, it was verified that methane from ethanol (δ¹³C = −11.1‰) is more enriched in ¹³C, with δ¹³C values ranging from −20‰ to −30‰, while the methane from toluene (δ¹³C = −28.5‰) had a carbon isotopic signature of −55‰. The field samples had δ¹³C values varying over a wide range (−10‰ to −80‰), and the δ¹³C values allowed the methane source to be clearly identified in five of the seven ethanol/gasoline sites. In the other two sites, methane appears to have been produced from both sources. Both gasoline and ethanol were sources of methane in potentially hazardous concentrations and methane could be produced from organic acids originating from ethanol along the groundwater flow system even after all the ethanol has been completed biodegraded.