A field evaluation of enhanced reductive dechlorination of chlorinated solvents in groundwater,New York Metropolitan Area

An initial assessment of an old manufacturing site with groundwater impacted by trichloroethene (TCE) contamination in the metropolitan New York area showed that the TCE was being removed naturally by reductive dechlorination. However, complete dechlorination was not expected at the site because the process was progressing too slowly under transitional to aerobic conditions at a degradation constant of –0.0013 and a TCE half life of 533 days. A pilot test was conducted at the site in which a carbohydrate substrate (molasses) was injected into the groundwater to create an In-Situ Reactive Zone (IRZ). Post-pilot test groundwater sampling and analysis indicated that an IRZ was created successfully as the total organic carbon (TOC) content and conductivity increased significantly while oxidation-reduction (REDOX) potential and dissolved oxygen (DO) decreased. The created IRZ caused enhanced reductive dechlorination of TCE at the site, found to proceed with a degradation constant of –0.0158 and a TCE half life of 44 days.     

Positive solution of two-dimensional solute transport in heterogeneous aquifers

The transport of contaminants in aquifers is usually represented by a convection-dispersion equation. There are several well-known problems of oscillation and artificial dispersion that affect the numerical solution of this equation. For example, several studies have shown that standard treatment of the cross-dispersion terms always leads to a negative concentration. It is also well known that the numerical solution of the convective term is affected by spurious oscillations or substantial numerical dispersion. These difficulties are especially significant for solute transport in nonuniform flow in heterogeneous aquifers. For the case of coupled reactive-transport models, even small negative concentration values can become amplified through nonlinear reaction source/sink terms and thus result in physically erroneous and unstable results. This paper includes a brief discussion about how nonpositive concentrations arise from numerical solution of the convection and cross-dispersion terms. We demonstrate the effectiveness of directional splitting with one-dimensional flux limiters for the convection term. Also, a new numerical scheme for the dispersion term that preserves positivity is presented. The results of the proposed convection scheme and the solution given by the new method to compute dispersion are compared with standard numerical methods as used in MT3DMS.     

Influence of the oceanographic conditions during spring 2003 on the transport of the Prestige tanker fuel oil to the Galician coast

Hydrographic data collected during the cruise HIDROPRESTIGE were combined with meteorological and dynamic data provided by remote sensors and drifting/moored buoys, to describe the surface circulation of the Northern Iberian basin in March-April 2003. Sea surface winds transported the floating Prestige oil slicks from the sinking area to the continental slope off the Rías Baixas in 1/2 month: the surface current intensity was 2% of the wind intensity and it was rotated clockwise 5 degrees from the wind direction. Mesoscale cyclonic and anticyclonic structures west of 10 degrees W increased the residence time of oil patches in the Northern Iberian basin, as compared with the expected southwards flow of the Iberian current (IC). On the other hand, the Iberian poleward current (IPC) formed a marked surface front with coastal waters, preventing the entry of fuel oil into the rías. PAHs in the surface layer during the cruise were <0.5 microgL(-1), except in the Galicia bank (approximately 1 microg L(-1); where the Prestige tanker was still leaking) and the vicinity of Cape Fisterra (approximately 1.5 microg L(-1); where the convergence front between the IPC and coastal waters vanished).     

Kinetics of the reactions of Cl atoms with 2-buten-1-ol, 2-methyl-2-propen-1-ol, and 3-methyl-2-buten-1-ol as a function of temperature

The reactions of three structurally similar unsaturated alcohols, 2-buten-1-ol (crotyl alcohol), 2-methyl-2-propen-1-ol (MPO221) and 3-methyl-2-buten-1-ol (MBO321) with Cl atoms, have been investigated for the first time, using a 400 l Teflon reaction chamber coupled with gas chromatograph-coupled with flame-ionization detection (GC-FID). The experiments were performed at atmospheric pressure and at temperatures between 255 and 298 K, in air or nitrogen as the bath gas. The obtained kinetic data were used to derive the Arrhenius expressions , , (in units of cm³ molecule⁻¹ s⁻¹). Finally, atmospheric lifetimes of those unsaturated alcohols with respect to OH, NO₃, O₃ and Cl have been calculated.     

STEREO/SECCHI Observations on 8 December 2007: Evidence Against the Wave Hypothesis of the EIT Wave Origin

Three-dimensional (3D) tomographic analysis of extreme ultraviolet (EUV) images is used to place empirical constraints on the corona’s temperature and density structure. The input data are images taken by the EUVI instrument on STEREO A and B spacecraft for Carrington Rotation 2069 (16 April to 13 May 2008). While the reconstructions are global, we demonstrate the capabilities of this method by examining specific structures in detail. Of particular importance are the results for coronal cavities and the surrounding helmet streamers, which our method allows to be analyzed without projection effects for the first time. During this rotation, both the northern and southern hemispheres exhibited stable polar crown filaments with overlying EUV cavities. These filaments and cavities were too low-lying to be well observed in white-light coronagraphs. Furthermore, due to projection effects, these cavities were not clearly discernible above the limb in EUV images, thus tomography offers the only option to study their plasma properties quantitatively. It is shown that, when compared to the surrounding helmet material, these cavities have lower densities (about 30%, on average) and broader local differential emission measures that are shifted to higher temperatures than the surrounding streamer plasma.     

Spectroscopy, morphometry, and photoclinometry of Titan's dunefields from Cassini/VIMS

Fine-resolution (500 m/pixel) Cassini Visual and Infrared Mapping Spectrometer (VIMS) T20 observations of Titan resolve that moon's sand dunes. The spectral variability in some dune regions shows that there are sand-free interdune areas, wherein VIMS spectra reveal the exposed dune substrate. The interdunes from T20 are, variously, materials that correspond to the equatorial bright, 5-μm-bright, and dark blue spectral units. Our observations show that an enigmatic “dark red” spectral unit seen in T5 in fact represents a macroscopic mixture with 5-μm-bright material and dunes as its spectral endmembers. Looking more broadly, similar mixtures of varying amounts of dune and interdune units of varying composition can explain the spectral and albedo variability within the dark brown dune global spectral unit that is associated with dunes. The presence of interdunes indicates that Titan's dunefields are both mature and recently active. The spectrum of the dune endmember reveals the sand to be composed of less water ice than the rest of Titan; various organics are consistent with the dunes' measured reflectivity. We measure a mean dune spacing of 2.1 km, and find that the dunes are oriented on the average in an east-west direction, but angling up to 10° from parallel to the equator in specific cases. Where no interdunes are present, we determine the height of one set of dunes photoclinometrically to be between 30 and 70 m. These results pave the way for future exploration and interpretation of Titan's sand dunes.     

Role of the Radiation Factor in Global Climatic Events of the Late Holocene

Izvestiya, Atmospheric and Oceanic Physics - On the basis of calculations of insolation and insolation characteristics, taking into account changes in solar activity, the causes of global climatic...     

Projections of climate change impacts on central America tropical rainforest

Tropical rainforest plays an important role in the global carbon cycle, accounting for a large part of global net primary productivity and contributing to CO₂ sequestration. The objective of this work is to simulate potential changes in the rainforest biome in Central America subject to anthropogenic climate change under two emissions scenarios, RCP4.5 and RCP8.5. The use of a dynamic vegetation model and climate change scenarios is an approach to investigate, assess or anticipate how biomes respond to climate change. In this work, the Inland dynamic vegetation model was driven by the Eta regional climate model simulations. These simulations accept boundary conditions from HadGEM2-ES runs in the two emissions scenarios. The possible consequences of regional climate change on vegetation properties, such as biomass, net primary production and changes in forest extent and distribution, were investigated. The Inland model projections show reductions in tropical forest cover in both scenarios. The reduction of tropical forest cover is greater in RCP8.5. The Inland model projects biomass increases where tropical forest remains due to the CO₂ fertilization effect. The future distribution of predominant vegetation shows that some areas of tropical rainforest in Central America are replaced by savannah and grassland in RCP4.5. Inland projections under both RCP4.5 and RCP8.5 show a net primary productivity reduction trend due to significant tropical forest reduction, temperature increase, precipitation reduction and dry spell increments, despite the biomass increases in some areas of Costa Rica and Panama. This study may provide guidance to adaptation studies of climate change impacts on the tropical rainforests in Central America.     

A GPS-Supported Gravity Survey in the Amazon of Ecuador

The growing importance and effectiveness of the Global Positioning System (GPS) in positioning geophysical surveys is now well established. In Ecuador's Amazon basin the use of GPS was the determining factor in the successful completion of a gravity survey carried out in 1994. Over 500 gravity stations, spaced from 1 to 5 km apart and positioned by GPS, were collected in the jungle environment in the country's southeast, where the preexisting data, of variable and unknown quality, were mostly confined to areas along the major rivers. This station densification, which includes the reoccupation of selected points, contributed to increase the gravity coverage; it also allowed us to evaluate and correct 1100 preexisting data points in the area, so that the different surveys can be merged. The composite data set is used for a geologic analysis of the study area, where previously unknown structures are discovered. ? 1999 John Wiley & Sons, Inc.     

Three‐term inversion of prestack seismic data using a weighted l2, 1 mixed norm

We present a new inversion method to estimate, from prestack seismic data, blocky P‐ and S‐wave velocity and density images and the associated sparse reflectivity levels. The method uses the three‐term Aki and Richards approximation to linearise the seismic inversion problem. To this end, we adopt a weighted mixed l_{2, 1}‐norm that promotes structured forms of sparsity, thus leading to blocky solutions in time. In addition, our algorithm incorporates a covariance or scale matrix to simultaneously constrain P‐ and S‐wave velocities and density. This a priori information is obtained by nearby well‐log data. We also include a term containing a low‐frequency background model. The l_{2, 1} mixed norm leads to a convex objective function that can be minimised using proximal algorithms. In particular, we use the fast iterative shrinkage‐thresholding algorithm. A key advantage of this algorithm is that it only requires matrix–vector multiplications and no direct matrix inversion. The latter makes our algorithm numerically stable, easy to apply, and economical in terms of computational cost. Tests on synthetic and field data show that the proposed method, contrarily to conventional l₂‐ or l₁‐norm regularised solutions, is able to provide consistent blocky and/or sparse estimators of P‐ and S‐wave velocities and density from a noisy and limited number of observations.