Experimental Study of the Effects of DNAPL Distribution on Mass Rebound     

Bart T. Wilking  Derrick R. Rodriguez  Tissa H. Illangasekare 《Ground water》2013,51(2):229-236

The release of stored dissolved contaminants from low permeability zones contributes to plume persistence beyond the time when dense nonaqueous phase liquid (DNAPL) has completely dissolved. This is fundamental to successfully meeting acceptable low concentrations in groundwater that are driven by site‐specific cleanup goals. The study goals were to assess the role of DNAPL entrapment morphology on mass storage and plume longevity. As controlled field studies are not feasible, two‐dimensional (2D) test tanks were used to quantify the significance of mass loading processes from source dissolution and stored mass rebound. A simple two‐layer soil domain representing a high permeable formation sand overlying a zone of lower permeability sand was used in the tests. DNAPL mass depletion through dissolution was monitored via X‐ray photon attenuation, and effluent samples were used to monitor the plume. These data enabled analysis of the DNAPL distribution, the dissolved plume, and the dissolved phase distribution within the low permeability layer. Tests in an intermediate tank showed that mass storage contributes substantially to plume longevity. Detectable effluent concentrations persisted long after DNAPL depletion. The small tank results indicated that the DNAPL morphology influenced the flow field and caused distinctive transport mechanisms contributing to mass storage. Zones of high DNAPL saturation at the interface between the low and high permeability layers exhibited flow bypassing and diffusion dominated transport into the low permeability layer. In the absence of a highly saturated DNAPL zone near the soil interface the contaminant penetrated deeper into the low permeability layer caused by a combination of advection and diffusion.  相似文献   

The imaging and slitless spectroscopy instrument for surveys (ISSIS) for the world space observatory-ultraviolet (WSO-UV)     

Ana I. Gómez?de?Castro  Jesús Maíz  Pablo Rodriguez  Maite Gómez  Juan A. Larruquert  José A. Méndez  Tomas Belenguer  Victor Rodrigo-Gudiel  Fátima López-Martínez  Paola Sestito 《Astrophysics and Space Science》2011,335(1):283-289

ISSIS is the Imaging and Slitless Spectroscopy Instrument for the World Space Observatory-Ultraviolet (WSO-UV) mission. ISSIS is a multipurpose instrument designed to carry out high resolution (<0.1 arcsec) imaging in the far UV with fields of view ≥2×2 arcmin². ISSIS has two acquisition channels: the High Sensitivity Channel (HSC) and the Channel for Surveys (CfS). The HSC is equipped with an MCP-type detector to guarantee high sensitivity in the 1150–1750 ? range and high rejection of lower energy radiation. The CfS is equipped with a large CCD detector (4k×4k) to obtain images from the far UV to the red (1150–8500 ?); the CfS is implemented to allow observing UV bright sources such as reflection nebulae or nearby massive star forming regions. The design drivers and the current status of the instrument are described in this contribution.  相似文献   

Titan’s cloud seasonal activity from winter to spring with Cassini/VIMS     

S. Rodriguez  S. Le Mouélic  P. Rannou  C. Sotin  R.H. Brown  J.W. Barnes  C.A. Griffith  J. Burgalat  K.H. Baines  B.J. Buratti  R.N. Clark  P.D. Nicholson 《Icarus》2011,216(1):89-110

Since Saturn orbital insertion in July 2004, the Cassini orbiter has been observing Titan throughout most of the northern winter season (October 2002–August 2009) and the beginning of spring, allowing a detailed monitoring of Titan’s cloud coverage at high spatial resolution with close flybys on a monthly basis. This study reports on the analysis of all the near-infrared images of Titan’s clouds acquired by the Visual and Infrared Mapping Spectrometer (VIMS) during 67 targeted flybys of Titan between July 2004 and April 2010.The VIMS observations show numerous sporadic clouds at southern high and mid-latitudes, rare clouds in the equatorial region, and reveal a long-lived cloud cap above the north pole, ubiquitous poleward of 60°N. These observations allow us to follow the evolution of the cloud coverage during almost a 6-year period including the equinox, and greatly help to further constrain global circulation models (GCMs). After 4 years of regular outbursts observed by Cassini between 2004 and 2008, southern polar cloud activity started declining, and completely ceased 1 year before spring equinox. The extensive cloud system over the north pole, stable between 2004 and 2008, progressively fractionated and vanished as Titan entered into northern spring. At southern mid-latitudes, clouds were continuously observed throughout the VIMS observing period, even after equinox, in a latitude band between 30°S and 60°S. During the whole period of observation, only a dozen clouds were observed closer to the equator, though they were slightly more frequent as equinox approached.We also investigated the distribution of clouds with longitude. We found that southern polar clouds, before disappearing in mid-2008, were systematically concentrated in the leading hemisphere of Titan, in particular above and to the east of Ontario Lacus, the largest reservoir of hydrocarbons in the area. Clouds are also non-homogeneously distributed with longitude at southern mid-latitudes. The n = 2-mode wave pattern of the distribution, observed since 2003 by Earth-based telescopes and confirmed by our Cassini observations, may be attributed to Saturn’s tides.Although the latitudinal distribution of clouds is now relatively well reproduced and understood by the GCMs, the non-homogeneous longitudinal distributions and the evolution of the cloud coverage with seasons still need investigation. If the observation of a few single clouds at the tropics and at northern mid-latitudes late in winter and at the start of spring cannot be further interpreted for the moment, the obvious shutdown of the cloud activity at Titan’s poles provides clear signs of the onset of the general circulation turnover that is expected to accompany the beginning of Titan’s northern spring. According to our GCM, the persistence of clouds at certain latitudes rather suggests a ‘sudden’ shift in near future of the meteorology into the more illuminated hemisphere. Finally, the observed seasonal change in cloud activity occurred with a significant time lag that is not predicted by our model. This may be due to an overall methane humidity at Titan’s surface higher than previously expected.  相似文献   

Organic sedimentary deposits in Titan’s dry lakebeds: Probable evaporite     

Jason W. Barnes  Jacob Bow  Jacob Schwartz  Robert H. Brown  Jason M. Soderblom  Alexander G. Hayes  Graham Vixie  Stéphane Le Mouélic  Sebastien Rodriguez  Christophe Sotin  Ralf Jaumann  Katrin Stephan  Laurence A. Soderblom  Roger N. Clark  Bonnie J. Buratti  Kevin H. Baines  Philip D. Nicholson 《Icarus》2011,216(1):136-140

We report the discovery of organic sedimentary deposits at the bottom of dry lakebeds near Titan’s north pole in observations from the Cassini Visual and Infrared Mapping Spectrometer (VIMS). We show evidence that the deposits are evaporitic, making Titan just the third known planetary body with evaporitic processes after Earth and Mars, and is the first that uses a solvent other than water.  相似文献   

An excerpt of X‐shooter operations at Paranal     

C. Martayan  A. Alvarez  P. Bristow  R. Castillo  H. Dekker  S. D'Odorico  V. Mainieri  E. Mason  A. Modigliani  S. Moehler  P. Moller  F. Patat  E. Pena  C. Reinero  J. Vernet  J. Vinther  M. Vuckovic  all the engineers who work a lot on X‐shooter 《Astronomische Nachrichten》2011,332(3):222-223

We report on the current performances and measurements done with X‐shooter during its first year of operations at Paranal. We present the current instrumental problems, recent and future improvements, and give advice for the observations (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Burning peat and reworking loess contribute to the formation and evolution of a large Carolina-bay basin     

Antonio B. Rodriguez  Matthew N. Waters  Michael F. Piehler 《Quaternary Research》2012,77(1):171-181

Carolina bays are nearly ubiquitous along ~ 1300 km of the North American Atlantic Coastal Plain, but relatively few bays have been examined in detail, making their formation and evolution a topic of controversy. The Lake Mattamuskeet basin, eastern North Carolina, USA, is a conglomeration of multiple Carolina bays that form a > 162 km² lake. The eastern shoreline of the lake is made up of a 2.9-km-wide plain of parabolic ridges that recorded rapid shoreface progradation. The lower shoreface deposit contains abundant charcoal beds and laminae dated 6465–6863 cal yr BP, corresponding with initiation of a lacustrine environment in the eastern part of the lake. A core from the western part of the lake sampled a 1541–1633 cal yr BP charcoal bed at the base of the lacustrine unit, indicating formation of this part of the basin postdates the eastern basin. Lake Mattamuskeet has no relationship to the Younger Dryas or a linked impact event because rim accretion significantly postdates 12,000 cal yr BP. The shoreline progradation, and association of charcoal beds with the oldest lake sediment in both main parts of the basin, suggest that fire and subsequent hydrodynamic processes were associated with initial formation of these Carolina bays.  相似文献   

One-year along-beach variation in the maximum depth of erosion resulting from irregular shoreline morphology     

Antonio B. Rodriguez  Patricia L. Rodriguez  Stephen R. Fegley 《Marine Geology》2012

Beach erosion and accretion occur across multiple time scales. Over long time scales (decades to millennia) the shoreface ravinement surface, which is recognized as a coarse lag deposit, forms at the shoreface toe as a result of wave- and current-induced erosion during shoreline transgression. Over short time scales (hours to days) the depth of sediment disturbance, which is recognized as coarse lamina and measured at the foreshore by devices and monitoring tracer beds, forms as a result of wave- and current-induced reworking during a tidal cycle. The maximum depth of erosion (MDOE), quantified here over 1 year, is modulated by processes that operate over a time scale that is between the drivers of short-term (e.g. tides and waves) and long-term (e.g. sea-level rise) beach erosion. The MDOE integrates the erosion that occurs over a discrete time interval and records the maximum depth of erosion that is likely principally induced by storms, which is difficult to quantify by other methods that rely on discrete observations (e.g. changes in elevation or movement of the mean high-water line). A novel technique for quantifying the MDOE, based on comparing the bedding and stratigraphy between cores collected at the same locations over a discrete time interval, is presented here and applied at Onslow Beach, NC, USA. This 12 km-long barrier island has irregular shoreline morphology, characterized by two embayments separated by a central headland. This shape is largely the result of variations in the depth of underlying rock strata and produces a steeper beachface at the headland than at the embayments. At each of the six sites examined along the barrier, the MDOE is found to increase from the backshore to the middle intertidal zone and is higher at the sites closer to the headland. These variations in the MDOE are likely due to the increase in average wave energy impacting the beachface from an offshore direction and steeper beaches (intermediate beach state) at the headland. Where the MDOE is within the beach facies, it is not associated with a coarsening, which is due to the heterolithic nature of the Onslow-beach strata. Where the MDOE is the contact between back-barrier and beach facies it is always associated with a coarsening and a gravel-rich lag deposit because in this case, the MDOE is an amalgamation of multiple erosional events, which is similar to the shoreface ravinement surface. Along-beach variation in the MDOE does not correspond with discrete observations of beach change over the same period and is likely a better indicator of erosion potential than long-term discrete observations, such as changes in surface elevation or the position of the mean high-water line.  相似文献   

Effect of coastal waves on sea level in Óbidos Lagoon,Portugal     

Madalena S. Malhadas  Paulo C. Leitão  Adélio Silva  Ramiro Neves 《Continental Shelf Research》2009

This study reports the role of waves, tide, wind and freshwater discharges over the sea level in Óbidos Lagoon, a coastal system connected to the sea through a narrow and shallow mobile inlet. To address the hydrodynamic features of this coastal system, the relative importance of different physical forcings were evaluated. For this purpose, observations together with realistic and idealized numerical modeling were used. Both model and measurements show that the lagoon sea level remains above offshore sea level during storm wave periods. Hence, a simplified inlet-lagoon idealized model was described through mathematical expressions, to understand and highlight the physical processes responsible for sea-level elevation.  相似文献   

Pine plantation bands limit seedling recruitment of a perennial grass under semiarid conditions     

J.A. Navarro-Cano  G.G. Barberá  A. Ruiz-Navarro  V.M. Castillo 《Journal of Arid Environments》2009,73(1):120-126

Pine plantations coexist with Stipa tenacissima grasslands in many semiarid western Mediterranean areas. We compared three microsites created by a 30-year-old Pinus halepensis plantation: below pine plantation line (BP), below canopy of pines (BC) and interline bare band (BA). They were evaluated in terms of soil properties, pine litter and suitability as recruitment niches for S. tenacissima. Next, in a manipulative experiment in growth chambers we tested the hypothesis that pine litter interferes with the seedling emergence of S. tenacissima. Three treatments in pots were compared: (a) soil from BA; (b) intact soil + litter from BP; and (c) soil + litter from BP, which was mixed in the laboratory (BPMX). In the field the main microsite differences were pine cover and litter cover and thickness. Seedling emergence was significantly greater in BA than in BP. Emergence and litter depth fits a linear regression model. In the growth chamber litter did not interfere with the emergence of S. tenacissima. However, seedlings grown without litter were 28% longer and their mass was 27% greater than in the litter treatments. The detected pine litter interference may be relevant for plant dynamics and might be considered in forestry management programs.  相似文献   

Thermo-mechanical ratcheting in soil–structure interfaces     

Pastén  César  Castillo  Emilia  Chong  Song-Hun 《Acta Geotechnica》2019,14(5):1561-1569

Acta Geotechnica - This paper proposes and validates a thermo-mechanical ratcheting mechanism that explains the cumulative displacement of soil–continuum interfaces when subjected to...  相似文献