904 research outputs found
Towards characterizing LNAPL remediation endpoints
Remediating sites contaminated with light non-aqueous phase liquids (LNAPLs) is a demanding and often prolonged task. It is vital to determine when it is appropriate to cease engineered remedial efforts based on the long-term effectiveness of remediation technology options. For the first time, the long term effectiveness of a range of LNAPL remediation approaches including skimming and vacuum-enhanced skimming each with and without water table drawdown was simulated through a multi-phase and multi-component approach. LNAPL components of gasoline were simulated to show how component changes affect the LNAPL\u27s multi-phase behaviour and to inform the risk profile of the LNAPL. The four remediation approaches along with five types of soils, two states of the LNAPL specific mass and finite and infinite LNAPL plumes resulted in 80 simulation scenarios. Effective conservative mass removal endpoints for all the simulations were determined. As a key driver of risk, the persistence and mass removal of benzene was investigated across the scenarios. The time to effectively achieve a technology endpoint varied from 2 to 6 years. The recovered LNAPL in the liquid phase varied from 5% to 53% of the initial mass. The recovered LNAPL mass as extracted vapour was also quantified. Additional mass loss through induced biodegradation was not determined. Across numerous field conditions and release incidents, graphical outcomes provide conservative (i.e. more prolonged or greater mass recovery potential) LNAPL remediation endpoints for use in discussing the halting or continuance of engineered remedial efforts
Quantifying the benefits of in-time and in-place responses to remediate acute LNAPL release incidents
Acute large volume spills from storage tanks of petroleum hydrocarbons as light non aqueous phase liquids (LNAPLs) can contaminate soil and groundwater and may have the potential to pose explosive and other risks. In consideration of an acute LNAPL release scenario, we explore the value of a rapid remediation response, and the value of installing remediation infrastructure in close proximity to the spill location, in effecting greater recovery of LNAPL mass from the subsurface. For the first time, a verified three-dimensional multi-phase numerical framework and supercomputing resources was applied to explore the significance of in-time and in-place remediation actions. A sand aquifer, two release volumes and a low viscosity LNAPL were considered in key scenarios. The time of commencement of LNAPL remediation activities and the location of recovery wells were assessed requiring asymmetric computational considerations. The volume of LNAPL released considerably affected the depth of LNAPL penetration below the groundwater table, the radius of the plume over time and the recoverable LNAPL mass. The remediation efficiency was almost linearly correlated with the commencement time, but was a non-linear function of the distance of an extraction well from the spill release point. The ratio of the recovered LNAPL in a well located at the centre of the spill/release compared to a well located 5 m away was more than 3.5, for recovery starting only 7 days after the release. Early commencement of remediation with a recovery well located at the centre of the plume was estimated to recover 190 times more LNAPL mass than a one-month delayed commencement through a well 15 m away from the centre of the LNAPL plume. Optimally, nearly 40% of the initially released LNAPL could be recovered within two months of commencing LNAPL recovery actions
Natural source zone depletion of LNAPL: A critical review supporting modelling approaches
Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) includes partitioning, transport and degradation of LNAPL components. NSZD is being considered as a site closure option during later stages of active remediation of LNAPL contaminated sites, and where LNAPL mass removal is limiting. To ensure NSZD meets compliance criteria and to design enhanced NSZD actions if required, residual risks posed by LNAPL and its long term behaviour require estimation. Prediction of long-term NSZD trends requires linking physicochemical partitioning and transport processes with bioprocesses at multiple scales within a modelling framework. Here we expand and build on the knowledge base of a recent review of NSZD, to establish the key processes and understanding required to model NSZD long term. We describe key challenges to our understanding, inclusive of the dominance of methanogenic or aerobic biodegradation processes, the potentially changeability of rates due to the weathering profile of LNAPL product types and ages, and linkages to underlying bioprocesses. We critically discuss different scales in subsurface simulation and modelling of NSZD. Focusing on processes at Darcy scale, 36 models addressing processes of importance to NSZD are investigated. We investigate the capabilities of models to accommodate more than 20 subsurface transport and transformation phenomena and present comparisons in several tables. We discuss the applicability of each group of models for specific site conditions
Calibrating CHIME, A New Radio Interferometer to Probe Dark Energy
The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a transit
interferometer currently being built at the Dominion Radio Astrophysical
Observatory (DRAO) in Penticton, BC, Canada. We will use CHIME to map neutral
hydrogen in the frequency range 400 -- 800\,MHz over half of the sky, producing
a measurement of baryon acoustic oscillations (BAO) at redshifts between 0.8 --
2.5 to probe dark energy. We have deployed a pathfinder version of CHIME that
will yield constraints on the BAO power spectrum and provide a test-bed for our
calibration scheme. I will discuss the CHIME calibration requirements and
describe instrumentation we are developing to meet these requirements
ALICE: The Ultraviolet Imaging Spectrograph aboard the New Horizons Pluto-Kuiper Belt Mission
The New Horizons ALICE instrument is a lightweight (4.4 kg), low-power (4.4
Watt) imaging spectrograph aboard the New Horizons mission to Pluto/Charon and
the Kuiper Belt. Its primary job is to determine the relative abundances of
various species in Pluto's atmosphere. ALICE will also be used to search for an
atmosphere around Pluto's moon, Charon, as well as the Kuiper Belt Objects
(KBOs) that New Horizons hopes to fly by after Pluto-Charon, and it will make
UV surface reflectivity measurements of all of these bodies as well. The
instrument incorporates an off-axis telescope feeding a Rowland-circle
spectrograph with a 520-1870 angstroms spectral passband, a spectral point
spread function of 3-6 angstroms FWHM, and an instantaneous spatial
field-of-view that is 6 degrees long. Different input apertures that feed the
telescope allow for both airglow and solar occultation observations during the
mission. The focal plane detector is an imaging microchannel plate (MCP) double
delay-line detector with dual solar-blind opaque photocathodes (KBr and CsI)
and a focal surface that matches the instrument's 15-cm diameter
Rowland-circle. In what follows, we describe the instrument in greater detail,
including descriptions of its ground calibration and initial in flight
performance.Comment: 24 pages, 29 figures, 2 tables; To appear in a special volume of
Space Science Reviews on the New Horizons missio
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Faint Radio Sources in the NOAO Bootes Field. VLBA Imaging And Optical Identifications
As a step toward investigating the parsec-scale properties of faint extragalactic radio sources, the Very Long Baseline Array (VLBA) was used at 5.0 GHz to obtain phase-referenced images of 76 sources in the NOAO Booetes field. These 76 sources were selected from the FIRST catalog to have peak flux densities above 10 mJy at 5'' resolution and deconvolved major diameters of less than 3'' at 1.4 GHz. Fifty-five of these faint radio sources were identified with accretion-powered radio galaxies and quasars brighter than 25.5 mag in the optical I band. On VLA scales at 1.4 GHz, a measure of the compactness of the faint sources (the ratio of the peak flux density from FIRST to the integrated flux density from the NVSS catalog) spans the full range of possibilities arising from source-resolution effects. Thirty of the faint radio sources, or 39{sub -7}{sup +9}%, were detected with the VLBA at 5.0 GHz with peak flux densities above 6 {sigma} {approx} 2 mJy at 2 mas resolution. The VLBA detections occur through the full range of compactness ratios. The stronger VLBA detections can themselves serve as phase-reference calibrators, boding well for opening up much of the radio sky to VLBA imaging. For the adopted cosmology, the VLBA resolution corresponds to 17 pc or finer. Most VLBA detections are unresolved or slightly resolved but one is diffuse and five show either double or core-jet structures; the properties of these latter six are discussed in detail. Eight VLBA detections are unidentified and fainter than 25.5 mag in the optical I band; their properties are highlighted because they likely mark optically-obscured active nuclei at high redshift
The state of the Martian climate
60°N was +2.0°C, relative to the 1981–2010 average value (Fig. 5.1). This marks a new high for the record. The average annual surface air temperature (SAT) anomaly for 2016 for land stations north of starting in 1900, and is a significant increase over the previous highest value of +1.2°C, which was observed in 2007, 2011, and 2015. Average global annual temperatures also showed record values in 2015 and 2016. Currently, the Arctic is warming at more than twice the rate of lower latitudes
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