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61.
Knowledge of groundwater residence time is important in understanding key issues in the evolution of water quality, whether this occurs due to water–rock interaction or simply by mixing or contamination. The build-up in the atmosphere of the trace gases chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) from the middle of the last century offers a convenient way of dating waters up to ∼60 a old. The gases are well-mixed in the atmosphere so their input functions are not area-specific as is the case with 3H. While any one of these trace gases can in principle provide a groundwater age, when two or more are measured on water samples the potential exists to distinguish between different modes of flow including piston flow, exponential flow and simple end-member mixing. As with all groundwater dating methods, caveats apply. Factors such as recharge temperature and elevation must be reasonably well-constrained. Primarily for SF6, the phenomenon of ‘excess air’ also requires consideration. Primarily for the CFCs, local sources of contamination need to be considered, as do redox conditions. For both SF6 and the CFCs, the nature and thickness of the unsaturated zone need to be factored into residence time calculations. However, as an inexpensive dating method, the trace gases can be applied to a wide range of groundwater problems where traditional age indicators might once have been used more sparingly. Examples include tracing flowlines, detecting small modern inputs in ‘old’ waters, and pollution risk assessment. In the future, with the main CFCs already declining in the atmosphere, new anthropogenic trace gases are likely to take their place. 相似文献
62.
The UK is committed to meeting a series of ‘high-level marine objectives’ which are required to satisfy national obligations and to meet international commitments for the marine environment. There are more than 16 such international high-level policy driven obligations and commitments, together with more than 18 European and more than 12 of national origin. In the UK, there is an assumption that the current and planned monitoring will provide evidence to demonstrate achievement against these high-level objectives. 相似文献
63.
Multiscale structure of UXO site characterization: spatial estimation and uncertainty quantification 总被引:1,自引:1,他引:0
G. Ostrouchov W. E. Doll L. P. Beard M. D. Morris D. A. Wolf 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(2):215-225
Unexploded ordnance (UXO) site characterization must consider both how the contamination is generated and how we observe that
contamination. Within the generation and observation processes, dependence structures can be exploited at multiple scales.
We describe a conceptual site characterization process, the dependence structures available at several scales, and consider
their statistical estimation aspects. It is evident that most of the statistical methods that are needed to address the estimation
problems are known but their application-specific implementation may not be available. We demonstrate estimation at one scale
and propose a representation for site contamination intensity that takes full account of uncertainty, is flexible enough to
answer regulatory requirements, and is a practical tool for managing detailed spatial site characterization and remediation.
The representation is based on point process spatial estimation methods that require modern computational resources for practical
application. These methods have provisions for including prior and covariate information. 相似文献
64.
A. M. Heras E. Wieprecht H. Feuchtgruber F. Lahuis K. Leech R. Lorente P. Morris A. Salama B. Vandenbussche 《Experimental Astronomy》2000,10(2-3):177-197
We present a trend analysis of the ISO-SWS detector performance and a study of the space radiation effects on the SWS detectors. In particular, dark currents, dark current noise and detector responses have been checked as a function of time through the mission and as a function of time in arevolution. The results show that these parameters were stable during the mission in all bandsbut for band 3 (Si:As). Dark currents and responses were found to be higherin the first hours following the start of the science window,especially in band 2 (Si:Ga). We have studied the impacts of cosmic rays and radiation belt particles on the SWS detectors, as well as of the only large solar proton event on November 6, 1997,that occurred during the ISO mission (operated during solar minimum).The observed glitch rates in all SWS bands are found to be between 2 and4 times higher than the value predicted by the CREME96 model for the cosmic ray flux in the period considered. The bands that registered the highest glitch rates showed also a correlation with the electron fluxes measured on theGOES 9 spacecraft. From the distribution of glitchheights (voltage jumps in the detector signal), we have derived the deposited energy distributions of the particles hits. Our results lead to the conclusion that secondaryparticles produced in the shield and the detectors contributed at least as much as cosmic rays to the observed glitch rate. The effects on the detectors of the November 6, 1997 event, which caused that all observationsin a revolution were declared failed, are described in detail. 相似文献
65.
Scott C. Chapman † Simon L. Morris Almudena Alonso-Herrero Heino Falcke 《Monthly notices of the Royal Astronomical Society》2000,314(2):263-272
We present near-infrared adaptive optics, Very Large Array (VLA) radio and Hubble Space Telescope ( HST ) optical imaging of the nearby Seyfert galaxy NGC 2992. Spiral structure and an extension to the west are traced down to the core region at the limiting resolution of our near-infrared images. A faint, diffuse loop of near-infrared and radio emission is also observed to the north, embedded within the prominent 2-arcsec radio loop previously observed to the north-west. Near-infrared colour maps and CO narrow-band imaging are then used to identify which regions may not be purely reddened stellar populations. Our new data provide evidence that the VLA radio-loop morphology in the shape of a figure of 8 represents two components superimposed: (1) outflow bubbles out of the plane of the disc, coincident with the extended emission-line region (EELR); (2) star formation along the spiral arm within the galaxy disc and through the dust lane. The near-infrared continuum emission associated with the outflowing radio bubbles suggests that the radio loops are driven by the active nucleus. 相似文献
66.
Salt marshes accrete both organic and inorganic sediments. Here we present analytical and numerical models of salt marsh sedimentation that, in addition to capturing inorganic processes, explicitly account for above- and belowground organic processes including root growth and decay of organic carbon. The analytical model is used to examine the bias introduced by organic processes into proxy records of sedimentation, namely 137Cs and 210Pb. We find that accretion rates estimated using 210Pb will be less than accretion rates estimated using the 137Cs peak in steadily accreting marshes if (1) carbon decay is significant and (2) data for 210Pb extend below the 137Cs peak. The numerical model expands upon the analytical model by including belowground processes such as compaction and root growth, and by explicitly tracking the evolution of aboveground biomass and its effect on sedimentation rates. Using the numerical model we explore how marsh stratigraphy responds to sediment supply and the rate of sea-level rise. It is calibrated and tested using an extensive data set of both marsh stratigraphy and measurements of vegetation dynamics in a Spartina alterniflora marsh in South Carolina, USA. We find that carbon accumulation in marshes is nonlinearly related to both the supply of inorganic sediment and the rate of sea-level rise; carbon accumulation increases with sea-level rise until sea-level rise reaches a critical rate that drowns the marsh vegetation and halts carbon accumulation. The model predicts that changes in carbon storage resulting from changing sediment supply or sea-level rise are strongly dependent on the background sediment supply: if inorganic sediment supply is reduced in an already sediment poor marsh the storage of organic carbon will increase to a far greater extent than in a sediment-rich marsh, provided that the rate of sea-level rise does not exceed a threshold. These results imply that altering sediment supply to estuaries (e.g., by damming upstream rivers or altering littoral sediment transport) could lead to significant changes in the carbon budgets of coastal salt marshes. 相似文献
67.
68.
69.
D. J. Wilman † M. L. Balogh ‡ R. G. Bower J. S. Mulchaey A. Oemler Jr R. G. Carlberg V. R. Eke I. Lewis S. L. Morris R. J. Whitaker 《Monthly notices of the Royal Astronomical Society》2005,358(1):88-100
We compare deep Magellan spectroscopy of 26 groups at 0.3 ≤ z ≤ 0.55 , selected from the Canadian Network for Observational Cosmology 2 field survey, with a large sample of nearby groups from the 2PIGG catalogue. We find that the fraction of group galaxies with significant [O ii ]λ3727 emission (≥5 Å) increases strongly with redshift, from ∼29 per cent in 2dFGRS to ∼58 per cent in CNOC2, for all galaxies brighter than ∼ M * + 1.75 . This trend is parallel to the evolution of field galaxies, where the equivalent fraction of emission-line galaxies increases from ∼53 to ∼75 per cent. The fraction of emission-line galaxies in groups is lower than in the field, across the full redshift range, indicating that the history of star formation in groups is influenced by their environment. We show that the evolution required to explain the data is inconsistent with a quiescent model of galaxy evolution; instead, discrete events in which galaxies cease forming stars (truncation events) are required. We constrain the probability of truncation ( P trunc ) and find that a high value is required in a simple evolutionary scenario neglecting galaxy mergers ( P trunc ≳ 0.3 Gyr−1 ) . However, without assuming significant density evolution, P trunc is not required to be larger in groups than in the field, suggesting that the environmental dependence of star formation was embedded at redshifts z ≳ 0.45 . 相似文献
70.