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761.
Analytical hydrologic models and the design of policy instruments for groundwater-quality management
This paper explores how analytical hydrologic models can inform the effective design and choice of policy instruments to manage groundwater quality by coupling a social-planner’s problem of optimal groundwater-quality management with analytical solutions from the hydrology literature. A theoretical analysis is performed in order to characterize the properties of an optimal emissions policy. The model is then applied in a numerical analysis of groundwater contamination by chloride from highway deicers, demonstrating the relevance of the theoretical results to practical management settings. This analytical approach can help determine which policy instruments are likely to be effective in controlling groundwater pollution, especially if costly numerical groundwater models are not available for the aquifer in question. Unlike previous economic studies of groundwater contamination, this approach defines optimal emissions policies as a function of geophysical parameters employed by hydrologists such as distance between source and sink, groundwater velocity, and aquifer dispersivity. The theoretical section of this paper also demonstrates that the relationship between geophysical parameters and optimal emissions levels may be ambiguous. 相似文献
762.
Appropriate Uses and Practical Limitations of 2D Numerical Analysis of Tunnels and Tunnel Support Response 总被引:5,自引:3,他引:2
Nicholas Vlachopoulos Mark S. Diederichs 《Geotechnical and Geological Engineering》2014,32(2):469-488
In spite of the gradual development of three-dimensional analysis packages utilizing finite element models or finite difference algorithms for stress–strain calculations, two-dimensional (2D) analysis is still used as the primary engineering tool for practical analysis of tunnel behavior and tunnel support performance for design—particularly at the preliminary stage of a project. The applicability of 2D finite element analysis or analytical convergence confinement solutions to staged support installation depend on the application of an assumed or validated longitudinal displacement profile. Convergence in commonly applied 2D staged models is controlled by boundary displacement or internal pressure relaxation. While there have been developments to improve this methodology, this often assumes independence between the ground reaction curve and the support resistance, independence between the longitudinal displacement profile to support application, and the assumption that non-isotropic stresses and non-circular geometries can be handled in the same way as circular tunnels in isotropic conditions. This paper examines the validity of these assumptions and the error inherent in these extensions to 2D tunnel analysis. Anisotropic stresses and lagged (staged) excavation present a particular problem. Practical solutions are proposed for support longitudinal displacement LDPs in simplified conditions. 相似文献
763.
Coralie?SiégelEmail author Nicholas?Arndt Stephen?Barnes Anne-Laure?Henriot Pierre?Haenecour Vinciane?Debaille Nadine?Mattielli 《Contributions to Mineralogy and Petrology》2014,168(6):1084
Two Archaean komatiitic flows, Fred’s Flow in Canada and the Murphy Well Flow in Australia, have similar thicknesses (120 and 160 m) but very different compositions and internal structures. Their contrasting differentiation profiles are keys to determine the cooling and crystallization mechanisms that operated during the eruption of Archaean ultramafic lavas. Fred’s Flow is the type example of a thick komatiitic basalt flow. It is strongly differentiated and consists of a succession of layers with contrasting textures and compositions. The layering is readily explained by the accumulation of olivine and pyroxene in a lower cumulate layer and by evolution of the liquid composition during downward growth of spinifex-textured rocks within the upper crust. The magmas that erupted to form Fred’s Flow had variable compositions, ranging from 12 to 20 wt% MgO, and phenocryst contents from 0 to 20 vol%. The flow was emplaced by two pulses. A first ~20-m-thick pulse was followed by another more voluminous but less magnesian pulse that inflated the flow to its present 120 m thickness. Following the second pulse, the flow crystallized in a closed system and differentiated into cumulates containing 30–38 wt% MgO and a residual gabbroic layer with only 6 wt% MgO. The Murphy Well Flow, in contrast, has a remarkably uniform composition throughout. It comprises a 20-m-thick upper layer of fine-grained dendritic olivine and 2–5 vol% amygdales, a 110–120 m intermediate layer of olivine porphyry and a 20–30 m basal layer of olivine orthocumulate. Throughout the flow, MgO contents vary little, from only 30 to 33 wt%, except for the slightly more magnesian basal layer (38–40 wt%). The uniform composition of the flow and dendritic olivine habits in the upper 20 m point to rapid cooling of a highly magnesian liquid with a composition like that of the bulk of the flow. Under equilibrium conditions, this liquid should have crystallized olivine with the composition Fo94.9, but the most magnesian composition measured by electron microprobe in samples from the flow is Fo92.9. To explain these features, we propose that the parental liquid contained around 32 wt% MgO and 3 wt% H2O. This liquid degassed during the eruption, creating a supercooled liquid that solidified quickly and crystallized olivine with non-equilibrium textures and compositions. 相似文献
764.
Nicholas J. Nidzieko Joseph A. Needoba Stephen G. Monismith Kenneth S. Johnson 《Estuaries and Coasts》2014,37(1):91-110
Optical in situ chemical sensors enable sampling intervals and durations that rival acoustic techniques used for measuring currents. Coupling these high-frequency biogeochemical and physical measurements in estuaries to address ecosystem-scale questions, however, is still comparatively novel. This study investigated how tides affect ecosystem metabolism in a mesotidal estuary in central California (Elkhorn Slough). Dissolved oxygen measurements were used to estimate the terms in a control volume budget for a tidal creek/marsh complex at tidal timescales over several weeks. Respiration rates were 1.6 to 7.3 g O2 m?2 day?1; net community production approached 20 g O2 m?2 day?1. We found that aquatic NCP integrated throughout the creek complex varied significantly over the spring-neap cycle. The intertidal contribution to aquatic metabolism was net heterotrophic during spring tides and generally in balance during neap tides because spring-tide marsh inundation was limited to nighttime, and therefore the marsh could not contribute any primary production to the water column. At the estuary scale, the fortnightly export of oxygen from the main channel to the intertidal was largely balanced by an advective flux up-estuary. 相似文献
765.
Shaun Cole Will J. Percival John A. Peacock Peder Norberg Carlton M. Baugh Carlos S. Frenk Ivan Baldry Joss Bland-Hawthorn Terry Bridges Russell Cannon Matthew Colless Chris Collins Warrick Couch Nicholas J. G. Cross Gavin Dalton Vincent R. Eke Roberto De Propris Simon P. Driver George Efstathiou Richard S. Ellis Karl Glazebrook Carole Jackson Adrian Jenkins Ofer Lahav Ian Lewis Stuart Lumsden Steve Maddox Darren Madgwick Bruce A. Peterson Will Sutherland Keith Taylor 《Monthly notices of the Royal Astronomical Society》2005,362(2):505-534
766.
The potential for western mosquitofish, Gambusia affinis, to negatively affect populations of black mudfish in New Zealand wetlands was investigated with annual fishing surveys of the Whangamarino wetland and by laboratory experiments evaluating competition and predation of mosquitofish on mudfish adults and larvae. Abundance of mudfish juveniles was reduced in the presence of mosquitofish at marginal wetland sites close to permanent water, but mosquitofish were not found at ephemeral adult mudfish habitats. In constructed wetland microcosms, mosquitofish reduced both growth and condition of post‐larval mudfish but did not attack or kill mudfish. In laboratory aquaria, mosquitofish rapidly consumed mudfish fry but survival time of the latter was significantly extended by an increase in density of submerged structures and by the presence of an alternative food source for mosquitofish. Because mosquitofish cannot survive in ephemeral wetlands that dry over summer, the impacts of mosquitofish on mudfish are likely to be minimal except in habitats where mosquitofish can survive year‐round owing to the presence of permanent standing water. 相似文献
767.
An integrated approach to shallow aquifer characterization: combining geophysics and geostatistics 总被引:1,自引:0,他引:1
We present a method of aquifer characterization that is able to utilize multiple sources of conditioning data to build a more
realistic model of heterogeneity. This modeling approach (InMod) uses geophysical data to delineate bounding surfaces within
sedimentary deposits. The depositional volumes between bounding surfaces are identified automatically from the geophysical
data by a region growing algorithm. Simple geometric rules are used to constrain the growth of the regions in 3-D. The nodes
within the depositional volume are assigned to categorical lithologies using geostatistical realizations and a dynamic lookup
routine that can be conditioned to field data. The realizations created with this method preserve geologically expected features
and produces sharp juxtapositions of high and low hydraulic conductivity lithologies along bounding surfaces. The realizations
created with InMod also have higher variance than models created only with geostatistics and honor the volumetric distribution
of sediments measured from field data. 相似文献
768.
Nicholas Deichmann John Clinton Stephan Husen Benjamin Edwards Florian Haslinger Donat Fäh Domenico Giardini Philipp Kästli Urs Kradolfer Iris Marschall Stefan Wiemer 《Swiss Journal of Geoscience》2010,103(3):535-549
This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2009. During this period, 450 earthquakes and 68 quarry blasts were detected and located in the region under consideration. The three strongest events occurred about 15 km NW of Basel in southern Germany (ML 4.2), near Wildhaus in the Toggenburg (ML 4.0) and near Bivio in Graubünden (ML 3.5). Although felt by the population, they were not reported to have caused any damage. With a total of 24 events with ML ≥ 2.5, the seismic activity in the year 2009 was close to the average over the previous 34 years. 相似文献
769.
Darcy F. Morey George M. Crothers Julie K. Stein James P. Fenton Nicholas P. Herrmann 《Geoarchaeology》2002,17(6):521-553
Indian Knoll is the largest Archaic shell midden excavated by WPA archaeologists in Kentucky. Situated in a large alluvial valley, the site is not associated with a known river shoal as might be expected, making its fluvial and geomorphic setting of interest. Based on sediment cores and auger samples, undisturbed portions of the site remain despite extensive excavations. In undisturbed portions, a shell‐bearing layer is overlain by a shell‐free midden layer. Profiles of organic matter and calcium carbonate content for both layers are similar to those of other Green River shell middens. New radiocarbon determinations date the shell deposit at 5590–4530 cal yr B.P. Analysis of mussel species collected from the Indian Knoll indicates that shell fishing took place in a swiftly flowing, shallow to moderately deep setting of the main river channel. Overall, the prehistoric river setting adjacent to Indian Knoll was characterized by deeper water on average with variable but finer‐grained substrate compared to other Green River shell midden sites. © 2002 Wiley Periodicals, Inc. 相似文献