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Salt marshes are important ecological areas and play a significant role in coastal flood defence schemes. In many areas of the UK they are adjacent to agricultural areas utilised for the growth of cereal crops, for which mecoprop is used as a selective herbicide in the control of broad-leaf weeds. This study measured concentrations of mecoprop in soils, drainage ditch waters and sediments and salt marsh sediments over a period of 138 days following spring application. Soil concentrations of up to 1827 microg/g were recorded after application, which demonstrated a half life for mecoprop of from 9 to 12 days, with first order kinetics. However, a major rainfall event 9 days after application resulted in significant transport of herbicide to the salt marsh via subsurface field drains, drainage ditches and discharge sluice. Mecoprop concentrations of up to 386 microg/l observed in water samples were above UK guidelines. 相似文献
95.
Hunt B 《Ground water》2008,46(4):529-30; discussion 530-1
96.
Ground water model calibration has made great advances in recent years with practical tools such as PEST being instrumental for making the latest techniques available to practitioners. As models and calibration tools get more sophisticated, however, the power of these tools can be misapplied, resulting in poor parameter estimates and/or nonoptimally calibrated models that do not suit their intended purpose. Here, we focus on an increasingly common technique for calibrating highly parameterized numerical models—pilot point parameterization with Tikhonov regularization. Pilot points are a popular method for spatially parameterizing complex hydrogeologic systems; however, additional flexibility offered by pilot points can become problematic if not constrained by Tikhonov regularization. The objective of this work is to explain and illustrate the specific roles played by control variables in the PEST software for Tikhonov regularization applied to pilot points. A recent study encountered difficulties implementing this approach, but through examination of that analysis, insight into underlying sources of potential misapplication can be gained and some guidelines for overcoming them developed. 相似文献
97.
C. Werner S. Hurwitz W.C. Evans J.B. Lowenstern D. Bergfeld H. Heasler C. Jaworowski A. Hunt 《Journal of Volcanology and Geothermal Research》2008
We characterize and quantify volatile emissions at Hot Spring Basin (HSB), a large acid-sulfate region that lies just outside the northeastern edge of the 640 ka Yellowstone Caldera. Relative to other thermal areas in Yellowstone, HSB gases are rich in He and H2, and mildly enriched in CH4 and H2S. Gas compositions are consistent with boiling directly off a deep geothermal liquid at depth as it migrates toward the surface. This fluid, and the gases evolved from it, carries geochemical signatures of magmatic volatiles and water–rock reactions with multiple crustal sources, including limestones or quartz-rich sediments with low K/U (or 40?Ar/4?He). Variations in gas chemistry across the region reflect reservoir heterogeneity and variable degrees of boiling. Gas-geothermometer temperatures approach 300 °C and suggest that the reservoir feeding HSB is one of the hottest at Yellowstone. Diffuse CO2 flux in the western basin of HSB, as measured by accumulation-chamber methods, is similar in magnitude to other acid-sulfate areas of Yellowstone and is well correlated to shallow soil temperatures. The extrapolation of diffuse CO2 fluxes across all the thermal/altered area suggests that 410 ± 140 t d− 1 CO2 are emitted at HSB (vent emissions not included). Diffuse fluxes of H2S were measured in Yellowstone for the first time and likely exceed 2.4 t d− 1 at HSB. Comparing estimates of the total estimated diffuse H2S emission to the amount of sulfur as SO42− in streams indicates ~ 50% of the original H2S in the gas emission is lost into shallow groundwater, precipitated as native sulfur, or vented through fumaroles. We estimate the heat output of HSB as ~ 140–370 MW using CO2 as a tracer for steam condensate, but not including the contribution from fumaroles and hydrothermal vents. Overall, the diffuse heat and volatile fluxes of HSB are as great as some active volcanoes, but they are a small fraction (1–3% for CO2, 2–8% for heat) of that estimated for the entire Yellowstone system. 相似文献
98.
Caroline Bouvet de Maisonneuve Olivier Bachmann Alain Burgisser 《Bulletin of Volcanology》2009,71(6):643-658
Silicic pumices formed during explosive volcanic eruptions are faithful recorders of the state of the magma in the conduit,
close to or at the fragmentation level. We have characterized four types of pumices from the non-welded rhyolitic Kos Plateau
Tuff, which erupted 161,000 years ago in the East Aegean Arc, Greece. The dominant type of pumice (>90 vol.%) shows highly
elongated tubular vesicles. These tube pumices occur throughout the eruption. Less common pumice types include: (1) “frothy”
pumice (highly porous with large, sub-rounded vesicles), which form 5–10 vol.% of the coarsest pyroclastic flow deposits,
(2) dominantly “microvesicular” and systematically crystal-poor pumices, which are found in early erupted, fine-grained pyroclastic
flow units, and are characterized by many small (<50 μm in diameter) vesicles and few mm-sized, irregular voids, (3) grey
or banded pumices, indicating the interaction between the rhyolite and a more mafic magma, which are found throughout the
eruption sequence and display highly irregular bubble shapes. Except for the grey-banded pumices, all three other types are
compositionally identical and were generated synchronously as they are found in the same pyroclastic units. They, therefore,
record different conditions in the volcanic conduit leading to variable bubble nucleation, growth and coalescence. A total
of 74 pumice samples have been characterized using thin section observation, SEM imagery, porosimetry, and permeametry. We
show that the four pumice types have distinct total and connected porosity, tortuosity and permeability. Grey-banded pumices
show large variations in petrophysical characteristics as a response to mingling of two different magmas. The microvesicular,
crystal-poor, pumices have a bimodal bubble size distribution, interpreted as reflecting an early heterogeneous bubble nucleation
event followed by homogeneous bubble nucleation close to fragmentation. Finally, the significant differences in porosity,
tortuosity and permeability in compositionally identical tube and frothy pumices are the result of variable shear rates in
different parts of the conduit. Differential shear rates may be the result of either: (1) pure shear, inducing a vertical
progression from frothy to tube and implying a relatively thick fragmentation zone to produce both types of pumices at the
same time or (2) localized simple shear, inducing strongly tubular vesicles along the wall and near-spherical bubbles in the
centre of the conduit and not necessarily requiring a thick fragmentation zone. 相似文献
99.
Doug Hunt 《GPS Solutions》2000,4(1):74-76
The GPS Toolbox is dedicated to highlighting algorithms utilized by GPS engineers and scientists. If you have an interesting
algorithm you would like to share with our readers or if you have a topic you would like to see covered in a future column,
contact us at gps-toolbox@ngs.noaa.gov. To comment on the algorithms presented here, or to leave a request for an algorithm
you may be looking for, visit our Web site (http://www.ngs.noaa.gov/gps-toolbox). ? 2000 John Wiley & Sons, Inc. 相似文献
100.
Jaume Vergés Yohann Poprawski Ylènia Almar Peter A. Drzewiecki Mar Moragas Telm Bover-Arnal Chiara Macchiavelli Wayne Wright Grégoire Messager Jean-Christophe Embry David Hunt 《Basin Research》2020,32(6):1653-1684
Integration of extensive fieldwork, remote sensing mapping and 3D models from high-quality drone photographs relates tectonics and sedimentation to define the Jurassic–early Albian diapiric evolution of the N–S Miravete anticline, the NW-SE Castel de Cabra anticline and the NW-SE Cañada Vellida ridge in the Maestrat Basin (Iberian Ranges, Spain). The pre shortening diapiric structures are defined by well-exposed and unambiguous halokinetic geometries such as hooks and flaps, salt walls and collapse normal faults. These were developed on Triassic salt-bearing deposits, previously misinterpreted because they were hidden and overprinted by the Alpine shortening. The Miravete anticline grew during the Jurassic and Early Cretaceous and was rejuvenated during Cenozoic shortening. Its evolution is separated into four halokinetic stages, including the latest Alpine compression. Regionally, the well-exposed Castel de Cabra salt anticline and Cañada Vellida salt wall confirm the widespread Jurassic and Early Cretaceous diapiric evolution of the Maestrat Basin. The NE flank of the Cañada Vellida salt wall is characterized by hook patterns and by a 500-m-long thin Upper Jurassic carbonates defining an upturned flap, inferred as the roof of the salt wall before NE-directed salt extrusion. A regional E-W cross section through the Ababuj, Miravete and Cañada-Benatanduz anticlines shows typical geometries of salt-related rift basins, partly decoupled from basement faults. These structures could form a broader diapiric region still to be investigated. In this section, the Camarillas and Fortanete minibasins displayed well-developed bowl geometries at the onset of shortening. The most active period of diapiric growth in the Maestrat Basin occurred during the Early Cretaceous, which is also recorded in the Eastern Betics, Asturias and Basque-Cantabrian basins. This period coincides with the peak of eastward drift of the Iberian microplate, with speeds of 20 mm/year. The transtensional regime is interpreted to have played a role in diapiric development. 相似文献