Characterising the distribution of near-shore submarine groundwater discharge along a coastline using 222Rn and electrical conductivity     

M. A. Short  S. Lamontagne  P. G. Cook  R. Cranswick 《Australian Journal of Earth Sciences》2013,60(2):319-331

The spatial and temporal distribution of near-shore fresh submarine groundwater discharge (SGD) was characterised from the coastal aquifers of the Willunga Basin, South Australia, an extensive aquifer system that supports an important viticultural region. Measurements of electrical conductivity (EC) and ²²²Rn (radon) activity were collected at 19 sites along the coastline during the Southern Hemisphere spring (2011) and summer (2013). At each site, samples were collected from the surf zone as well asporewater from beach sediment in the intertidal zone. Surf-zone radon activity ranged from <5 to 70mBq L^–1, and intertidal porewater radon ranged over two orders of magnitude (220–36 940 mBq L^–1) along the Willunga Basin coastline during both surveys. Overall, surf-zone and porewater EC was lower in the spring 2011 survey than in the summer 2013 survey. Porewater EC was similar to that of coastal water at most sites along the coastline, except at three sites where porewater EC was found to be lower than coastal water during both surveys, and three sites where evaporated seawater was observed in the summer survey. Based on the patterns in radon and EC along the coastline, two sites of localised fresh SGD were identified, in addition to a groundwater spring that is known to discharge to the coast. The results indicate that near-shore fresh SGD occurs as localised seeps rather than diffuse seepage along the entire coastline. The apparent absence of groundwater discharge at most locations is also consistent with current evidence suggesting that extensive groundwater pumping within the basin has resulted in seawater intrusion across much of the coastline. These observations also suggest that previous studies are likely to have over-estimated SGD rates from the Willunga Basin because they assumed that SGD occurred along the entire coastline.  相似文献   

Discretizing the fracture-matrix interface to simulate solute transport   总被引：1，自引：0，他引：1  

Weatherill D  Graf T  Simmons CT  Cook PG  Therrien R  Reynolds DA 《Ground water》2008,46(4):606-615

This article examines the required spatial discretization perpendicular to the fracture-matrix interface (FMI) for numerical simulation of solute transport in discretely fractured porous media. The discrete-fracture, finite-element model HydroGeoSphere ( Therrien et al. 2005 ) and a discrete-fracture implementation of MT3DMS ( Zheng 1990 ) were used to model solute transport in a single fracture, and the results were compared to the analytical solution of Tang et al. (1981) . To match analytical results on the relatively short timescales simulated in this study, very fine grid spacing perpendicular to the FMI of the scale of the fracture aperture is necessary if advection and/or dispersion in the fracture is high compared to diffusion in the matrix. The requirement of such extremely fine spatial discretization has not been previously reported in the literature. In cases of high matrix diffusion, matching the analytical results is achieved with larger grid spacing at the FMI. Cases where matrix diffusion is lower can employ a larger grid multiplier moving away from the FMI. The very fine spatial discretization identified in this study for cases of low matrix diffusion may limit the applicability of numerical discrete-fracture models in such cases.  相似文献   

Estimating Flow Using Tracers and Hydraulics in Synthetic Heterogeneous Aquifers   总被引：2，自引：0，他引：2  

Marie Larocque  Peter G. Cook  Klaus Haaken  Craig T. Simmons 《Ground water》2009,47(6):786-796

Regional ground water flow is most usually estimated using Darcy's law, with hydraulic conductivities estimated from pumping tests, but can also be estimated using ground water residence times derived from radioactive tracers. The two methods agree reasonably well in relatively homogeneous aquifers but it is not clear which is likely to produce more reliable estimates of ground water flow rates in heterogeneous systems. The aim of this paper is to compare bias and uncertainty of tracer and hydraulic approaches to assess ground water flow in heterogeneous aquifers. Synthetic two-dimensional aquifers with different levels of heterogeneity (correlation lengths, variances) are used to simulate ground water flow, pumping tests, and transport of radioactive tracers. Results show that bias and uncertainty of flow rates increase with the variance of the hydraulic conductivity for both methods. The bias resulting from the nonlinearity of the concentration–time relationship can be reduced by choosing a tracer with a decay rate similar to the mean ground water residence time. The bias on flow rates estimated from pumping tests is reduced when performing long duration tests. The uncertainty on ground water flow is minimized when the sampling volume is large compared to the correlation length. For tracers, the uncertainty is related to the ratio of correlation length to the distance between sampling wells. For pumping tests, it is related to the ratio of correlation length to the pumping test's radius of influence. In regional systems, it may be easier to minimize this ratio for tracers than for pumping tests.  相似文献   

Spatial patterns of sub-tidal seagrasses and their tissue nutrients in the Torres Strait,northern Australia: Implications for management     

James K. Sheppard  Alex B. Carter  Len J. McKenzie  C. Roland Pitcher  Robert G. Coles 《Continental Shelf Research》2008

The distribution and nutritional profiles of sub-tidal seagrasses from the Torres Strait were surveyed and mapped across an area of 31,000 km². Benthic sediment composition, water depth, seagrass species type and nutrients were sampled at 168 points selected in a stratified representative pattern. Eleven species of seagrass were present at 56 (33.3%) of the sample points. Halophila spinulosa, Halophila ovalis, Cymodocea serrulata and Syringodium isoetifolium were the most common species and these were nutrient profiled. Sub-tidal seagrass distribution (and associated seagrass nutrient concentrations) was generally confined to northern-central and south-western regions of the survey area (<longitude 142.60), where mean water depth was relatively shallow (approximately 13 m below MSL) and where sediments were comprised primarily muddy sand to gravelly sand. Seagrass nitrogen and starch content, the most important nutrients for marine herbivores, were significantly correlated with species and with the plant component (above or below ground). For all seagrass species, the above-ground component (shoots and leaves) possessed greater nitrogen concentrations than the below-ground component (roots and rhizomes), which possessed greater starch concentrations. S. isoetifolium had the highest total nitrogen concentrations (1.40±0.05% DW). However, it also had higher fibre concentrations (38.2±0.68% DW) relative to the other four species. H. ovalis possessed the highest starch concentrations (2.76±0.12% DW) and highest digestibility (83.24±0.66% DW) as well as the lowest fibre (27.2±0.66% DW). The high relative abundance (found at 55% of the sites that had seagrass) and nutrient quality characteristics of H. ovalis make it an important source of energy to marine herbivores that forage sub-tidally in the Torres Strait. There were two regions in Torres Strait (north-central and south-western) where sub-tidal seagrass meadows were prevalent and of relatively higher nutritional value. This spatial and nutritional information can be used by local agencies to manage and to protect the ecological, economic and cultural values of the sub-tidal seagrass ecosystems and associated fisheries of the Torres Strait.  相似文献   

Productivity,carbon assimilation and intra-annual change in tropical reef platform seagrass communities of the Torres Strait,north-eastern Australia     

Michael A. Rasheed  Kara R. Dew  Len J. McKenzie  Robert G. Coles  Simon P. Kerville  Stuart J. Campbell 《Continental Shelf Research》2008

Detailed data on seagrass distribution, abundance, growth rates and community structure information were collected at Orman Reefs in March 2004 to estimate the above-ground productivity and carbon assimilated by seagrass meadows. Seagrass meadows were re-examined in November 2004 for comparison at the seasonal extremes of seagrass abundance. Ten seagrass species were identified in the meadows on Orman Reefs. Extensive seagrass coverage was found in March (18,700 ha) and November (21,600 ha), with seagrass covering the majority of the intertidal reef-top areas and a large proportion of the subtidal areas examined. There were marked differences in seagrass above-ground biomass, distribution and species composition between the two surveys. Major changes between March and November included a substantial decline in biomass for intertidal meadows and an expansion in area of subtidal meadows. Changes were most likely a result of greater tidal exposure of intertidal meadows prior to November leading to desiccation and temperature-related stress.  相似文献   

North Atlantic marine 14C reservoir effects: Implications for late-Holocene chronological studies     

P.L. Ascough  G.T. Cook  A.J. Dugmore 《Quaternary Geochronology》2009,4(3):171-180

We investigated surface ocean–atmosphere ¹⁴C offsets for the later Holocene at eight locations in the eastern North Atlantic. This resulted in 11 new ΔR assessments for the west coast of Ireland, the Outer Hebrides, the north coast of the Scottish mainland, the Orkney Isles and the Shetland Isles over the period 1300–500 BP. Assessments were made using a robust Multiple Paired Sample (MPS) approach, which is designed to maximize the accuracy of ΔR determinations. Assessments are placed in context with other available data to enable reconstruction of a realistic picture of surface ocean ¹⁴C activity over the Holocene period within the North Atlantic region.  相似文献   

TAOS – The Taiwanese‐American Occultation Survey     

M. J. Lehner  C. Alcock  T. Axelrod  F. Bianco  Y.‐I. Byun  W.‐P. Chen  K. H. Cook  R. Dave  I. de Pater  J. Giammarco  S.‐K. King  T. Lee  J. Lissauer  S. L. Marshall  S. Mondal  T. Nihei  J. Rice  M. Schwamb  A. Wang  S.‐Y. Wang  C.‐Y. Wen  Z.‐W. Zhang 《Astronomische Nachrichten》2006,327(8):814-817

The Taiwanese‐American Occultation Survey (TAOS) seeks to determine the number and size spectrum for small (∼3 km) bodies in the Kuiper Belt. This will be accomplished by searching for the brief occultations of bright stars (R ∼ 14) by these objects. We have designed and built a special purpose photometric monitoring system for this purpose. TAOS comprises four 50 cm telescopes, each equipped with a 2048 × 2048 pixel CCD camera, in a compact array located in the central highlands of Taiwan. TAOS will monitor up to 2 000 stars at 5 Hz. The system went into scientific operation in the autumn of 2005. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

Exploring coronal structures with SOHO     

M. Karovska  B. Wood  J. Chen  J. Cook  R. Howard 《Journal of Astrophysics and Astronomy》2000,21(3-4):403-406

We applied advanced image enhancement techniques to explore in detail the characteristics of the small-scale structures and/or the low contrast structures in several Coronal Mass Ejections (CMEs) observed by SOHO. We highlight here the results from our studies of the morphology and dynamical evolution of CME structures in the solar corona using two instruments on board SOHO: LASCO and EIT.  相似文献   

High Angular Resolution Studies of Coronal Structures with SOHO     

Margarita Karovska  Brian Wood  John W. Cook  James Chen  Russ Howard 《Astrophysics and Space Science》2000,273(1-4):7-15

Since January 1996, the Solar and Heliospheric Observatory (SOHO)has been providing unprecedented views of the extended solar coronato heliocentric distances of up to 32 solar radii. During the past threeyears we carried out studies of the morphology and dynamical evolutionof various structures in the solar corona using two instruments on boardSOHO: the Large Angle Spectrometric Coronagraph (LASCO) and the ExtremeUltraviolet Imaging Telescope (EIT). We have applied advanced image resolutionenhancement techniques to explore in detail the characteristics of thesmall-scale structures and/or the low contrast structures in the solarcorona. We describe here the results from these high-angular resolutionstudies, including of the kinematics of several Coronal Mass Ejections andpolar jets.  相似文献   

Azimuthal impact directions from oblique impact crater morphology     

D. Wallis †  M. J. Burchell  A. C. Cook  C. J. Solomon  N. McBride 《Monthly notices of the Royal Astronomical Society》2005,359(3):1137-1149

Planetary impact craters have a high degree of radial symmetry. This hampers efforts to identify the azimuthal impact direction for most craters – the radially symmetric component of an impact crater swamps any asymmetries that may be present. We demonstrate how the asymmetric component can be isolated and the direction of the asymmetries quantified using a two-dimensional eigenfunction expansion over a circular domain. The complex coefficients of expansion describe the magnitude and phase (angular alignment) of each term. From the analysis of hypervelocity impact craters formed in the laboratory, with impact angles ranging from 0° to 50° from the surface normal, we show that asymmetries which reveal the impact direction are still present at just 10° from the surface normal, and that the phase of one complex coefficient of expansion, c ₁₁, indicates the impact direction. Analysis of the lunar crater Hadley shows bilateral symmetry in the radially asymmetric component, which may be due to oblique impact. The 31-km lunar ray crater Kepler has morphological features that indicate the azimuthal impact direction. Coefficient c ₁₁ gives an azimuthal impact direction similar to that expected from the morphology, although post-impact gravitational collapse and slumping obscure the result to some degree. Ray craters may provide a means of testing the method for smaller 'simple' craters when data are available.  相似文献