Hydro-geochemical processes in an area with saline groundwater in lower Shire River valley, Malawi: An integrated application of hierarchical cluster and principal component analyses   总被引：2，自引：0，他引：2  

Maurice Monjerezi  Rolf D. VogtPer Aagaard  John D.K. Saka 《Applied Geochemistry》2011,26(8):1399-1413

Our ability to adapt to changes in groundwater quality, arising from a changing climate and/or local pressures, is dependent on comprehension of the governing controls of spatial variation in groundwater chemistry. This paper presents results of an assessment of dominant hydro-geochemical processes controlling groundwater chemical composition, using an integrated application of hierarchical cluster analysis (HCA) and principal component analysis (PCA) of a major ion dataset of groundwater from lower Shire River valley, Malawi. The area is in the southernmost part of the western section of the East African Rift System (EARS) and has localised occurrence of saline groundwater. HCA classified samples into three main clusters (C1-C3) according to their dominant chemical composition: C1 (dominant composition: Na-Cl; median TDS: 3436 mg L⁻¹), C2 (dominant composition: Na-HCO₃; median TDS: 966 mg L⁻¹) and C3 (dominant composition: alkali earths-HCO₃; median TDS: 528 mg L⁻¹). These clusters were in turn described by the principal components PC1, PC3 and PC2, respectively, resulting from the PCA. The results of the PCA and geochemical interpretation suggest that the spatial variation of groundwater quality in the area is influenced by the following processes: C3 samples result mainly from H₂CO₃ weathering of aluminosilicate minerals by percolating water supersaturated with CO₂. In addition to aluminosilicate weathering, C2 samples are influenced by the processes of cation exchange of Ca²⁺ and Mg²⁺ in the water for Na⁺ on clay minerals, and carbonate precipitation. The increase in ionic strength of C2 samples is attributed to mixing with high TDS groundwater in proximity with C2 samples. The saline/brackish C1 groundwater results from the processes of evaporation (for samples with high water table close to the Shire marshes) and dissolution of Cl⁻ and SO₄-evaporative salts followed by mineralised seep from sedimentary Karoo and Cretaceous Lupata sandstones.  相似文献   

Distribution of arsenic and its mobility in shallow aquifer sediments from Ambikanagar, West Bengal, India   总被引：1，自引：0，他引：1  

Joyanto Routh  Per Hjelmquist 《Applied Geochemistry》2011,26(4):505-515

Sediments from a core retrieved during installation of a shallow drinking water well in Ambikanagar (West Bengal, India) were analyzed for various physical and chemical parameters. The geochemical analyses included: (1) a 4-step sequential extraction scheme to determine the distribution of As between different fractions, (2) As speciation (As³⁺ vs. As⁵⁺), and (3) C, N and S isotopes. The sediments have a low percentage of organic C and N (0.10-0.56% and 0.01-0.05%, respectively). Arsenic concentration is between 2 and 7 mg kg⁻¹, and it is mainly associated with the residual fraction, less susceptible to chemical weathering. The proportion of As³⁺ in these sediments is high and ranges from 24% to 74%. Arsenic in the second fraction (reducible) correlates well with Mn, and in the residual fraction As correlates well with several transition elements. The stable isotope results indicate microbial oxidation of organic matter involving SO₄ reduction. Oxidation of primary sulfide minerals and release of As from reduction of Fe-(oxy)hydroxides do not seem important mechanisms in As mobilization. Instead, the dominance of As³⁺ and presence of As⁵⁺ reducing microorganisms in this shallow aquifer imply As remobilization involving microbial processes that needs further investigations.  相似文献   

Upwelling in the Alaskan Beaufort Sea: Atmospheric forcing and local versus non-local response     

Robert S. Pickart  Michael A. Spall  G.W.K. Moore  Thomas J. Weingartner  Rebecca A. Woodgate  Knut Aagaard  Koji Shimada 《Progress in Oceanography》2011,88(1-4):78-100

The spin up and relaxation of an autumn upwelling event on the Beaufort slope is investigated using a combination of oceanic and atmospheric data and numerical models. The event occurred in November 2002 and was driven by an Aleutian low storm. The wind field was strongly influenced by the pack-ice distribution, resulting in enhanced winds over the open water of the Chukchi Sea. Flow distortion due to the Brooks mountain range was also evident. Mooring observations east of Barrow Canyon show that the Beaufort shelfbreak jet reversed to the west under strong easterly winds, followed by upwelling of Atlantic Water onto the shelf. After the winds subsided a deep eastward jet of Atlantic Water developed, centered at 250 m depth. An idealized numerical model reproduces these results and suggests that the oceanic response to the local winds is modulated by a propagating signal from the western edge of the storm. The disparity in wave speeds between the sea surface height signal—traveling at the fast barotropic shelf wave speed—versus the interior density signal—traveling at the slow baroclinic wave speed—leads to the deep eastward jet. The broad-scale response to the storm over the Chukchi Sea is investigated using a regional numerical model. The strong gradient in windspeed at the ice edge results in convergence of the offshore Ekman transport, leading to the establishment of an anti-cyclonic gyre in the northern Chukchi Sea. Accordingly, the Chukchi shelfbreak jet accelerates to the east into the wind during the storm, and no upwelling occurs west of Barrow Canyon. Hence the storm response is fundamentally different on the Beaufort slope (upwelling) versus the Chukchi slope (no upwelling). The regional numerical model results are supported by additional mooring data in the Chukchi Sea.  相似文献   

Model of outgrowths in the spiral galaxies NGC 4921 and NGC 7049 and the origin of spiral arms     

Per Carlqvist 《Astrophysics and Space Science》2013,343(2):689-712

NGC 4921 and 7049 are two spiral galaxies presenting narrow, distinct dust features. A detailed study of the morphology of those features has been carried out using Hubble Space Telescope archival images. NGC 4921 shows a few but well-defined dust arms midway to its centre while NGC 7049 displays many more dusty features, mainly collected within a ring-shaped formation. Numerous dark and filamentary structures, called outgrowths, are found to protrude from the dusty arms in both galaxies. The outgrowths point both outwards and inwards in the galaxies. Mostly they are found to be V-shaped or Y-shaped with the branches connected to dark arm filaments. Often the stem of the Y appears to consist of intertwined filaments. Remarkably, the outgrowths show considerable similarities to elephant trunks in H ii regions. A model of the outgrowths, based on magnetized filaments, is proposed. The model provides explanations of both the shapes and orientations of the outgrowths. Most important, it can also give an account for their intertwined structures. It is found that the longest outgrowths are confusingly similar to dusty spiral arms. This suggests that some of the outgrowths can develop into such arms. The time-scale of the development is estimated to be on the order of the rotation period of the arms or shorter. Similar processes may also take place in other spiral galaxies. If so, the model of the outgrowths can offer a new approach to the old winding problem of spiral arms.  相似文献   

Berm formation and dynamics on a gently sloping beach; the effect of water level and swash overtopping     

Stine G. Jensen  Troels Aagaard  Tom E. Baldock  Aart Kroon  Michael Hughes 《地球表面变化过程与地形》2009,34(11):1533-1546

Berm formation and morphological development of the beach face have been observed during a neap–neap tidal cycle on the gently sloping and accreting beach at Vejers, Denmark. During the field campaign, an intertidal bar migrated onshore and stabilized as a berm on the foreshore. A new intertidal bar occurred on the lower beach face, migrated onshore on the rising tide and finally merged with the pre‐existing berm. As the tide continued to rise, the new berm translated further onshore as an intertidal bar to the uppermost part of the foreshore. The sediment transport during the berm transition was onshore directed in the upper swash and offshore directed in the lower swash. This berm development can be described through both the neap‐berm, ridge‐and‐runnel and berm‐ridge development concepts proposed by Hine (Sedimentology 1979; 26: 333–351), and all three stages were observed during only three tidal cycles. The main factors controlling this fast transformation were the gentle slope of the cross‐shore profile, rapid water level translation rates, substantial swash overtopping of the berm, and low infiltration rates. Despite the onshore migration of intertidal bars and berm formation, no net foreshore accretion took place during the field campaign. This was largely due to the formation of rip channels with strong rip currents cutting through the intertidal bars and the berm, which acted as a sediment drain in the profile. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Godzilla from a Zoological Perspective     

Per Christiansen 《Mathematical Geology》2000,32(2):231-245

The 1998 TriStar movie Godzilla proved to be a major blockbuster, although not the financial success that was initially anticipated and movie critics have been rather unforgiving. Apart from a radically different external morphology compared to the classic Japanese movie monster, the new Godzilla character apparently was made different from the old version on a number of key points to make him more biologically probable. However, calculations show that his limbs and limb muscles must have been severely undersized to move his huge bulk around at even a leisurely pace, and most other biological problems with the old Godzilla, e.g., growth rates and reptilian physiology at such a massive size, have remained unaltered. The old Godzilla was actually the more plausible from a biomechanical point of view.  相似文献   

Sputtering in the outflows of cool stars     

Carl Covatto  Per A. Aannestad 《Monthly notices of the Royal Astronomical Society》2000,318(1):67-72

Radiation pressure acts to accelerate dust grains and, by transfer of momentum through collisions with the gas, drives the outflows of late-type stars. Some of these dust–gas collisions may be energetic enough to remove atoms from the dust grains. From an assumed initial size distribution for the dust grains, the method of Krüger et al. is used to study the evolution of a sample of spherical amorphous carbon grains under conditions typical of a late-type star. The size distribution of dust grains is presented for various sets of model parameters. One set of models assumes an initial Mathis, Rumpl & Nordsieck (MRN) distribution for the dust grains. The high-luminosity ( L ∗), high-effective temperature ( T _eff) set of parameters has a terminal velocity ( v _term) that is near, but above , the upper limit of observed outflow velocities for carbon stars (∼30 km s⁻¹ for the assumed ̇ of 5×10⁻⁶ M_⊙ yr⁻¹). The low L ∗, T _eff model has a v _term that lies near, but below , the upper limit of observed velocities. A significant amount of sputtering occurs in the high L ∗, T _eff model with ∼40 per cent of the grain mass sputtered. About ∼1 per cent of the dust mass is sputtered in the low L ∗, T _eff. Another set of models assumes that the dust forms with a log-normal distribution. Here, v _term is nearly the same for the high L ∗, T _eff model as for the low L ∗, T _eff model. This is a result of the large amount of dust mass loss (∼75 per cent) by sputtering in the high L ∗, T _eff model.  相似文献   

Sediment-chemistry response to land-use change and pollutant loading in a hypertrophic lake, southern Sweden   总被引：3，自引：0，他引：3  

Siv Olsson  Joachim Regnéll  Anders Persson  Per Sandgren 《Journal of Paleolimnology》1997,17(3):275-294

Responses to recent land-use changes and pollutant loading in the sediment of a hypertrophic lake in southern Sweden were studied by comparison of geochemical, pollen and magnetic records with historical land-use data. A chronology was constructed for the last two centuries by correlating changes in the pollen diagram to major events in the land-use history. Sediment accumulation was low (mean c. 0.2 g cm^-2 yr^-1) prior to 1800 AD, when less than 25% of the catchment was arable land. Reorganization of the agrarian system during the 19th century increased the annually tilled area by 300%, which accelerated soil erosion and substantially increased the accumulation of allochtonous matter in the lake. Since the turn of the century 90% of the catchment has been ploughed every year. The deposition of clastic matter in the lake has, however, decreased due to a gradual rerouting of the drainage system, which has reduced the effective catchment area by c. 85%.Authigenic vivianite (Fe3(PO4)2.8H2O) is a major P phase in the preindustrial non-sulphidic sediments, which suggests that the sediments at that time served as a fairly efficient sink for P. The arable expansion, increased manuring and, eventually, the introduction of artificial fertilizers during the 19th century led to a massive influx of nutrients, which elevated primary production in the lake. Subsequent development of bottom water anoxia around 1900, in combination with an additional pollutant burden of sulphate within the lake basin, led to major alterations of the biogeochemical cycles. The most critical change in the post-1900 sediments involved the cycling of Fe and P. The linkage between the lacustrine P and Fe cycles can explain that FeS formation was paralleled by a release of P from the sedimentary pool. This supply of P to the lake basin must have supplemented the nutrient supply by modern agriculture and contributed to recent hypertrophication. The bacterial sulphate reduction also affected the generation of alkalinity which supported a significant calcite precipitation in the post-1900 sediments.S is enriched 10-fold in the post-1900 sediments compared to preindustrial values. Along with the rise in S, soot particles derived from fossil fuel combustion appear in the sediments for the first time. Therefore, Bussj¨osj¨on is thought to be a good example of how a well-buffered, highly productive lake may respond to the pollution by sulphur from acid rain.  相似文献   

Water-Well Siting in Hard-Rock Areas: Identifying Promising Targets Using a Probabilistic Approach     

Per Sander 《Hydrogeology Journal》1997,5(3):32-43

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Late Holocene tephrochronology of the northern Antarctic Peninsula     

Svante Bjrck  Per Sandgren  Rolf Zale 《Quaternary Research》1991,36(3)

Andesitic and basaltic andesitic tephra layers are abundant in Holocene deposits from the Antarctic Peninsula. Visually discernible tephra horizons occur in three lakes on Livingston Island. Tephra in two other lakes and in a moss bank on Elephant Island, with very low ash concentrations, were detected magnetically. Deception Island is the most likely volcanic source for the tephra. With direct ¹⁴C dating, age/depth curves, and cross-correlations at least 14 tephra horizons dating to between ca. 4700 and 250 yr B.P. were identified and now form the basis for a preliminary regional tephrochronology that will be a valuable dating tool for investigating the Holocene climatic history of Antarctica.  相似文献