Saltwater intrusion and nitrate pollution in the coastal aquifer of Dar es Salaam, Tanzania   总被引：6，自引：5，他引：1  

Yohana Mtoni  Ibrahimu Chikira Mjemah  Charles Bakundukize  Marc Van Camp  Kristine Martens  Kristine Walraevens 《Environmental Earth Sciences》2013,70(3):1091-1111

Dar es Salaam Quaternary coastal aquifer is a major source of water supply in Dar es Salaam City used for domestic, agricultural, and industrial uses. However, groundwater overdraft and contamination are the major problems affecting the aquifer system. This study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of Dar es Salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality. Major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors: dissolution of calcite and dolomite, weathering of silicate minerals, seawater intrusion due to aquifer overexploitation, and nitrate pollution mainly caused by the use of pit latrines and septic tanks. High enrichment of Na⁺ and Cl^? near the coast gives an indication of seawater intrusion into the aquifer as also supported from the Na–Cl signature on the Piper diagram. The boreholes close to the coast have much higher Na/Cl molar ratios than the boreholes located further inland. The dissolution of calcite and dolomite in recharge areas results in Ca–HCO₃ and Ca–Mg–HCO₃ groundwater types. Further along flow paths, Ca²⁺ and Na⁺ ion exchange causes groundwater evolution to Na–HCO₃ type. From the PHREEQC simulation model, it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals. The results of this study provide important information required for the protection of the aquifer system.  相似文献   

Drought impacts on long-term hydrodynamic behavior of groundwater in the tertiary–quaternary aquifer system of Shahrekord Plain, Iran     

Mahdi Radfar  Marc Van Camp  Kristine Walraevens 《Environmental Earth Sciences》2013,70(2):927-942

Shortage of water resources in arid and semi-arid areas causes water supply to be one of the most important subjects and major concerns within NGO and governments’ policies in recent years. The Shahrekord Plain aquifer system is located in a semi-arid area and acts as a key source of water supply. Groundwater management in this area is thus very important. Although change in the climatological factors is not possible, long-term fluctuation studies can help in managing the available water resources to overcome from drought or decrease its negative impact. The hydrodynamic study of the aquifer system coupled with the drought indices in each region can be useful in making decisions related to the hydro-ecosystem management of that region. In this article, hydrodynamics of the aquifer system of the Shahrekord Plain coupled with the ratio of P/PET as a drought index, are assessed on the long term. In Shahrekord Plain aquifer, there is a short-term seasonal fluctuation, which is increased by overexploitation during the dry season, when water is needed for irrigation. The hydrodynamic behavior of the plain aquifer on the long term is changing. This fluctuation at first is a function of time. Secondly, it is spatially dependent. Groundwater behavior is directly sensitive to the variation of drought index, both seasonally and on the long term.  相似文献   

Geoheritage values in the Cat Ba islands, Vietnam     

Ta Hoa Phuong  Nguyen Huu Cu  Tran Duc Thanh  Bui Van Dong 《Environmental Earth Sciences》2013,70(2):543-548

Geological wonders have been generally known as natural wonderful products. Resulted from geological processes, geological wonders are diverse in size that have geoheritage values that should be protected from damaging of substance, form and natural development. In a large scale, geological wonders can be geoheritage areas, containing several geodiversity elements that are geologically important or in a smaller scale, they can be geosites of heritage values (or geoheritage sites). In the delimitation of areas, having geoheritage values and the establishment of geoparks, the first thing is to recognise them as geosites and geoheritage areas that indicate great geological values. Besides the Ha Long bay, the world natural heritage with its outstanding aesthetic and geological values, the Cat Ba islands are typical and grandeur karst landscapes formed in tropical condition. Based on the geodiversity elements with their own geoheritage values on aesthetics, uniqueness and grandeur in the Cat Ba islands, the authors have recognised three geoheritage areas: the south cape of the Cat Ba embayment, Tung Gau (shelter), and the Lan Ha bay. Sites where Brachiopods, Crinoids and Tetracorals are exposed on the way through the island are considered as palaeontological geosites. The folds of limestone layers in the northern part of Cat Co 3 beach, with typical turbidite structures in carbonate formations are considered as a lithological geosite. The Devonian-Carboniferous boundary near the Cat Co 3 beach is regarded as a stratigraphical geosite while Que Kem and Turtle islands, etc. are considered as geomorphological geosites.  相似文献   

Accumulation and distribution of petroleum hydrocarbons found in mussels (Mytilus galloprovincialis) in the canals of Venice, Italy   总被引：3，自引：0，他引：3  

Wetzel DL  Van Vleet ES 《Marine pollution bulletin》2004,48(9-10):927-936

Petroleum contamination was assessed in indigenous and transplanted mussels for three types of environments within the Venice Lagoon and its associated interior canals. Indigenous and transplanted mussels were least impacted by petroleum contamination at open-water stations, but more affected in partially-enclosed areas indicating that physical processes of tidal or wind activities can remove organic contaminants such as petroleum from these areas more effectively. Limited tidal flushing of the interior canals resulted in contaminated locations unable to support indigenous mussels. Clean mussels transplanted to these highly impacted sites accumulated the highest levels of petroleum hydrocarbons. The distribution of polycyclic aromatic hydrocarbons (PAHs) indicated mainly combustion-related activities from sources such as the nearby industrial zones and boat traffic, with some evidence of fresher petroleum inputs. Total hydrocarbons found in all mussels could be related to concentrations found in the surrounding sediments as reported in an earlier study.  相似文献   

Processes controlling solubility of biogenic silica and pore water build-up of silicic acid in marine sediments   总被引：1，自引：0，他引：1  

Suvasis Dixit  Philippe Van Cappellen  A. Johan van Bennekom 《Marine Chemistry》2001,73(3-4)

Dissolution experiments in batch and flow-through reactors were combined with data on sediment composition and pore water silicic acid profiles to identify processes controlling the solubility of biogenic silica and the build-up of silicic acid in marine sediments. The variability of experimentally determined biogenic silica solubilities is due, in part, to variations in specific surface area and Al content of biosiliceous materials. Preferential dissolution of delicate skeletal structures and frustules with high surface areas leads to a progressive decrease of the specific surface area. This may cause a reduction of the solubility of deposited biosiliceous debris by 10–15%, relative to fresh planktonic assemblages. Dissolution of lithogenic (detrital) minerals in sediments releases dissolved aluminum to the pore waters. This aluminum becomes structurally incorporated into deposited biogenic silica, further decreasing its solubility. Compared to Al-free biogenic silica, the solubility of diatom frustules is lowered by as much as 25% when one out of every 70 Si atoms is substituted by an Al(III) ion.The build-up of silicic acid in pore waters of sediments with variable proportions of detrital matter and biogenic silica was simulated in batch experiments using kaolinite and basalt as model detrital constituents. The steady-state silicic acid concentrations measured in the experiments decreased with increasing detrital-to-opal ratios of the mixtures. This trend is similar to the observed inverse relationship between asymptotic pore water silicic acid concentrations and detrital-to-opal ratios in Southern Ocean sediments. Flow-through reactor experiments further showed that in detrital-rich sediments, precipitation of authigenic alumino-silicates may prevent the pore waters from reaching equilibrium with the dissolving biogenic silica. This agrees with data from Southern Ocean sediments where, at sites containing more than 30 wt.% detrital material, the pore waters remain undersaturated with respect to the experimentally determined in situ solubility of biogenic silica.The results of the study show that interactions between deposited biogenic silica and detrital material cause large variations in the asymptotic silicic acid concentration of marine sediments. The production of Al(III) by the dissolution of detrital minerals affects the build-up of silicic acid by reducing the apparent silica solubility and dissolution kinetics of biosiliceous materials, and by inducing precipitation of authigenic alumino-silicate minerals.  相似文献   

Librations and obliquity of Mercury from the BepiColombo radio-science and camera experiments     

Gregor Pfyffer  Tim Van Hoolst 《Planetary and Space Science》2011,59(9):848-861

A major goal of the BepiColombo mission to Mercury is the determination of the structure and state of Mercury's interior. Here the BepiColombo rotation experiment has been simulated in order to assess the ability to attain the mission goals and to help lay out a series of constraints on the experiment's possible progress. In the rotation experiment pairs of images of identical surface regions taken at different epochs are used to retrieve information on Mercury's rotation and orientation. The idea is that from observations of the same patch of Mercury's surface at two different solar longitudes of Mercury the orientation of Mercury can be determined, and therefore also the obliquity and rotation variations with respect to the uniform rotation.The estimation of the libration amplitude and obliquity through pattern matching of observed surface landmarks is challenging. The main problem arises from the difficulty to observe the same landmark on the planetary surface repeatedly over the MPO mission lifetime, due to the combination of Mercury's 3:2 spin-orbit resonance, the absence of a drift of the MPO polar orbital plane and the need to combine data from different instruments with their own measurement restrictions.By assuming that Mercury occupies a Cassini state and that the spacecraft operates nominally we show that under worst case assumptions the annual libration amplitude and obliquity can be measured with a precision of, respectively, 1.4 arcseconds (as) and 1.0 as over the nominal BepiColombo MPO lifetime with about 25 landmarks for rather stringent illumination restrictions. The outcome of the experiment cannot be easily improved by simply relaxing the observational constraints, or increasing the data volume.  相似文献   

Exact first integrals for a Lane–Emden equation of the second kind modeling a thermal explosion in a rectangular slab     

Robert A. Van Gorder 《New Astronomy》2011,16(8):492-497

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Photoabsorption in Ganymede’s atmosphere     

Gaël Cessateur  Jean Lilensten  Mathieu Barthélémy  Thierry Dudok de Wit  Cyril Simon Wedlund  Guillaume Gronoff  Hélène Ménager  Matthieu Kretzschmar 《Icarus》2012,218(1):308-319

In the framework of future space missions to Ganymede, a pre-study of this satellite is a necessary step to constrain instrument performances according to the mission objectives. This work aims at characterizing the impact of the solar UV flux on Ganymede’s atmosphere and especially at deriving some key physical parameters that are measurable by an orbiter. Another objective is to test several models for reconstructing the solar flux in the Extreme-UV (EUV) in order to give recommendations for future space missions.Using a Beer–Lambert approach, we compute the primary production of excited and ionized states due to photoabsorption, neglecting the secondary production that is due to photoelectron impacts as well as to precipitated suprathermal electrons. Ions sputtered from the surface are also neglected. Computations are performed at the equator and close to the pole, in the same conditions as during the Galileo flyby. From the excitations, we compute the radiative relaxation leading to the atmospheric emissions. We also propose a simple chemical model to retrieve the stationary electron density. There are two main results: (i) the modelled electron density and the one measured by Galileo are in good agreement. The main atmospheric visible emission is the atomic oxygen red line at 630 nm, both in equatorial and in polar conditions, in spite of the different atmospheric compositions. This emission is measurable from space, especially for limb viewing conditions. The OH emission (continuum between 260 and 410 nm) is also probably measurable from space. (ii) The input EUV solar flux may be directly measured or reconstructed from only two passbands solar observing diodes with no degradation of the modelled response of the Ganymede’s atmosphere. With respect to these results, there are two main conclusions: (i) future missions to Ganymede should include the measurement of the red line as well as the measurement of OH emissions in order to constrain the atmospheric model. (ii) None of the common solar proxies satisfactorily describes the level of variability of the solar EUV irradiance. For future atmospheric planetary space missions, it would be more appropriate to derive the EUV flux from a small radiometer rather than from a full-fledged spectrometer.  相似文献   

Oxygen isotope composition of meteoritic ablation debris from the Transantarctic Mountains: Constraining the parent body and implications for the impact scenario     

M. Van GINNEKEN  C. SUAVET  C. CORDIER  L. FOLCO  P. ROCHETTE  C. SONZOGNI  N. PERCHIAZZI 《Meteoritics & planetary science》2012,47(11):1738-1747

Abstract– Microscopic meteoritic ablation spheres recently found on top of the Victoria Land in Transantarctic Mountains, and in the L2 Dome C and DF2691 Dome Fuji ice core layers document a major impact of a 10⁸ kg (or larger) cosmic body in the Antarctic region about 480 kyr ago. Although of broadly chondritic composition, the exact nature of the impactor is unknown, and whether the impactor struck the Antarctic ice sheet or exploded in the atmosphere is a matter of debate. Based on oxygen isotope analyses of ablation spheres from the Transantarctic Mountains by means of IR‐laser fluorination coupled with mass spectrometry, we suggest that they represent the debris of an atmospheric airburst of a primitive asteroid of CV, CO, or CK composition, or a comet with composition similar to the short‐period comet 81P/Wild 2.  相似文献   

Navigating to the Moon along low-energy transfers     

M. Vetrisano  W. Van der Weg  M. Vasile 《Celestial Mechanics and Dynamical Astronomy》2012,114(1-2):25-53

This paper presents a navigation strategy to fly to the Moon along a Weak Stability Boundary transfer trajectory. A particular strategy is devised to ensure capture into an uncontrolled relatively stable orbit at the Moon. Both uncertainty in the orbit determination process and in the control of the thrust vector are included in the navigation analysis. The orbit determination process is based on the definition of an optimal filtering technique that is able to meet accuracy requirements at an acceptable computational cost. Three sequential filtering techniques are analysed: an extended Kalman filter, an unscented Kalman filter and a Kalman filter based on high order expansions. The analysis shows that only the unscented Kalman filter meets the accuracy requirements at an acceptable computational cost. This paper demonstrates lunar weak capture for all trajectories within a capture corridor defined by all the trajectories in the neighbourhood of the nominal one, in state space. A minimum Δv strategy is presented to extend the lifetime of the spacecraft around the Moon. The orbit determination and navigation strategies are applied to the case of the European Student Moon Orbiter.  相似文献