Groundwater evolution, chemical sedimentation and carbonate brine formation on an island in the Okavango Delta swamp, Botswana

The Okavango Delta of semi-arid northern Botswana is a large alluvial fan (22,000 km²) covered by permanent and seasonal swamps from which 96% of the annual discharge is lost by evapotranspiration. Many small islands (1ha) within the permanent swamps are the sites of accumulation of sodium carbonate salts and many contain saline pans. The associated alkaline soils are toxic to vegetation. An understanding of the processes involved in alkalinization could be of potential benefit to long-term conservation planning in this unique ecosystem. The relation between soil chemistry and mineralogy, and swamp and groundwater chemistry were investigated on an island in the swamps. The study revealed that the water table beneath the island is depressed and swamp water enters the groundwater regime of the island from the margins and below, and flows toward the centre. The water becomes progressively more saline, initially owing to transpiration by trees and ultimately by evaporation in the central parts of the island. As a result of increasing salinity, amorphous SiO₂ and magnesium calcite precipitate in the soils beneath the marginal zone of the island, raising the land surface, while the more soluble alkali carbonates are concentrated in the centre of the island as surface crusts and brine ponds. Leaching of these salts into the soil during the rainy season and gravity-driven flow of saline brines in the dry season causes the downward movement of Al and Fe in the central zone of the island. K-feldspar and possibly amorphous allophane develop in the deeper soils under the central zone of the island.     

Cyanophycées marines endolithes colonisant les surfaces rocheuses dénudées (Etages Supralittoral et Médiolittoral de la région de Marseille)

Résumé Etude des Cyanophycées endolithes, recolonisant les surfaces rocheuses dénudées, au niveau des Etages Supralittoral et Médiolittoral de la région de Marseille. Apparition de différentes formes évolutives de l’espèceEntophysalis deusta en fonction de l’intensité des facteurs du milieu. Niveau de l’Etage Supralittoral, humectation réduite, formes épilithes et peu pénétrantes; niveau de l’Etage Médiolittoral, humectation intense, formes endolithes filamenteuses. Il est à noter que ces formes se succèdent toujours dans le même ordre, seul le stade final est différent selon la condition du milieu envisagé. Ceci va donc à l’encontre d’une espèce unique avec divers stades évolutifs. La position extrême adoptée parF. Drouet etW.A. Daily au sujet de la systématique des Cyanophycées endolithes entra?ne de nombreuses réserves et seule une étude basée sur des cultures unialgales permettra de conna?tre avec précision la marge de variation des taxons.

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Summary The object of this study was the endolithic blue-green algae of the upper and middle litoral zone of the rocky coast near Marseille. Difficulties in determining these algae cropped up very soon because of their strong plasticity. To ascertain whether there were different species of algae or only different ecological types of the same species, two series of experiments were carried out in the region around Marseille: 1. small rock splinters from various places were cemented into place in the above-mentioned zones. 2. The regrowth of rock surfaces that were cleared of algae was observed. Moreover cultures of the individual species were made to find an unequivocal solution to the systematic and ecological problems of these algae. Only the results of the tests on recolonization of the rock surface by algae are dealt with in this paper. According to our observations, theEntophysalis deusta (Men.) Drouet and Daily developed in different patterns depending on the degree of moisture. In the upper litoral zone where the amount of moisture is less significant it takes on epilithic forms that do not penetrate into the rock. In the more humid middle zone they prevail in an endolithic stringy form. It is interesting to note that the various forms always appear in the same order; thus with increasing moisture one finds:Gloeocapsa, Entophysalis, Scopulonema (forms of the upper litoral zone) andHyella caespitosa, Hyella tenuior (forms of the middle zone). Other species such as theHormathonema that grow in the same biotope as theEntophysalis seem to react differently to outside stimuli. The conjectures concerning the plasticity of theEntophysalis deusta confirmDrouet andDaily’s theory. Yet this hypothesis needs to be confirmed through cultures of the forms observed.

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Zusammenfassung Die endolithischen Blaualgen der supralitoralen und mediolitoralen Zone an der felsigen Küste in der Region von Marseille wurden untersucht. Schon zu Beginn der Arbeit zeigten sich Schwierigkeiten bei der Bestimmung dieser Algen, da sie zum Teil eine starke Plastizit?t aufzuweisen scheinen. Um abzukl?ren, ob verschiedene Arten oder nur unterschiedliche ?kotypen ein, und derselben Art vorliegen, wurden zwei Untersuchungsserien in der Region von Marseille durchgeführt: 1. Kleine Felssplitter verschiedener Herkunft wurden in den genannten Zonen festzementiert. 2. Es wurde beobachtet, wie Stellen, die man von Algen entbl?sste, wieder besiedelt wurden. Ferner begann man Kulturen der einzelnen Typen herzustellen, um die systematisch-?kologischen Probleme dieser, Blaualgen eindeutiger zu l?sen. In dieser Mitteilung sind nur die Resultate derWiederbesiedlungsversuche dargestellt. Die Beobachtungen ergaben im wesentlichen, dass die ArtEntophysalis deusta (Men.) Drouet et Daily sich je nach Intensit?t der Benetzung in verschiedenen Formen entwickelt. So tritt sie in der supralitoralen Zone, die einen reduzierten Feuchtigkeitsgrad aufweist, in epilithischen Formen auf, die wenig in den Fels eindringen. In der mediolitoralen Zone mit grosser Feuchtigkeit findet man sie als endolithische f?dige Formen. Interessant ist, dass diese verschiedenen Erscheinungsformen stets in gleicher Reihenfolge auftreten, und zwar begegnet man mit zunehmender Feuchtigkeit folgenden Formen:Gloeocapsa, Entophysalis, Scopulonema (= Formen der supralitoralen Zone),Hyella caespitosa, Hyella tenuior (= Formen der mediolitoralen Zone). Andere Gattungen, wie z. B.Hormathonema, die im gleichen Biotop wieEntophysalis auftreten, scheinen anders auf die Umweltfaktoren zu reagieren. Die Vermutungen über die Plastizit?t vonEntophysalis deusta best?tigen eigentlich die Auffassung vonDrouet undDaily. Doch diese Hypothese verlangt eine Best?tigung mittels Kulturen der beobachteten Erscheinungsformen.

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Contribution à l’étude des Cyanophycées lithophytes des Etages Supralittoral et Médiolittoral (région de Marseille), Tethys1 (1), 119–172 (1969).     

Formation, prevention and breaking of sea water in crude oil emulsions ‘chocolate mousses’

During the combatting of oil spills, the handling of the stable sea water in crude oil emulsions called ‘chocolate mousses’, either at sea or on the beach, is often a major problem.

A recent laboratory study of the formation and properties of such emulsions revealed the key roles played by asphaltenes and crystallized wax in their stability.

Viscosity measurements showed that mousses with 70% vol. sea water are thixotropic and may solidify after having been pumped into the hold of a salvage vessel or into a storage tank. During a search for novel chemicals for combatting oil spills we found a type of chemical additive that is effective in preventing the formation of mousses, in greatly improving the pumpability of mousses once formed and in decreasing their water content after recovery or during storage. The additive, designated LA 1834, has been tested successfully in the laboratory, on a semi-technical scale in cooperation with the Netherlands State Waterways Board and in the large-scale clean-up operation after a recent oil spill in Greece. The compound keeps oil from dispersing vertically into the water column, when applied to spilled oil or mousse on a sea water surface and holds promise of environmental acceptability.     

The slope movements within the Mondorès graben (Drôme, France); the interaction between geology, hydrology and typology

The Mondorès graben in the south-eastern French Alps is an uncommon structural feature, which originates in a complex polyphasic tectonic evolution. In contrast with its immediate surroundings, with hardly any huge landslides, the Mondorès graben is characterised by various types of landslides. A huge sagging caused part of the limestone cliff to subside some 50 m within 50 years. Two recent mud flows that occurred were considered a potential threat to some inhabited places downstream. The hydrologic aspects of the Boulc-Mondorès landslide resemble the geology: infiltration (and karst input?) in the rock-sliding zone and exfiltration in the marls resulting in slumps and mud flows. The mass movement interactions could be explained by a structural geology analysis with geodetic monitoring using different techniques. It is also shown that hydrochemistry as well as geophysical surveys are of importance in unravelling the hydrologic systems and the geological subsurface structures. The present paper aims at explaining the geological control of the different slides in view of estimating their potential danger. Understanding the geological structure and its evolution therefore is a necessary prerequisite.     

Sustainable groundwater resources exploration and management in a complex geological setting as part of a humanitarian project (Mahafaly Plateau,Madagascar)

Southwestern Madagascar is a semi-arid region and a hot-spot of global change. On the Mahafaly plateau, people live with quasi-permanent water stress and groundwater, the only available resource, is difficult to exploit due to a complex hydrogeological environment. A methodology (suitable for humanitarian projects; <?40 k€) was developed in four phases to assess the sustainable exploitation of the water resource: (A) regional scale exploration, (B) village scale exploration, (C) drilling campaign, and (D) hydro-climatic monitoring. This integrated hydrogeophysical approach involves geophysical measurements (262 TEM-fast soundings, 2588 Slingram measurements, 35 electrical soundings), hydrochemical analyses (112 samples), and a piezometric survey (127 measurements). Two groundwater resources were identified, one deep (below 150 m) and one shallow (<?20 m). Hydrochemical results highlighted the vulnerability of both resources: anthropic contamination for the shallower and seawater intrusion for the deeper. Therefore, subsequent geophysical surveys supported the siting of six boreholes and three wells in the shallow aquifer. This methodological approach was successful in this complex geological setting and requires testing at other sites in and outside Madagascar. The study demonstrates that geophysical results should be used in addition to drilling campaigns and to help monitor the water resource. In fact, to prevent over-exploitation, piezometric and meteorological sensors were installed to monitor the water resource. This unique hydro-climatic observatory may help (1) non-governmental organization and local institutions prevent future water shortages and (2) scientists to understand better how global change will affect this region of the world.     

Avalanche Hazard Mitigation Strategies Assessed by Cost Effectiveness Analyses and Cost Benefit Analyses—evidence from Davos,Switzerland

This paper demonstrates the application of cost effectiveness analysis and cost benefit analysis to alternative avalanche risk reduction strategies in Davos, Switzerland. The advantages as well as limitations of such analysis for natural hazards planning are discussed with respect to 16 avalanche risk reduction strategies. Scenarios include risk reduction measures that represent the main approaches to natural hazards planning in Switzerland, such as technical, organisational, and land use planning measures. The methodologies used outline how concepts and techniques from risk analysis, hazard mapping, Geographic Information System, and economics can be interdisciplinary combined. The results suggest important considerations, such as possible sources of uncertainty due to different choices in the calculation of cost effectiveness ratio and net present value. Given the parameters and assumptions, it seems as if the current approach to avalanche risk reduction in the study area approximates to economic and cost efficiency and serves the aim of reducing risk to human fatalities.     

Regional Assessment of the Effects of Land-Use Change on Landslide Hazard By Means of Physically Based Modelling

Physically based models are capable of evaluating the effects of environmental changes through adaptations in their parameters. For landslide hazard zonation, this gives them an edge over traditional, statistically based techniques that require large datasets and often lack the objectivity to achieve the same purpose. Therefore, physical models can be valuable tools for hazard assessment and planning purposes.The usefulness of the model prognosis depends largely on the ability of the physical model to mimic the landscape system. This implies that the model should be calibrated and validated and that the imposed changes do not lead to a radical departure from the present situation.Under the recognition of these constraints, a physically based model has been applied to a 1.5 km² catchment in the Alcoy region (SE Spain) to evaluate the effects of land use change on landslide activity. The model couples a transient, distributed hydrological model with a probabilistic assessment of the slope stability. Thus, it is able to assess the spatial and temporal activity of slope instability. For the present situation, validation demonstrates that the probability of failure returns a conservative estimate of the spatial frequency of landsliding. The model has subsequently been applied to two hypothetical land use change scenarios that extrapolate present and likely trends. For these scenarios, the model results indicate a marginal decrease in the spatial frequency of landsliding (aerial extent of instability). However, the decrease in the temporal activity (is total duration of instability over a given period) is substantial under the altered land use conditions. The forecasted change in landslide activity not only affects the relative weight of slope processes in the region. It also has implications for the perceived hazard levels and the landslide hazard zonation of the area.     

On the effect of clearcuts on turbulence structure above a forest canopy

Summary The modification of the flow structure arising from the removal of large patches of trees in a managed forest plantation near Gainesville, Florida is described. Using wavelet analysis of turbulence measurements taken above a forest canopy hundreds of meters downwind from the forest gap and well outside the footprint, the present paper examines changes in flow characteristics and demonstrates that the presence of the nearby clearcut introduces extraneous coherent events passing by the eddy-covariance flux measurement system.     

Results from 44 months of observations with a superconducting gravimeter at Moxa/Germany

Results for more than 42 months of observations with the superconducting gravimeter CD-034 at the Geodynamic Observatory Moxa are discussed. Moxa observatory is one of the newer stations within the ‘Global Geodynamics Project’ (GGP). A special feature of the gravimeter at Moxa is its dual sensor system; differences in the results obtained from the two sensor recordings are generally well within the standard deviations of the tidal analysis. One significant difference concerns the slightly different drift rates of 31 and 49.5 nm/s² per year for upper and lower sensor; both sensor drifts can be fitted by a linear function. We find that the noise levels are close to the ‘New Low Noise Model’ for the seismic-modes and are also low in the tidal bands. Due to this low noise, Moxa is a station well suited to search for small geodynamic signals. The long-period variation in the gravity residuals correlates well with the polar motion.The difference signal between the two sensor recordings has a peak-to-peak amplitude of about 6 nm/s² and shows systematic variations. Its spectrum is characterised by instrumental noise between 0.2 and 0.4 cph. The noise level of the difference and of the sum of the two residual datasets are clearly lower, respectively, higher than the noise contents of the gravity residuals themselves. This is a strong indication for the existence of broadband signals common to the two residual datasets, leading to the conjecture that the reduction of environmental effects is still not sufficient.Our results once more emphasize the necessity to correct the data for barometric pressure effects when analyzing the data for seismic modes. The reduction visibly increases the signal-to-noise ratio in the low frequencies of the mode band and helps to avoid misinterpreations of peaks. Besides the well known barometric pressure influence we can establish hydrological effects in the data which are probably caused by soil moisture and groundwater table variations as well as by batch-wise water movement within the weathering layer. As the major part of the observatory surroundings is above gravimeter level, an anticorrelation between hydrological and gravity changes is observed. In addition, it can be shown that global hydrological effects reach an order of magnitude that makes it necessary to consider these effects when analyzing long-period signals like polar motion. Vice versa these effects are large enough to be detectable in the gravity data. A first joint analysis of five datasets from the GGP network shows no indications for signals related to the Slichter triplet or core modes.