Heavy metal pollution in the Arcachon Basin (France) : Bonding states

Information available (since 1977) from a great many recent surveys on the Arcachon Basin has enabled us to determine three characteristic environments and study the behaviour of heavy metals (Ni, Cu, Zn, Pb, Cd, Hg). The relationship between heavy metals and grain-size fractions has been determined after having specified whether metals were preferentially organically or inorganically bound. This study has allowed us to establish the origin (natural or anthropogenic) of metallic enrichment and assess the extent of pollution.     

Non-linear soil-structure-interaction analysis using dynamic stiffness or flexibility of soil in the time domain

The basic equation of motion to analyse the interaction of a non-linear structure and an irregular soil with the linear unbounded soil is formulated in the time domain. The contribution of the unbounded soil involves convolution integrals of the dynamic-stiffness coefficients in the time domain and the corresponding motions. Alternatively, a flexibility formulation for the contribution of the unbounded soil using the dynamic-flexibility coefficients in the time domain, together with the direct-stiffness method for the structure and the irregular soil can be applied. The dynamic-stiffness or flexibility coefficient in the time domain is calculated as the inverse Fourier transform of the corresponding value in the frequency domain. The dynamic-stiffness coefficient's asymptotic behaviour for high frequencies determines the singular part whose transformation exists only in the sense of a distribution. As the dynamic-flexibility coefficient converges to zero for the frequency approaching infinity, the corresponding coefficient in the time domain is simpler to calculate, as no singular part exists. The salient features of the dynamic-stiffness and flexibility coefficients in the time domain are illustrated using a semi-infinite rod with exponentially increasing area. The dynamic-flexibility coefficients in the time domain are calculated for a rigid circular disc resting on the surface of an elastic halfspace and of a layer built-in at its base. Material damping is also introduced using the three-parameter Kelvin and the Voigt models.     

Non-linear soil-structure-interaction analysis using green's function of soil in the time domain

The contribution of the (linear) unbounded soil to the basic equation of motion of a non-linear analysis of soil-structure interaction consists of convolution integrals of the displacement-force relationship in the time domain and the history of the interaction forces. The former is calculated using the indirect boundary-element method, which is based on a weighted-residual technique and involves Green's functions. As an example of a non-linear soil-structure-interaction analysis, the partial uplift of the basemat of a structure is examined. As the convolution integrals have to be recalculated for each time step, the computational effort in this rigorous procedure is substantial. A reduction can be achieved by simplifying the Green's function by ‘concentrating’ the region of influence. Alternatively, assuming a specified wave pattern, a coupled system of springs and dashpots with frequency-independent coefficients can be used as an approximation.     

The effect of urbanization in an arid region: Formation of a perched water table that causes environmental damages

Construction in a new neighborhood in the israeli town of Dimona, situated in an arid region in the south of the country (150 mm average annual rainfall), resulted in a rise in groundwater levels during the subsequent rainy seasons This caused flooding of shelter basements, soil sliding, and sagging which permanently damaged walls and buildings The neighborhood had been built on continental sands and marls blanketed by loess, on a valley slope near a rocky anticlinal dip-slope Subsurface studies, using piezometer holes and groundwater analyses, revealed the presence of sand lenses alternating with plastic marls, which act as seasonal aquifers with perched water tables Groundwaters obtain high SO ₄ ⁻² and Cl⁻ corrosivity through contact with these nonflushed marls of the Neogene valley fill (Hazeva Formation) The reasons for the rising of groundwater were found to be (a) artificial interference with the natural (pre-construction) drainage system—interception of the hillside runoff by building plots, roads, etc, (b) partial denudation of the loess blanket, increasing the local infiltration and the build-up of local, perched water tables, and (c) corrosion of concrete and steel pipelines, as well as foundations, by prolonged contact with corrosive groundwater, resulting in haphazard but massive leakage Guidelines are proposed for an environmental improvement plan, which would include terracing and planting of the watershed above town to increase evapotranspiration, lowering of the water table by pumping, and diverting the water to suburban parks (groves of saltresistant trees), and replacement of steel and cement pipes by a non-corrodable plastic pipe system     

Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences

A pyrope-quartzite originally described by Vialon (1966) from the Dora Maira massif was resampled and reinvestigated. Garnet (up to 25 cm in size), phengite, kyanite, talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz. The garnet is a pyrope-almandine solid solution with 90 to 98 mol % Mg end-member. It contains inclusions of coesite which has partially inverted to quartz, resulting in a typical radial cracking of the host garnet around the inclusions. Several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite. The formidable pressures of formation implied (≧28 kbar) are independently indicated by i) the coexistence of nearly pure pyrope with free silica and talc, ii) the coexistence of jadeite with kyanite, iii) the high Si content of phengite. Water activity must have been low. The stability of talc-phengite and the presence of rare glaucophane inclusions in pyrope point to low formation temperatures (about 700 °C) and to a probable Alpine age for the assemblage. This is evidence that low temperature gradients, how essentially transient they are, may nevertheless persist to considerable depths. Moreover, the upper crustal (evaporite-related?) origin of the quartzite and its interbedding within a continental unit implies that continental crust may also be subducted to depths of 90 km or more. The return back to the surface is problematic; the retrograde assemblages observed show that it must be tectonic. If the rocks remain at depth, new perspectives open for the genesis of intermediate to acidic magmas. Eventually, the role of continental crust in geodynamics may have to be reconsidered.     

Suevite of the ries crater,Germany: Source rocks and implications for cratering mechanics

Suevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement, glass inclusions and a small amount of sedimentary clasts. Data are given of the modal composition of fall-out suevites (deposited at isolated points around the crater) and crater suevite (forming a layer below post-impact lake sediments in the crater cavity). Fall-out suevites contain aerodynamically shaped bombs which are absent in crater suevite. Taking into account not only large glass fragments and bombs, but also the finer fractions, the glass content of fall-out and crater suevites amounts to 47 and 29 vol%, respectively. Crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks (mainly granites) and a lower series of gneisses and amphibolite. Based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined. Suevites contain on the average 46 % igneous and 54 % metamorphic clasts. In constrast, weakly shocked and unshocked crystalline ejecta of the Ries structure consist of 82 % igneous and 18 % metamorphic rocks. From 138 analyses of crystalline rock samples average compositions of the major rock types were calculated. Comparison of these averages with the average glass composition leads to the conclusion that suevite glasses were formed by shock melting of gneisses in deeper levels of the basement. Suevite matrices consist in most cases of 80 to 90 % montmorillonite, in special cases of celadonite. Chemical analyses are given of some matrices and montmorillonite formulas calculated. It is supposed that montmorillonite was formed by early hydrothermal alteration of rock flour or fine glass particles. In the latter case the original glass content of suevites was higher than at present. Of all ejecta from the Ries crater only crystalline rocks contained in suevites occur in all stages of shock metamorphism up to complete fusion. The overwhelming majority of the ejecta from the sedimentary sequence (about 580 m) show no indications of shock pressures above 10 GPa. The same holds true for crystalline megablocks and breccias around the crater which consist mainly of granites from upper levels of the basement. We assume that the Ries impact can be approximated by a deep-burst model: The projectile penetrated through the sedimentary cover into the basement in such a way that the highest pressures and temperatures developed within the gneiss complex below the upper, predominately granitic layer and that rocks of the sedimentary sequence experienced weak shock compression. Numerical data are given for such a model of the Ries impact on transient crater geometry and volumes of vaporized, melted, shocked and excavated rocks. Fall-out suevites are supposed to have been lifted from the central zone by an expanding plume of vaporized rocks and deposited as fluidized turbulent masses outside the crater whereas the main mass of crater suevite was not removed from the crater cavity.