Prévision rapide de l''extension d''une pollution dans quelques cas d''écoulement et de mode d''injection

The relationship between maximum concentration and longitudinal distance, independent of time, can determine the size of the area to be contaminated, under well-defined conditions of injection and flow in a saturated medium. The envelope curve for maximal concentrations is easily obtained as a function of the axial distance in non-dimensional variables in the cases of one- and two-dimensional uniform flow, as well as in those of radially symmetric converging or diverging flow.

Graphs providing relations between dimensionless variables (as described in the text) are given for the following cases:

1. (1) uniform flow with instantaneous discharge of a pollutant; measurement downstream of the injection point, no transverse dispersion;

2. (2) uniform flow with continuous injection of a pollutant; measurement downstream of the injection point, influence of transverse dispersion included;

3. (3) convergent radial flow towards a well; pollution caused by an instantaneous pointsource at some distance; and

4. (4) divergent radial flow from an injection point; pollution caused by instantaneous injection of a pollutant.

Résumé

L'expression de la concentration maximale en fonction de la distance et indépendamment du temps permet de déterminer l'extension d'une zone susceptible d'être contaminée lors de conditions bien définies d'injection d'un produit dangereux et d'écoulement en milieu saturé. La courbe enveloppe des maxima de concentration en fonction de la distance axiale est facilement exprimable sous forme adimensionnelle (abaques) dans le cas d'écoulements uniformes mono- et bi-dimensionnels (réponse dans l'axe), comme dans ceux d'écoulements purement cylindriques convergent ou divergent.     

Boundary-layer ozone depletion as seen in the Norwegian Arctic in spring

Several years of measurements of ozone, hydrocarbons, sulphate and meteorological parameters from Spitsbergen in the Norwegian Arctic are presented. Most of the measurements were taken on the Zeppelin Mountain at an altitude of 474 m a.s.l. The focus is the episodes of ozone depletion in the lower troposphere in spring, which are studied in a climatological way. Episodes of very low ozone concentrations are a common feature on the Zeppelin Mountain in spring. The low ozone episodes were observed from late March to the beginning of June. When the effect of transport direction was subtracted, the frequenty of the low ozone episodes was found to peak in the beginning of May, possibly reflecting the seasonal cycle in the actual depletion process. Analyses based on trajectory calculations show that most of the episodes occurred when the air masses were transported from W-N. Ozone soundings show that the ozone depletion may extend from the surface and up to 3–4 km altitude. The episodes were associated with a cold boundary layer beneath a thermally stable layer, suppressing mixing with the free troposphere. The concentration of several individual hydrocarbons was much lower during episodes of low ozone than for the average conditions. The change in concentration ratio between the hydrocarbons was in qualitative agreement with oxidation of hydrocarbons by Br and Cl atoms rather than by OH radicals.     

Atmospheric concentrations of nonmethane hydrocarbons at a North European coastal site

Two years of individual nonmethane hydrocarbon (NMHC) measurements at a rural site close to the south coast of Norway show that there was a distinct annual cycle with a late winter maximum and late summer minimum in the slowly reacting NMHCs acetylene, ethane, propane and i- and n-butane. The average January—March concentrations were a factor 2–4 higher than the July-September concentrations. Also ethene, propene and the pentanes show a similar annual cycle, but the individual scatter in the measurements in particular of propene, is large. The highest concentrations of NMHC were found in winter for easterly transport on a regional scale (out to 1500 km from the site), and for southeasterly transport in the summer.     

Rapid biotic molecular transformation of fulvic acids in a karst aquifer

The study of molecular transformation processes of dissolved organic carbon (DOC) in the environment significantly contributes to a better understanding of the global biogeochemical organic matter cycle. In an oxic karst groundwater system, in which the most powerful abiotic DOC degradative reactions, photodegradation and metal-mediated redox chemistry, are at best marginal contributors, a near complete turnover of fulvic acids (FAs) has been observed within decades (∼60 years). Depletion of oxygen for a very extensive range of aliphatic and aromatic carbon chemical environments has been confirmed as well as the formation of novel classes of compounds, suggesting a major contribution from biotic processes. From these results we infer that FAs must be perceived as a rather active participant in the global carbon cycle. Molecular-level alterations of such magnitude and rapidity on such short-time scales ought to be considered as widespread in the processing of “refractory” DOC in the environment.     

TL age-estimates for the Middle Palaeolithic layers at Theopetra cave (Greece)

The lowest stratigraphic unit of Theopetra cave (Greece) contains a Middle Palaeolithic sequence radiocarbon dated to between 46 and 35 ka BP; at the limit of this dating method. The upper part of this sequence has yielded late Middle Palaeolithic lithic assemblages containing several early Upper Palaeolithic artifacts. To get more precise dates for the human occupation of the cave, burnt flint specimens from these Middle Palaeolithic layers were dated by thermoluminescence. The dates obtained are coherent and indicate that the first human occupation of the cave took place at the end of isotopic stage 6 or more probably at the beginning of stage 5, much earlier than previously proposed. In the light of this new evidence, it is reasonable to assume that: (a) Theopetra contains the oldest dated Middle Palaeolithic deposits of Greece and (b) that the assemblages first interpreted as “Transitional Middle/Upper Palaeolithic” industries may be the result of post-depositional mixing of cultural material.     

Spatio-temporal changes in the abundance of the populations of the gastrotrich community in a shallow lake of tropical Africa

Studies on the biodiversity and population dynamics of freshwater planktonic Gastrotricha have been carried out in conjunction with a physical–chemical analysis of the water in the Yaounde Municipal Lake (Cameroon, Central Africa) over a 14 months period (November 1996–December 1997). The results obtained allow to consider the Yaounde Municipal Lake as an eutrophic lake. It harbours eight species of Gastrotricha belonging to four genera (Chaetonotus, Dasydytes, Neogossea and Polymerurus) of the order Chaetonotida. This community was characterized by high abundances of populations, and was dominated by the genus Neogossea and Chaetonotusreaching up to 2000 ind. L⁻¹. Polymerurus was mostly abundant at the almost anoxic bottom layers. The highest abundances were found mostly during the rainy season, when there is an important sedimentation process of organic matter, and were influenced by several different environmental factors such as dissolved oxygen, temperature and pH of the water.

Finally this community which may play an important role in the water bodies, is a potential water quality indicator.