FR :

La gestion des systèmes hydroélectriques s'effectue de plus en plus dans un contexte multi-objectifs car l'eau sert non seulement à produire de l'électricité mais bien à d'autres fins comme la navigation, le flottage du bois, et l'alimentation des municipalités. Dans cet article, nous montrons comment ces autres fins peuvent être incorporées sous forme de contraintes probabilistes dans un modèle de gestion à long terme des installations d'une vallée. Le problème devient alors de non seulement résoudre le problème de gestion mais bien d'ajuster la valeur des contraintes probabilistes de façon à ce que la probabilité de ne pas satisfaire les besoins des autres usagers soit acceptable. Le choix de la probabilité de dépassement des contraintes a naturellement un effet sur la production d'électricité. Une étude effectuée sur la rivière St-Maurice permettra de montrer l'importance de celle-ci.

EN :

The paper deals with the problem of determining the amount of water to release from several reservoirs located in the same river basin, so as to satisfy the demand for electricity at minimal cost white respecting a set of constraints related to environment protection, flood control, wood floating and navigation. Would the demand for electricity and the river inflows be known for the entire period studied, that the problem could be written

Minimize {Z (D - H (X)) / AX ≽ Ɵ, X ɛ ɼ} (1)

Where Z is the cost function, D the demand for electrical energy, H(X) the hydroelectric production, AX ≽ Ɵ the set of constraints mentioned above, and r the set of admissible solutions. However, since the demand for electricity and the river inflows cannot be forecasted a long time in advance, and since they change significantly from one year to the next, the constraints AX ≽ Ɵ cannot always be respected. For instance, maintaining the level of reservoir i above Ɵi may be impossible in a period of very low flow. In order to find a solution to the problem, the constraints AX ≽ Ɵ were replaced by the probabilistic constraints Pr(AX ≽ Ɵ) ≼ B and the problem rewritten

Minimize E {Z (D - H (X))} (2)

s.t. Pr(AX < Ɵ) ≼ B (3)

X ɛ ɼ (4)

Where E denotes the mathematical expectation and Pr the probability. In the solution procedure, however, constraints (3) are not directly taken into account, but are replaced by additional terms in the objective function which penalize violations of the constraints. The parameters of these penalty terms are adjusted iteratively until constraints (3) are respected. More precisely, values are chosen for the parameters and the problem is solved by stochastic dynamic programming. Next, the operation of the reservoirs is simulated over hundreds of years to find out whether constraints (3) are respected with the parameters initially selected. If the constraints are too often violated or not enough, the values of the parameters are changed and the problem solved again. This approach has been applied to the St-Maurice River in Canada, where upper limits on the flow exist at different points along the river to prevent floods. The aim of the study was to find an operating policy that respects these upper limits 95 % of the time, and to determine what effects such policy has on the production of hydroelectric energy.

FR :

En vue d'optimiser l'aération de substrats liquides à forte charge organique dissoute, un système d'aération fonctionnant selon te principe du "cheminement prolongé" a été conçu et construit à l'échelle pilote (3 000 L). Le principe mis à l'essai consiste à injecter de l'air à la base de longs tuyaux enroulés. La dynamique hydraulique du système à cheminement prolongé a été caractérisée. Un système d'aération plus conventionnel, soit un injecteur d'air de type poreux, a été utilisé comme base de comparaison. Les performances d'oxygénation des deux systèmes ont été mesurées dans l'eau, sous les mêmes conditions, puis comparées entre elles et aux valeurs rapportées dans la littérature pour d'autres systèmes.

Les principaux résultats montrent que le coefficient de transfert d'oxygène (K_L.a) est bon à l'intérieur des tuyaux de l'aérateur à cheminement prolongé (jusqu'à 230 h^-1). Le nouveau prototype démontre également une forte capacité d'emprisonnement de l'air puisque le gonflement atteint 20 %. Finalement, de bonnes efficacités énergétiques ont été obtenues : des valeurs brute de 1.9 kg O₂/kWh et nette de 9.2 kg O₂/kWh ont été atteintes. Le prototype, tel que construit, a offert des performances. d'oxygénation globales similaires à cettes démontrées par l'injecteur d'air de type poreux, le volume interne des tuyaux, qui présente un intérêt particulier pour le transfert d'oxygène, ne représentant que 26 % du volume liquide total.

L'optimisation de l'aérateur à cheminement prolongé passe donc par l'accroissement maximal du volume interne des tuyaux par rapport au volume liquide total. Cette mesure se traduirait par une augmentation de la capacité d'oxygénation totale et des efficacités physique et énergétique du transfert d'oxygène, ainsi que par une diminution des puissances spécifiques requises.

Les performances du système à cheminement prolone dans un liquide chargé en substrat organique restent à étudier, mais les résultats préliminaires obtenus dans l'eau du robinet sont encourageants.

EN :

Aeration represents the major operating cost in the various aerobic treatment processes intended for wastewaters or other effluents showing a high dissolved organic load, such as swine manure; it is therefore necessary to maximize the utilization of the oxygen provided to any treatment system.

Various types of air injectors and turbines have been studied or built in order to achieve an adequate oxygenation of liquid wastes to be treated : porous and non-porous injectors, immersed and surface turbines, the latter rotating vertically or horizontally. The aeration performances of these systems appear to be variable; moreover, these systems are not always well adapted to a substrate such as swine manure. With a view to possible application to swine manure treatment, we have designed an "extended contact" pilot-scale aerator (3000 L).

The strategy involved injecting air at the basal end of long coiled pipes in which the manure to be treated was circulated. The objective was to increase the time of contact between the air bubbles and the liquid and, consequently, oxygen transfer to the latter. The internal surface of the pipes was large and could become an effective physical support for microbial attachment, thus promoting further digestion.

We evaluated the performance of the system with tap water and compared it with a more conventional aeration system, i.e. a porous-type injector also known as a mini-bubble diffusor.

Eight oxygenation parameters were evaluated for each of the two systems oxygen transfer coefficient (K_L-a), swelling (SWEL), standard oxygenation capacity (SOC), specific standard oxygenation capacity (SSOC), physical efficiency of oxygen transfer [ɛ, cf. Equation (1)], power consumed (P), specific power injected (SP) and energy efficiency (E). A theoretical simulation of the performances of a hybrid system combining the two aeration principles was carried out. The hydraulic dynamics of the extended-contact system were established through the evaluation of the recirculated flows due to the aerator. The best results obtained were finally compared to those taken from the literature.

The main results show the following positive aspects of the extended-contact aerator :

1 - high oxygen transfer coefficients inside the pipes;

2 - high air retention capacity;

3 - good energy efficiency for low air flows.

Inside the pipes, values of 230 h^-1 were obtained for K_L.a. The new prototype retained up to 546 L of air which corresponds to a swelling of 20 %. A raw energy efficiency of 1.9 kg O₂/kWh was obtained, and the net value (based open the net power) was 9.2 kg O₂/kWh.

The prototype, as constructed, gave oxygenation performances globally similar to those obtained with the porous type air injector despite the fact that the internal volume of the pipes, which is of special interest for oxygen transfer, was only 26 % of the total liquid volume.

The optimization of the extended-contact system therefore calls for a maximal increase of the internal volume of the pipes as compared to the total liquid volume. Ideally, the internal volume should represent 100 % of the total volume. By doing so, oxygenation performances should be improved because of :

1 - increased total oxygenation capacity due to a better overall K_L.a;

2 - increased physical efficiency of oxygen transfer (ɛ) in relation to the length of the pipes (in theory, ɛ could reach 100 %);

3 - decreased injected specific power (SP);

4 - increased energy efficiency of oxygen transfer (E).

With such improvements in view and considering the requirement : of the aerobic treatment of highly loaded affluents such as swine manure, the extended-contact aerator appears to be a promising aeration technique that still has, however, to be tested under practical conditions.

FR :

Les auteurs comparent les communautés d'hyphomycètes aquatiques récoltés dans l'écume de 7 cours d'eau du Béarn et des Landes. L'abondance de spores observées dans les deux régions diffère considérablement (174-1175 spores/µl en Béarn et 2-57 spores/µl dans les Landes) tandis que les variations de richesse spécifique apparaissent faibles. L'influence de la végétation riveraine et du pH de l'eau est discutée.

Des expériences de dégradation in vitro montrent que des souches isolées de Tetracladium marchalianum, Heliscus lugdunensis et dans une moindre mesure Anguillospora longissima présentent une activité cellulolytique. T. marchalianum et H. lugdunensis, participent activement à la décomposition de litière de saule blanc, avec respectivement 21,7 et 18,2 % de dégradation après 5 semaines à 18°C.

EN :

Aquatic hyphomycete communities from the foam of seven streams in the Bearn and Landes regions of France were compared at four different dates. The total number of species was similar in the two regions, but common species (> 5 spores/µl) in the Bearn were twice as abundant as in the Landes. Mean spore concentrations in the Bearn and Landes streams were in a ratio of 10 : 1. In the Bearn, spore concentration and the number of fungal species increased considerably in autumn, subsequent to the fall of leaf litter into the streams. Analysis of variance of spore concentrations (original or transformed data) in the two regions showed that date, station and the interaction of these two factors were highly significant parameters (P ANOVA < 0.0001). Alatospora acuminate was the commonest species both in the Bearn (77 % annual mean) and in the Landes (41 %). After A. acuminata, Clavatospora stellata (7 %) and Tetracladium marchalianum (4 %,) in the Bearn, Flagellospora curvula (28 %) and Clavastospora longibrachiata (20 %) in the Landes were the dominant species of the mycoflora. Five species new to France were noted : Actinospora megalospora Ingold, Camposporium pellucidum (Grave) Hugues, Diplocladiella scalaroides Arnaud, Flabellospora acuminata Descals and Triscelophorus monosporus Ingold. The difference in fungal richness between the two regions was suggested to be due to the composition of the riparian vegetation, the phenology of the litter fall, the presence (or absence) of plant matter accumulation in the stream, and the pH of the stream water. Litter deposits in the Landes streams were rare and made up exclusivey of pine needles, the few deciduous leaves which fall being exported to surrounding lakes and to the ocean. Water pH was always low (5.0-5.5). The riparian vegetation of the Bearn streams, on the contrary, was abundant and varied Cash, common locust-tree, alder, willow, oak, chestnut, poplar). Moreover the neutral or weakly acid pH of the Bearn streams (6.3-7.0) seemed to favour fungal diversity (cf. BÄRLOCHER and ROSSET, 1981; WOOD-EGGENSCHWILLER and BARLOCHER, 1983).

Single spore isolates of Anguillospora langissima (de Wild.) Ingold, Heliscus lugdunensis Saccardo et Therry and Tetraclacium marchalianum De Wild were obtained from decomposing leaf litter in the Bearn streams. In laboratory experiments, the degradation activity of each species was tested both on sterilized paper cellulose and white willow leaf litter. The cellutolytic activity of fungal cultures was significant when compared with controls (t test). After 5 weeks at 18 °C, mass loss was 8.6 % for A. longissima and 10.0 % for H. lugdunensis and T. marchalianum. Aeration stimulated cellulose degradation for T. marchalianum (16.6 %) only. With willow leaves as substrate, degradation was greater for T. marchalianum (22.7 %) than for N. lugdunensis (11.8 %) and A. longissima (6.2 %, non-significant).

FR :

Au cours des dix dernières années, de nombreuses méthodes ont été proposées dans la littérature, afin de mesurer l'activité des bactéries hétérotrophes en milieu aquatique. Parmi celles-ci, la mesure de l'incorporation de thymidine tritiée dans le DNA bactérien semble être, à l'heure actuelle, la méthode la plus utilisée. Elle offre, en effet, l'avantage de sa spécificité et d'un protocole expérimental simple. Néanmoins, la conversion des résultats expérimentaux en production de biomasse bactérienne pose un certain nombre de problèmes quant à l'interprétation correcte de cette méthode. Cet article fait le point sur les réponses théoriques et expérimentales qui peuvent être apportées à ces problèmes, ainsi que sur les diverses possibilités d'utilisation de cette méthode.

EN :

During the fast ten years, numerous methods have been proposed in the literature to mesure the activity of heterotrophic bacteria in aquatic ecosystems. Among these methods, the measurement of tritiated thymidine incorporation proposed by FUHRMAN and AZAM (1980) seems to be, at the present time, the most useful. It offers in fact the advantage of its specificity for bacteria and its simple experimental procedure.

This method is based on the fact that, in bacteria, DNA synthesis is directly proportional to the division rate. The close relation between growth and DNA synthesis means that measurement of the rate of DNA synthesis is a good way to measure the bacterial growth rate. The DNA synthesis rate is estimated from the incorporation rate of methyl-³H thymidine. Thymidine is one of the four nucleoside precursors of DNA, but it is not a precursor of RNA. At the nanomolar concentrations of (methyl-³H) thymidine used in this experiment only heterotrophic bacteria utilize exogenous thymidine and all active heterotrophic bacteria utilize thymidine. The usual experimental procedure used by the various authors working with this method is that of the FUHRMAN and AZAM (1982). A 5 nM thymidine concentration has been recommended by these authors for the marine environment, but it has been shown that in more eutrophic ecosystems higher concentrations are needed to saturate the incorporation process.

In fact, the conversion of experimental data into bacterial production raises some problems. Among these, two questions are important :

- Which is the relative part of exogenous and endogenous thymidine used for DNA synthesis? In other words, which is the isotopic dilution factor?

- The cold trichloroacetic acid (TCA) fraction collected in this experiment includes, besides DNA, proteins and RNA. As some catabolic products of thymidine could be incorporated into RNA or proteins, how mach radioactivity is really incorporated into DNA?

Some theoretical and experimental answers can be given to these questions.

- Comparing their results of thymidine and H³²PO₄⁼ incorporation in DNA of marine bacteria, FUHRMAN and AZAM (1982) found an isotopic dilution factor in the 3-7 range. MORIARTY and POLLARD (1981) have proposed a kinetic approach for estimating the internal pool of thymidine but the accuracy of this method was critized by RIEMANN et al., (1982) and FUHRMAN and AZAM (1982).

- Usually, in order to determine the part of radioactivity incorporated into DNA, RNA and proteins, the biochemical procedure of LURIA (1960) is used. SERVAIS et al., (1987), after testing this procedure with labelled macromolecules, have concluded that this method is not accurate for such determinations. These authors have proposed an enzymatic procedure based on the use of DNase tell breakage. With this method, they have showed important fluctuations in the percentage of thymidine incorporated into DNA from one ecosystem to another.

The first authors using thymidine incorporation calculated conversion factors based on theoretical data and various assumptions to convert thymidine incorporation data into bacterial cell production. These factors were in the range of 0.2 to 4 10¹⁸ bacteria produced per mole of thymidine incorporated in the cold TCA insoluble material. More recently, because of the uncertainty or the difficulty in estimating some of the parameters (isotopic dilution, part of thymidine incorporated into DNA, part of thymidine residues in DNA, quantity of DNA per bacterial cells) required for the calculation of the conversion factor, most of the authors have used an experimental conversion factor. It was estimated from the comparison of cell number increase and thymidine incorporation in sterilized, and reinoculated samples. These experimental conversion factors were usually in the range of 0.5 to 10 10¹⁸ bacteria produced per mole of thymidine incorporated.

The conversion of cells production into bacterial biomass production expressed in carbon - which is finally the important flux to know for the study of the first trophic levels dynamic - requires a knowledge of the average bacterial biovolume and the carbon content par unit of cell volume. The first parameters can be estimated from the observation of bacteria by epifluorescence microscopy. Most authors use 1.2 10^-13 gC.µm^-3 to calculate carbon content of bacteria proposed by WATSON et al., (1977).

The use of the tritiated thymidine incorporation method is not limited to measure bacterial production in the water column; it is also used to measure the activity of fixed bacteria, to study the grazing of bacteria by microzooplankton and in ecotoxicological studies.

FR :

L'indice de réflexion ou réflectance donne une mesure de la teinte du sédiment entre le blanc (100 %) et le noir (0 %). Sa valeur est directement liée à la teneur en matières organiques.

Les relations entre l'indice de réflexion et les teneurs en carbone organique, en azote Kjeldahl et en carbonates sont données pour les sédiments de dix lacs différents.

Une étude particulière de prélèvements superficiels et de prélèvements profonds montre qu'une simple mesure de la réflectance donne une valeur approchée de la composition chimique.

EN :

Thorough studies on many lakes in Franche-Comté, especially chemical and physical investigations concerning sediments, have been carried out for several years. Seventy-one sediment samples have been extracted at different points and depths of ten lakes in Franche-Comté. The upper stratum between 0 and - 2,5 cm were isolated and studied.

A comparison of visual observations and results of chemical analysis have shown that the darkest sediments are those which contain the greatest amount of organic matter. This finds its origin in a particularity of Franche-Comté lake sediments, which are often composed of dark black organic matter in different evolutionary states, and clear mineral compounds such as calcium carbonate and detrital particles.

Differences in colouration were particularly difficult to determine, so colours were defined by the reflexion factor of the sediment surfaces, after the sediments had been well dried and homogeneized.

The reflectance index was measured with a reftectometer, which is generally used for determinations of dark smokes in air pollution. The reflectance index was defined between the values 0 and 100. The value 0 was obtained with a standard black glass and the value 100 with a white filter paper.

Comparing reflectance indices and ignition loss has shown that there is an exponential relationship between these two parameters. These relationships were found to exist between the reflectance index and organic carbon, Kjeldahl nitrogen and carbonates.

Ignition loss, organic carbon and Kjeldahl nitrogen proportions increase, but, on the contrary, carbonate proportions decrease when the reflexion index increases.

The values calculated and observed were then compared and generally the difference does not appear to be too great. Once, relationships have been established, it becomes possible to use them, after measuring reflexion indices, to obtain quickly and easily a good approximation of the composition.

Special relationships may also be calculated for an individual lake, as shown for St-Point Lake. If one mineral constituent prevails in the composition, it is possible to obtain a good estimation of this component.

For this lake twenty-eight sediment samples were extracted and for each sample three strata were separated. The reflectance index was again defined between the values 0 and 100, but the value 100 was now obtained by using a flat calcium carbonate surface, as the principal mineral part of St-Point Lake consists of calcium carbonate.

Recent results obtained from two lake sediments of the Massif Central indicate that such relationships also exist for these lakes and that this method of estimating the chemical composition can be generalised.

The calculated composition may often be sufficient to estimate values required for others studies on the same lakes, especially biological studies, thus avoiding laborious and extensive chemical determinations.

FR :

L'étude porte sur un magasin aquifère calcaréo-dolomitique très karstifié (karst à pitons ou tourelles) d'une région à climat tropical humide de la vallée de Li-Jiang.

L'échantillonnage hydrochimique concerne les zones saturée et non saturée de différents secteurs du karst.

La détermination des pressions de CO₂ et des coefficients de saturation par rapport à la calcite ont fait l'objet d'une attention particulière. Les concentrations en calcium et bicarbonates (éléments prépondérants) varient respectivement dans les intervalles 3,1 à 4,7 et 2,5 à 4,8 mé.l^-1.

La plus grande partie des échantillons de la zone saturée est en équilibre avec la calcite et la dolomite, sous des pressions de CO₂ de l'ordre de 1 à 2.10^-2 atm.

Les eaux de la zone non saturée sont celles de différents suintements recueillis au plafond d'une galerie naturelle. Elles se distinguent par une inégalité entre les teneurs en bicarbonates et en calcium, interprétée comme étant due à des échanges de bases avec les résidus de colmatage des fissures.

Les résultats hydrochimiques font supposer l'existence, dans le karsts de plusieurs domaines aquifères liés à la morphologie, interconnectés, avec drainage général par le Li-Jiang.

EN :

This paper deals with a research program aimed at studying the hydrogeologic properties of the water-rich carbonate rocks in Southern China. We focus our attention on the Guilin area (Guangxi province). Our results concern the hydrochemistry and we show how a study of time- and space-variations in the chemical composition of groundwater (saturated and unsaturated zone) may help in understanding the hydrodynamic conditions in a karstic aquifer.

The Guilin karstic area is mainly composed of a limestone and dolomitic limestone series extending in age from Mid-Devonian to early Triassic, reaching 3 000 m in thickness. In the Li-Jiang valley, the quaternary caver (alluvial deposits) is 10 to 20 meters thick. Geomorphology is named "tower karst", as it may be observed in various tropical regions. In fact, two different types of landform appear : the "fonglin" (peak forest) and the "fongchong" (peak cluster). In the former, isolated peaks (mean altitude : 250-300 m) rise in the plain (altitude around 150 m). In the latter, the peaks join together and delimit closed depressions (attitude up to 600 m in the Guilin area). Deep karstification is as important as surface karstification, which is related to the tectonic context (rocks are intensely fractured), to the subtropical wet climate (mean annual rainfall : 1 900 mm) and also to intense groundwater circulation.

Our hydrochemical study has been carried out on various types of water points (springs, sinkholes, large diameter wells, boreholes, an underground river, oozing points from the unsaturated zone). They are located in both the fonglin and the fongchong area, but the highest density is close to the contact fine between the two landforms. The chemical analyses are interpreted in three steps :

1) calculation (if possible) of the saturation coefficient relative to calcite and dolomite and of the CO₂ equilibrium pressure,

2) study of time variations in the composition of water sampled at oozing points from the unsaturated zone and,

3) statistic study of space variations in the composition of water sampled at alt points.

Hydrochemistry is dominated by Ca²⁺ and HCO₃^- ions. Except for two aberrant points (probably resulting from a mixture with surface water), Ca²⁺ and HCO₃^- concentration varies in a fairly narrow range (respectively 3.12-4.72 meq.l^-1 and 2.53-4.80 mea.l^-1). Most of samples from the saturated zone are close to equilibrium with calcite and often with dolomite, under CO₂ pressures about 0.01-0.02 atm. Small scale variations in lithology (presence of dolomite) and in CO₂ pressure may therefore explain space variations in hydrochemistry and enable waters of the same origin to be identified.

The study of the unsaturated zone consists in sampling the water at three oozing points located in three fractures of the roof of a natural cave. Althrough the points are quite near to each other, water composition differs clearly between them. The most interesting result though is provided by the following experiment. One of the three points was initially dry. The flow occurred when breaking the concretion obstructing it. For six hours we studied the discharge and the water-composition variations at that point. The initial value of the discharge (6 ml.mn^-1) rapidly decreases and reaches its stable value (4 ml.mn^-1) after 1 h 30 min. Ca²⁺, Mg²⁺, Na⁺, K⁺ and HCO₃^- concentration show erratic variations, but the fact that Na⁺ and K⁺ concentration are linked suggests that these fluctuations are related to adsorption-desorption phenomena. It should be noticed that SO₄^2- concentration is not erratic : it decreases from 0.44 meq.l^-1 and reaches a stable value (0.08 meq.l^-1) after four hours, i.e. much later than the discharge. This proves that the oozing point yields water from two different origins : a small reservoir and a network of connected fractures reaching a given potential boundary, both being initially static.

The statistic study of space variations identifies a mountain aquifer, in the Fongchong zone, and proves that relationships do exist between points which may be located quite far apart. Such relationships should be discussed within the tectonic setting : it has been shown that fracturation develops mainly in three directions (N000-020, N060-080, N120-130); the fracture opening is maximum in the N060-080 direction which corresponds to anormal and strike-slip faults. An open fracture in that direction, joining two points, may explain the relation between them. It also appears that the water composition of springs in the plain, far from the contact fine with the fongchong zone, may be similar to the fongchong one. This proves that water from the fongchong does not only yield springs on the contact fine between fonglin and fongchong. Thus, a groundwater flow occurs from fongchong to fonglin and the hydraulic boundary conditions between the two landforms way be very complex.

Finally, we propose the most probable flow pattern in the area studied. The various elements (unsaturated zone, mountain karst aquifer, fongchong aquifer, deep plain aquifer, surface plain aquifer) appear to be closely linked. Further work is underway, in order to estimate the depth of karstification, to identify the groundwater flow more precisely and to quantify if.

FR :

Plusieurs corps d'eau lagunaires et continentaux égyptiens sont comparés quant à l'abondance en oxygène-18 et la concentration saline (ici appréciée par la teneur en Cl¯) de leurs eaux superficielles.

En Egypte, les eaux continentales de surface proviennent toutes du système nilotique dont les eaux, après leur longue traversée du grand Desert Oriental, se trouvent très évaporées et fortement enrichies en oxygène-18 à leur arrivée dans le delta.

Dans les bassins alimentés uniquement par la mer (lagunes de Mer Rouge et du Golfe de Suez) soit par l'eau du Nil (Lac du Fayoum), les deux paramètres considérés augmentent conjointement depuis les zones d'alimentation vers les reculées marginales selon le déplacement des eaux.

Dans les bassins à alimentation mixte, (Partie terminale du delta, lac Temsah sur le Canal de Suez), les eaux les plus diluées sont aussi les plus riches en isotopes lourds.

Ainsi, dans le contexte climatique et géographique très particulierde l'Egypte, il est possible de reconnaître les eaux continentales, marines ou paraliques à partir des deux paramètres étudiés mais non à partir d'un seul, notamment la teneur en oxygène-18.

EN :

The aim of this paper is to study some aspects of the geochemical behaviour (¹⁸O, (Cl^-)) of waters of some basins in Egypt. Several types of basin are studied (figure 1).

1) Basin with marine seawater input only, either wide open to the Red sea such as Zeit Bay (27°45'N, 33°25'E), relatively open such as Guesmah Lagoon (27°40'N, 33°30'E) or indirectly linked to the sea through a coastal sand bar such as the pools of sebkha Melaha (28°10'N, 33°10'E).

2) Basin supplied by continental water without outflow such as Fayoum Lake (= Birket Karoum) (29°25'N, 30°30'E) which receives fresh waters from a Nile diversion.

3) Water bodies with a mixed water input (continental and marine waters) such as the Nile delta and Temsah Lake (= Ismaïla Lake), (30°25'N, 31°30'E), the katter being on the Suez Canal and receiving waters from Mediterranean Sea and from the eastern branch of the Nile.

Local means climatic parameter are those of Cairo, Alexandria, Queseir and Louxor (table 1).

RESULTS AND DISCUSSION

1 - Nile delta (figure 2 and 3)

Sampling period was performed in August 1984.

- At the beginning of the Nile delta the water at Beni-Suez (station 14) presents a low chloride concentration (0.1 to 0.2 g l^-1) and a relatively high ¹⁸O content (+ 3 ‰) due to evaporation of the Nile water during its downstream course from the Aswan High Dam which collects waters in the upper part of the Nile watershed.

- The content of both (Cl^-) and ¹⁸O stightly increases in the delta region in relation to the anastomosis of branches of the Nile due to an intensive irrigation network.

- In the coastal region, due to mixing with sea water or/and brackish water, the isotope content decreases while the chloride concentration increases.

2 - Temsah or Ismaïla Lake

Two periods of sampling : March and August 1984. The general trend is a decrease of ¹⁸O and an increase of (Cl^-) from the Nile waters to the central part of the lake, and an increase of both ¹⁸O and (Cl^-) at the fringes of the lake, due to evaporation.

3 - Coastal basins from the Red Sea

Zeit Bay and Guesmah Lagoon. Two sampling periods : March and August 1984. The isotope content and chloride concentration act in parallel and depend principally on the movement of water bodies. For the surface waters wind velocity and direction play a major rote in the spatial distribution of the chlorinity and isotope content. A slight enrichment is noted during the summer period.

4 - Pools in Sebkha Melaha

The isotope composition and chloride content of the ponds water fed by sea water through the sand salt bar increased during the summer as it is the case in the first stages of evaporation of salt pans (PIERRE and FONTES, 1982).

5 - Fayoun Lake : Birket Karoum

Two sampling periods : March and August 1984. Because of a lack of sufficient data (salinity and isotopic composition of the input, water flux of the incoming water, local values of the relative humidity and temperature of the atmosphere, isotopic composition of the water vapour...) a water mass balance coupled with a chloride and isotopic mass balance was not possible. Nevertheless, with our data measured (¹⁸O, (Cl^-) lake levels), some estimations were made of the temperature, evaporation, wind velocities (from meteorological tables) and the isotopic composition of the input : δA in the lake (SIMPSON et al., 1987), and if we assume that the lake presents a long term dynamic equilibrium, it is possible to estimate with reasonable accuracy that : i) the total isotopic enrichment factor (ɛ) is between 8 ‰ and 24 ‰ (figure 12) : ɛ= ɛ_e + ɛ_k , with ɛ_e = equilibrium isotopic enrichment factor; ɛ_k = kinetic isotopic enrichment factor. ii) the salt content of the incoming water δ_A is ≈20 g l^-1. The principal consequence is that the important loss of water observed between the sampling periods (1.5 m difference in water level) was not due to evaporation only, but more by seepage from the bottom of the take (≈ 560 10⁶ m³ for a total volume of ≈800 10⁶ m³).

CONCLUSIONS

1) On a diagram δ¹⁸O - CL^- (figure 13 and 14) where all the data for the Egyptian basins studied are represented, the similarity in the geochemical behaviour of the basins with a single input, is noted; the relation between isotopic composition and chloride content is approximatively logarithmic. In the case of a mixed input basin (Temsah Lake) two branches on the diagram are distinct; the first branch shows the effect of mixing in the west part of the lake, between evaporated Nile water and water from the Mediterranean Sea. The shape of this branch is very similar to that of the Nile delta: the second branch represents evaporation in the east part of the take (figure 13 and 14).

2) In the case of a semi-arid environment, this study shows that with two simple parameters only, such as isotopic composition and the chloride content of waters, it is possible to distinguish the marine, paralic and continental domain (figure 15).