Teneurs en 18O et concentration saline d'eaux paraliques et continentales égyptiennes O18 content and sait concentration of lagoonal and continental egyptian waters

Résumé

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.

Mots-clés:

• Oxygène-18,
• salinité,
• paralique,
• continental,
• Egypte,
• actuel

Abstract

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).