The city of El-Oued though located in the arid Grand Erg Oriental basin, have seen during these last twenty years intense growth around its large cultivated oases of the north eastern of the Algerian Sahara.

Three independent aquifer systems are under exploitation in this area. These are respectively : the deep Continental Intercalaire (CI) one of the largest confined aquifers in the world, the overlying Complexe Terminal (CT) and the superficial phreatic waters (PW).

Phreatic aquifers are presently causing serious problems to the local public authorities and especially to the water resources managers. Over the past decade and besides the severe deterioration of the water chemical and bacteriological quality, the piezometric level of such aquifers has not ceased to rise.

Consequently, this phenomenon has provoked the death and disappearance of several thousands of date-palms. The latter constitute the main agricultural resource of this region where these trees are grown in a sort of inter-dune dug crater-shaped gardens, locally called "Ghoots". In effect, the oases of the Oued-Souf region produce the world wide known and unique 'Deglet Nour' date. At present, even the architectural foundations of the buildings themselves are endangered in some places.

The spatial and temporal variation of measured groundwater hydrochemical parameters (major ion contents, electric conductance, ...) has enabled us to infer the water mineralisation origin. In the absence of a sanitation waters' network, high nitrate concentrations which reached at some sampling points figures greater than 200 mgl-1 were thus observed. These facts led us to suspect a probable contribution to the phreatic aquifers, both from sewage waters and also from irrigation waters.

Bacteriological analyses performed on a chosen number of El-Oued phreatic water samples have revealed the presence of various germs : coliforms, colibacillus and streptococcus faecalis, therefore confirming the above mentioned pollution hypothesis. Despite that fact, some of the suburban local population is unfortunately still using traditional dug well waters for their daily domestic purposes.

Stable isotopes measured in the groundwaters occurring in the investigated area have led us to determine the origin of water mineralisation and also the extent of mixing of the different water reservoirs. On the other side, tritium has shown that the phreatic aquifers are presently replenished by rapid infiltration of the episodic and rare precipitation taking place in the Grand Erg Oriental basin.
 

The Grand Erg Oriental basin extends over much of Algeria and southern Tunisia ) and is today an arid region with rainfall mainly below 100 mm yr-1. It is underlain by two major aquifer systems, the Continental Intercalaire (CI) and the Complexe Terminal (CT), which contain moderately fresh water. The CI extends across the Algerian-Tunisian border and the principal areas of recharge are in the South Atlas Mountains of Algeria and Tunisia and the Dahar mountains of Tunisia. The main discharge area is in Tunisia, in the Chotts area or the Gulf of Gabes. Integrated geochemical methods (major and trace elements, stable and radioisotopes) have been used to follow the changes taking place along selected flow paths, to better understand the controls on water quality and the processes of groundwater mineralisation, as well as any discontinuities in the system. Inert natural tracers, such as Cl and d2H and d18O are used together with radiocarbon to determine the extent of modern recharge, the history of recharge, groundwater residence time and information relating to palaeoclimate. The results from the CI aquifer are used here to illustrate the scientific approach and the essential findings.

There is a smooth increase in salinity in the CI with increase in total dissolved solids ranges from 964 to 3377 mg l-1. Sulphate is also an important component of the total mineralisation and ranges in concentration from a minimum of around 500 to a maximum of around 1750 mg l-1 SO4 .The salinity is mainly derived from evaporites as shown by the Br/Cl ratio. Oxidising conditions exist for some 300 k along the flow line from the Atlas mountains, but a redox boundary provides an important control on the chemistry, especially of Fe and other metals.

The radiocarbon and other isotope results show that modern recharge can be recognised in the aquifer to a distance of some 50-100 k from the Atlas recharge area. Water which recharged during the Holocene or late Pleistocene can be recognised by detectable radiocarbon some 300 k along the west east flow line in Algeria and also in most of the waters discharging in the Taxer-Nefta area of Tunisia. This indicates two converging lines of flow supplied by palaeorecharge, one from the Atlas and one most likely from the Dahar of southern Tunisia. In the centre of the basin the age of the waters is likely to be well in excess of 50000 yr BP and noble gas ratios indicate that these waters had a recharge temperature some 6o lower than at the present day.

Results confirm the overall continuity of the flow system in the CI aquifer, but emphasize the two contributions from the south-east and from the west. Although "modern" recharge can be recognised these amounts are small and the resources must be regarded as non-renewable. The aquifer is bounded by more saline waters in the formations above and below, as well as in the discharge area of the Chotts and therefore the waters are vulnerable to pollution through increased salinity caused by over-pumping which would induce excessive drawdowns. Salinity is relatively high, near to the limits of potability. However, the water is suitable mainly for agricultural use providing scaling is accounted for. It nevertheless forms a huge reserve of relatively fresh water in this region which with suitable technology (e.g. solar) can be treated and used for human consumption.