falajes of united Arab Emirates: geological setting and hydrogeological characteristics

الملخص EN

The falaj is a man-made stream which intercepts groundwater at the footslopes mountains and brings it to the surface at a lower level for irrigation purposes.

The object of this study is to discuss the effect of climate, geology and hydrological conditions on chemistry and quality of water in the U.A.E.

falajes.

Between 1978 and 1995, total falaj discharges ranged from 9.0 to 31.2 x 106 m3, which represents 2.8 to 9.7% of the total water use in the country.

The location, aqu storage, seepage loss from falaj channels, and rainfall, are the main factors affecting f; discharges in the U.A.E.

All U.A.E.

falajes are confined to the northern Oman mountains and the surround gravel plains.

The Electrical Conductance (EC) of falaj waters are generally low to medii varying between 450 micro-Seimens per centimeters (|iS/cm) in Falaj Asimah (A1 Fujaii and 10 940 (iS/cm in Falaj Ain Sukhnah (A1 Ain).

Iso-EC map shows that the EC of f waters is minimal near the water divide of the mountains and increases to the east west.

In open-channel falajes (A1 Gheli type), the EC increases with increasing falaj len but in the tunnel-type falajes (A1 Daudi type), EC is generally low irrespective to the 1 length.

Falaj waters have high concentrations of Mg2+ because they drain magnesium-mantle sequence rocks of the lower Semail nappe.

The Mg/(Ca + Mg) ratio is >0.5 ii falajes out of 33, indicating the dissolution of Mg-rich rocks which are the main constiti of the ultramafic rocks of the Semail ophiolites.

These conditions also favor the precipits of calcite (CaC03) and possibly huntite (Mg3Ca(C03)4) which can be altered late magnesite (MgC03).

According to EC and Sodium Adsorption Ratio (SAR), water o: Gheli falajes in ophiolitic rocks are good for irrigation, whereas the water in the D and Hadouri falajes draining limestone rocks are fair to poor for irrigation purposes.