HYDROGEOLOGY OF TERTIARY AQUIFERS OF CENTRAL NEJD AREA, SULTANATE OF OMAN

Sultanate of Oman is situated in the eastern edge of the Arabian Peninsula. The most important physiographic features of the Sultanate are the Oman Mountains to the north and Dhofar Mountains to the south. Nejd is a gently sloping plain lying between the two mountain ranges and is known to be the driest region of Oman. The water resource potential of this vast area has recently been ignored and poorly understood. A detailed hydrogeologic evaluation based on potentiometric, hydrochemical, and well hydrograph data was made to assess the hydrogelogical as well as hydrogeochemical behavior of Nejd aquifers.

Three principal aquifers appear to exist in the Nejd area. They are located within the Dammam and Rus Formations (Aquifer A), the Upper Umm Er Rhaduma Formation (Aquifer B), and the Lower Umm Er Rhaduma Formation (Aquifer C), at depths ranging between 16 m to 280 m below ground surface. The major aquifer horizons are normally associated with limestone units of Tertiary sequence. In most cases, their permeability is enhanced many folds because of the presence of karst, thus making them anisotropic. As a result, transmissivity of the aquifers varies greatly and ranges from 10 m²/day to 8,300 m²/day.

The findings based on the geologic and hydrologic data (such as stratigraphic, hydraulic parameters, and well construction) show that Aquifer A is an unconfined aquifer, Aquifer B is generally of a confined nature, and Aquifer C is confined over the entire Nejd area. Flowing artesian conditions were found to prevail in the north and north-west. The groundwater quality in the Nejd varies substantially, but the general trend for all aquifers display an increase in salinity towards the north and northeast. This trend corresponds to the groundwater flow direction and probably reflects increasing residence time and dissolution of salts into the groundwater. There is no distinct chemistry associated with the different Nejd aquifers. However, more saline water of Na⁺Cl^- type and few water samples containing Na⁺SO₄^2- were found at several locations north of the study area. In general, flouride content was found to be higher while going down towards the base of the Tertiary aquifers thus making Aquifer A an important source of potable water for the Nejd area.