A REVIEW OF THE GEOLOGICAL AND HYDROGEOLOGICAL REGIMES AT THE HIMALAYAN FOOTHILLS IN NORTHWEST PAKISTAN
The Peshawar intermontane basin is a broad, oval shaped depression comprising of a thick sequence of lacustrine, deltaic and fluvial sediments overlain by loess and alluvial deposits, which are dated at 2.8 to 0.6 Ma. These sediments form well-productive aquifers and consist mainly of sand and gravel in the north and south of the basin. However, in the central part, the coarse sediments are interbedded with clay, silt and sandy silt attaining its maximum thickness and producing several semi-confined aquifers. Khyber, Attock-Cherat and Lower Swat-Bunner piedmont aquifers occur on the periphery of the basin, while flood plain and lacustrine aquifers occupy the central part of the basin. Depth to the water table is < 5 m, except on the margins of the basin and in the southeast where it ranges from 5 to > 30 m. The EC values range from 800 mS/cm near the margins to a maximum of 7800 mS/cm in the center of the basin. Similar is the case of mineralization. Majority of the groundwater has low TDS with dominant bicarbonate and sulfate types of the alkaline earths and alkalies. However, in the central part of the basin, there is a transition from the fresh bicarbonate to highly mineralized sulfate-chloride water. Such waters are either a product of evaporative process or a "squeeze out" from deeper evaporite formations.