Abstract
Isotopic studies were carried out on the right banks of the River Indus, downstream of Tarbela, to study the effect of Tarbela reservoir on the groundwater system. Tarbela dam, the giant multipurpose dam, completed in 1976, has been built across the River Indus. The reservoir is 97 km long, 260 km super(2) in area and has a gross storage capacity of 17 109 000 m super(3) at the maximum lake elevation of 472 m, a residual capacity of 2 802 000 m super(3) at the assumed level of maximum drawdown of 396 m and a net capacity of 14 307 000 m super(3). The total catchment area above Tarbela is spread over 168 000 m super(2), which largely brings in snowmelt supplies in addition to some monsoon rains. Initially, hundreds of sinkholes and cracks developed in the dam site bed during the first filling of the reservoir. Excessive seepage through the dam foundation and its abutment also presented serious problems. These were, later on, rectified. However, it is observed that the water table rises in the downstream areas when the reservoir attains its highest level in the monsoon season. The water table rise may be associated with the rise of the reservoir level and the creation of sinkholes in the dam/reservoir bed. Another incident was the appearance of water in the ponds near Gadoon Amazai on the right side of Tarbela reservoir, related to the (probable) seepage from the reservoir. Bearing this in mind, it was decided to apply isotopic techniques to attempt to determine the situation. The main objectives of the study were to determine the hydraulic connection, if any, between Tarbela Lake and the groundwater appearing in the ponds near Gadoon Amazai, to see the effect of Tarbela dam on the groundwater system in the downstream areas, and to estimate the relative contribution of different recharge sources to the groundwater system.