Application of electrical resistivity inversion to delineate salt and freshwater interfaces in quaternary sediments of northwest Himalaya, Pakistan

Delineation of saline water and freshwater interfaces is a primary objective in groundwater studies and groundwater management. Karak valley, NW Himalaya of Pakistan is situated in a semiarid region where the continuous supply of freshwater is challenging because of high salinity values in groundwater. In this study, vertical electrical resistivity and borehole data are used to mark the freshwater and saline water interfaces. Borehole data collected from different locations in the study area is used to interpret various subsurface lithologies and to calibrate the modeled resistivity curves. The electrical resistivity data indicates that a thick cover of Quaternary sediments is present in the valley. The re-inversion of electrical resistivity data with modern analysis techniques is highly effective to map the boundary between fresh and saline water aquifers in the valley. Inversion technique is applied to generate 2D and 3D subsurface resistivity maps for the delineation of fresh and saltwater surfaces. The presence of saline water aquifer with clay rich sediments is identified by very low resistivity values on the northern part of the valley whereas relatively high resistivity values with sand and gravel sediments on the southeastern part of the valley indicate the presence of freshwater aquifer. The generated 2D resistivity surface maps at different depth levels above and below the water table and formation resistivity distribution map are effectively used to mark the boundary between fresh and saline water zones. The zone of brackish water is clearly seen in the resistivity inverse model at a depth of 5–30 m.