Evaluation of Fresh Groundwater Potentials for Future Urban Planning of Sandwip, an Offshore Island in Bangladesh

Abstract

Sandwip, an offshore island of Chittagong District, Bangladesh, requires a substantial amount of fresh groundwater mainly for industrial and household uses. This study integrates resistivity and borelog data to explore fresh groundwater potential in a complex deltaic environment for sustainable urban development. A total of 15 Vertical Electrical Soundings (VES) and two Electrical Resistivity Tomography (ERT) profiles were acquired using Schlumberger and Wenner arrays, respectively. Resistivity data, interpreted through computer modeling and calibrated with borehole lithology, reveal highly heterogeneous subsurface conditions with frequent aquifer–aquitard alternations. The topsoil (2.35–12.16 Ωm) is underlain by silty clay (Aquitard 1). A shallow aquifer (Aquifer 1) is located at depths of 10–60 m, composed of fine to medium sand with resistivity of 14.51–94.24 Ωm. A second aquitard (5–61 m thick) separates Aquifer 1 from an intermediate aquifer (Aquifer 2), encountered at 60–130 m with a thickness of 4.15–82.13 m. A deeper clay unit (Aquitard 3) underlies Aquifer 2. Although sand layers are present, both aquifers exhibit resistivity signatures of brackish to saline water, with only isolated pockets of freshwater. Hence, large-scale groundwater development within 130 m depth appears limited. Future investigations should employ extended electrode spreads to explore aquifers below 300 m, where more prolific freshwater reserves may occur.