Source, Transport, and Evolution of Saline Groundwater in a Shallow Holocene Aquifer on the Tidal Deltaplain of Southwest Bangladesh

Abstract

Deltas are dynamic natural and socio-economic environments that are hypersensitive to the effects of urbanization, natural disasters and climate driven sea level change. Deltaic groundwater resources are often susceptible to degradation from saline water through seawater encroachment, tidal channel mixing and interaction with brackish paleowaters. The Ganges-Brahmaputra-Meghna River delta, located in Bangladesh, is a particularly vulnerable area with an estimated twenty million coastal inhabitants directly affected by saline drinking water. The shallow groundwater of the coastal regions is primarily brackish with pockets of fresher water. A small scale hydrologic investigation of groundwater salinity beneath a tidal channel island was undertaken to explore possible hydrogeological explanations of the distribution of water salinities in the shallow aquifer. This study employs a combination of 3H and 14C dating, electromagnetic (EM) subsurface mapping, and a 2D solute transport model. The EM and isotope data suggests the development of freshwater lenses, sustained by meteoric recharge, above paleo-brackish estuarine water that was deposited during the Holocene. The transport model indicates an advection dominated system which is insensitive to small changes in aquifer architecture.