Shallow groundwater, the primary water source in the Bengal Basin, contains up to 100 times the World Health Organization (WHO) drinking-water guideline of 10g l 1 arsenic (As), threatening the health of 70 million people. Groundwater from a depth greater than 150m, which almost uniformly meets the WHO guideline, has become the preferred alternative source. The vulnerability of deep wells to contamination by As is governed by the geometry of induced groundwater flow paths and the geochemical conditions encountered between the shallow and deep regions of the aquifer. Stratification of flow separates deep groundwater from shallow sources of As in some areas. Oxidized sediments also protect deep groundwater through the ability of ferric oxyhydroxides to adsorb As. Basin-scale groundwater flow modelling suggests that, over large regions, deep hand-pumped wells for domestic supply may be secure against As invasion for hundreds of years. By contrast, widespread deep irrigation pumping might effectively eliminate deep groundwater as an As-free resource within decades. Finer-scale models, incorporating spatial heterogeneity, are needed to investigate the security of deep municipal abstraction at specific urban locations. ?? 2010 Macmillan Publishers Limited. All rights reserved.