In the Bengal Basin, the land surface exposed during the last lowstand of sea level around 20 ka, and now buried by Holocene sediment, is capped by an effectively impermeable clay paleosol that we term the Last Glacial Maximum paleosol (LGMP). The paleosol strongly affects groundwater flow and controls the location of arsenic pollution in the shallow aquifers of our study site in southern West Bengal and, by implication, in shallow aquifers across the Bengal Basin and As-polluted deltaic aquifers worldwide. The presence of the LGMP defines paleointerfluvial areas; it is absent from paleochannel areas. A paleosol model of pollution proposed here predicts that groundwater in paleochannels is polluted by arsenic, while that beneath paleointerfluvial areas is not: paleointerfluvial aquifers are unpolluted because they are protected by the LGMP from downward migration of arsenic and from downward migration of organic matter that drives As-pollution via reductive dissolution of As-bearing iron oxyhydroxides. Horizontal groundwater flow carries arsenic from paleochannels toward paleointerfluvial aquifers, in which sorption of arsenic minimizes the risk of pollution.