Reduction of iron oxyhydroxide (FeOOH) and release of its sorbed arsenic load to solution is an important mechanism by which groundwater worldwide becomes polluted with arsenic. In the Bengal Basin of Bangladesh and West Bengal (India), it is the main mechanism by which arsenic pollutes groundwater. Arsenic pollution does not arise from oxidation of sedimentary sulfides nor from ion-exchange with phosphorus derived from fertilizer (or other sources). The concentration of arsenic in the sediments of the Bengal Basin is not exceptional and the occurrence of reducing conditions alone is insufficient to explain the extreme degree and extent of arsenic pollution. Extreme pollution by arsenic occurs because biodegradation of buried peat deposits drives extreme degrees of FeOOH reduction and supplies high concentrations of arsenic to groundwater (hundreds of μg l1). Sources of organic matter other than peat are neither reactive enough nor abundant enough to generate the amount and degree of reduction necessary to cause such severe arsenic pollution but may account for pervasive low-level contamination (< 50 μg l1). The distribution of known peat basins, and their ages, correlates to some degree with the surface distribution and depth profiles of arsenic. The distribution of peat and arsenic can be related to the Late Pleistocene and Holocene evolution of the Bengal Basin. Because this evolution is controlled largely by climatic fluctuations and sea-level changes, a general model emerges for predicting the occurrence of groundwater that is naturally polluted by arsenic.