Reduction of iron oxyhydroxide (FeOOH) and release of its sorbed arsenic load to solution is an importantmechanism by which groundwater worldwide becomes polluted with arsenic. In the Bengal Basin ofBangladesh 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 withphosphorus derived from fertilizer (or other sources). The concentration of arsenic in the sediments of theBengal Basin is not exceptional and the occurrence of reducing conditions alone is insufficient to explainthe extreme degree and extent of arsenic pollution. Extreme pollution by arsenic occurs becausebiodegradation of buried peat deposits drives extreme degrees of FeOOH reduction and supplies highconcentrations of arsenic to groundwater (hundreds of ?g l-1). Sources of organic matter other than peat areneither reactive enough nor abundant enough to generate the amount and degree of reduction necessary tocause such severe arsenic pollution but may account for pervasive low-level contamination (< 50 ?g l-1).The distribution of known peat basins, and their ages, correlates to some degree with the surfacedistribution and depth profiles of arsenic. The distribution of peat and arsenic can be related to the LatePleistocene and Holocene evolution of the Bengal basin. Because this evolution is controlled largely byclimatic fluctuations and sea-level changes, a general model emerges for predicting the occurrence ofgroundwater that is naturally polluted by arsenic.