Reduction of iron oxyhydroxides (FeOOH), and its sorbed load of AS(V), pollutes groundwater in Hungary, Taiwan, the Bengal Basin, and Vietnam. In these areas, the process leads to extreme pollution (As > 1 mg/L) because it is driven by subsurface peat deposits, which are documented in Vietnam and the Bengal Basin. A model is given whereby fermentation of buried peat provides small organic moities (amines, sugars, carboxylic acids) that are carried downwards into the aquifer to drive reduction of FeOOH and arsenate. This model accounts for the vertical and lateral variation in arsenic pollution and its correlation to some degree with the distribution of known peat basins in the Bengal Basin and Vietnam. The effects of urbanization and unsewered sanitation can confuse interpretation of natural groundwater compositions and contribute to the lateral patchiness of arsenic distributions. Dissolved ammonium in aquifers is contributed by the microbial fermentation of buried peat deposits, so ammonium is not an infallible indicator of faecal contamination of groundwater in tropical deltaic settings. Simple redox stoichiometry shows that arsenic pollution at concentrations of tens of microgrammes per litre may arise from pyrite oxidation in sedimentary aquifers but this does not occur in the specific examples examined. Arsenic pollution by competitive exchange with phosphorus (from any source) or bicarbonate is a process not naturally important, but it may operate where anthropogenic influences disturb local environments.