The fossil record is our only data source for studying Earth's long-term biotic dynamics, but such studies require careful hypothesis testing. In Bangladesh and West Bengal, arsenic contamination in groundwater from alluvial Ganges aquifers is a major public health issue. Arsenic is derived from the reductive dissolution of arsenic-rich iron oxyhydroxides, which form from the weathering of base-metal sulphides. This process is driven by sedimentary organic matter, with concentrations up to 6%. Arsenic-rich iron oxyhydroxides are now being reduced in anoxic groundwater, releasing arsenic into solution. The arsenic is mainly found in alluvial aquifers of the Ganges delta, with its source in the Ganges basin upstream of Bangladesh. Weathering of base-metal deposits in the Ganges basin has supplied arsenic-rich iron oxyhydroxides to downstream sediments. The arsenic-rich groundwater is mostly restricted to alluvial aquifers, and the source of arsenic-rich iron oxyhydroxides lies in the Ganges source region. Understanding the sedimentary architecture and distribution of iron, arsenic, and reductant carbon in Ganges alluvial sediments can help develop a predictive model to guide future aquifer development, minimizing arsenic pollution. Simple aeration of anoxic groundwater followed by settling can remove a considerable amount of arsenic from solution. This treatment could be performed on a household or village scale. Although the disposal of arsenic-rich iron oxyhydroxides would require special arrangements, it is preferable to the current widespread poisoning or the return to contaminated surface water. The study highlights the importance of understanding sedimentary processes in managing arsenic contamination.The fossil record is our only data source for studying Earth's long-term biotic dynamics, but such studies require careful hypothesis testing. In Bangladesh and West Bengal, arsenic contamination in groundwater from alluvial Ganges aquifers is a major public health issue. Arsenic is derived from the reductive dissolution of arsenic-rich iron oxyhydroxides, which form from the weathering of base-metal sulphides. This process is driven by sedimentary organic matter, with concentrations up to 6%. Arsenic-rich iron oxyhydroxides are now being reduced in anoxic groundwater, releasing arsenic into solution. The arsenic is mainly found in alluvial aquifers of the Ganges delta, with its source in the Ganges basin upstream of Bangladesh. Weathering of base-metal deposits in the Ganges basin has supplied arsenic-rich iron oxyhydroxides to downstream sediments. The arsenic-rich groundwater is mostly restricted to alluvial aquifers, and the source of arsenic-rich iron oxyhydroxides lies in the Ganges source region. Understanding the sedimentary architecture and distribution of iron, arsenic, and reductant carbon in Ganges alluvial sediments can help develop a predictive model to guide future aquifer development, minimizing arsenic pollution. Simple aeration of anoxic groundwater followed by settling can remove a considerable amount of arsenic from solution. This treatment could be performed on a household or village scale. Although the disposal of arsenic-rich iron oxyhydroxides would require special arrangements, it is preferable to the current widespread poisoning or the return to contaminated surface water. The study highlights the importance of understanding sedimentary processes in managing arsenic contamination.