The chapter discusses the arsenic poisoning issue in groundwater in Bangladesh and West Bengal, where alluvial Ganges aquifers are contaminated with naturally occurring arsenic. The arsenic is derived from the reductive dissolution of arsenic-rich iron oxyhydroxides, which are formed through the weathering of base-metal sulphides. This process is driven by the reduction of arsenic-rich iron oxyhydroxides in anoxic groundwater, leading to increased arsenic concentrations. The contamination affects millions of people, causing widespread health issues and deaths. The authors suggest that sedimentological studies can guide the placement of new water wells to avoid arsenic-contaminated water. Additionally, simple aeration and settling of anoxic groundwater can remove significant amounts of arsenic, providing a potential solution for household and village-scale treatment. The arsenic-rich iron oxyhydroxides, if properly disposed of, could help mitigate the ongoing poisoning problem.The chapter discusses the arsenic poisoning issue in groundwater in Bangladesh and West Bengal, where alluvial Ganges aquifers are contaminated with naturally occurring arsenic. The arsenic is derived from the reductive dissolution of arsenic-rich iron oxyhydroxides, which are formed through the weathering of base-metal sulphides. This process is driven by the reduction of arsenic-rich iron oxyhydroxides in anoxic groundwater, leading to increased arsenic concentrations. The contamination affects millions of people, causing widespread health issues and deaths. The authors suggest that sedimentological studies can guide the placement of new water wells to avoid arsenic-contaminated water. Additionally, simple aeration and settling of anoxic groundwater can remove significant amounts of arsenic, providing a potential solution for household and village-scale treatment. The arsenic-rich iron oxyhydroxides, if properly disposed of, could help mitigate the ongoing poisoning problem.