On the basis of our survey and analysis of arsenic in the hand tube wells (n = 250,000) from the Ganga–Meghna–Brahmaputra (GMB) plain (area: 569,749 km²; population: above 500 million) over the last 23 years, we revealed that some portions of all the states in Ganga–Brahmaputra plains in India and Bangladesh are arsenic affected. The source of arsenic is geogenic. The Himalaya and Tibet Plateau is considered to be the arsenic source. The exact nature of mobilization process from the source in groundwater is still unknown. On an average, 50% of the water samples had arsenic above the World Health Organization (WHO) guideline value of 10 μg/L.