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Arsenic contamination of groundwater has been identified in Cambodia, where some 100,000 family-based wells are used for drinking water needs. We conducted a comprehensive groundwater survey in the Mekong River floodplain, comprising an area of 3700 km² (131 samples, 30 parameters). Seasonal fluctuations were also studied. Arsenic ranged from 1 to 1340 μg L-1 (average 163 μg L-1), with 48% exceeding 10 μg L-1. Elevated manganese levels (57% >0.4 mg L-1) are posing an additional health threat to the 1.2 million people living in this area. With 350 people km-2 potentially exposed to chronic arsenic poisoning, the magnitude is similar to that of Bangladesh (200 km-2). Elevated arsenic levels are sharply restricted to the Bassac and Mekong River banks and the alluvium braided by these rivers (Kandal Province). Arsenic in this province averaged 233 μg L-1 (median 100 μg L-1), while concentrations to the west and east of the rivers were <10 μg L-1. Arsenic release from Holocene sediments between the rivers is most likely caused by reductive dissolution of metal oxides. Regions exhibiting low and elevated arsenic levels are co-incident with the present low relief topography featuring gently increasing elevation to the west and east of a shallow valley—understood as a relict of pre-Holocene topography. The full georeferenced database of groundwater analysis is provided as Supporting Information. [ABSTRACT FROM AUTHOR]