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Targeting uncontaminated deep aquifers is a popular mitigation option although its long-term impact remains unknown.

Here we present the alarming results of a large-scale groundwater survey covering the entire Red River Delta and a unique probability model based on three-dimensional Quaternary geology.

, with both rural and urban populations being affected by toxic levels of As.

The highest concentrations are present in a 20 km wide band along the NW-SE boundary of the delta plain, to the SW of the modern Red River course, and coinciding with the location of the palaeo-Red River channel (9,000 y B. The spatial distribution of As in this region matches a pattern of elevated ) where dissolved sulfate levels are low, i.e., where sulfate reduction accompanied by As sequestration in sulfide minerals is limited (20).

Correspondingly, geogenic groundwater pollution in the Red River Delta poses a serious long-term health threat to about seven million people.