Groundwater arsenic content in quaternary aquifers of the Red River delta, Vietnam, controlled by the hydrogeological processes

•High As levels in the Pleistocene aquifer imposed by natural hydrogeological processes.•Preferential flow paths created due to the low subsidence rate in a river delta.•Flow of high As groundwater from Holocene to Pleistocene aquifers.•Surface derived organic matter seeping through thick, permeable clay layers.•Groundwater As levels in Holocene aquifers related to the flushing extent.