Natural groundwater contamination ny arsenic has become a crucial water quality problem in many parts ofthe world, particularly in the Ganges delta (Bangladesh and West Bengal, India) [1, 6, 9, 13, 14, 18]. Here we report the first time on the occurence of arsenic of the Red River alluvial tract in the city of Hanoi and in the surronding rural districts. The analysed waters were taken either form individual home drilling wells or from water treatment palnts and urban distributon system. Spatial and season variations of arsenic in the aquifers can partly be explained by hydro-geological and geochemical processes.