The Mekong Delta region is comprised of alluvial Holocene sediments
(depths of 8–40 m) overlying Pleistocene sediments (50–80 m;
Fig. 3). Arsenic above 10 ppb was typically
found in relatively shallow wells from both the Holocene and Pleistocene
aquifers. Two deep wells (N200 m) of the Lower Pliocene aquifer
were exceptional and also showed elevated As. Both deep wells are located
in the Northern sub-group