Satellite images at a much smaller scale show that the surface geology of these floodplain sediments is very complex. Abandoned channels, oxbow lakes, point bar progressions are intersecting into a complex mosaic. Previous studies have shown that particularly abandoned channels may be associated with high arsenic groundwater which indicates that the sedimentary architecture may have an important effect on the groundwater arsenic content (Sahu & Saha, 2015). At a small scale we have studied localities with high and low arsenic groundwater with the objective of resolving the controlling parameters for the groundwater arsenic content (Postma et al., 2007, 2012). By dating the burial age of the aquifer sediment, using optical stimulated luminescence, we could show that young sediments (400-600 years) contain groundwater with a much higher arsenic content than older (3,500-6,000 years) sediments. From this observation it follows that the geochemical properties of these very young aquifer sediments
The biggest unsolved problem concerning the contamination of groundwater in the floodplains of SE Asia with arsenic concerns the high spatial variability of the arsenic content. Wells with a high arsenic concentration in the groundwater may be located within a few hundreds of meters from wells containing very little arsenic without any apparent explanation. In order to obtain effective management strategies for the arsenic contamination problem, we clearly need a proper understanding of the mechanisms controlling the spatial arsenic variability. Here we use the Red River flood plain in Vietnam as the example to address this issue.