Depth Transects of Sediment Age, Reflectance, and Aquifer Arsenic in the Bac Bo Plain, Vietnam: Implications for Groundwater Arsenic Heterogeneity in the Red River Delta

Shallow aquifer groundwater arsenic heterogeneity is well documented in many of the fluvial regions of Asia. To this day, the cause for the heterogeneity remains poorly understood in part because of the heterogeneity of sediment properties inherent to a young floodplain depositional environment. In April 2006, a needle-sampler device was used to obtain depth transects of both sediment and porewater samples as the first step towards understanding the heterogeneous subsurface environment. Depth transects were taken between sites with opposing trends in tube-well arsenic, e.g. low As cluster to high As cluster, in a stable, fault controlled river bend in Van Phuc, Vietnam, to determine how sediment properties such as grainsize and reflectance relate to dissolved arsenic. Luminescence dating of aquifer sands and He3/H3 dating of the groundwater was conducted to investigate how aquifer redox conditions and groundwater arsenic evolve in Van Phuc's river-bend over time. Results indicate that localized zones of sediments enriched in leachable arsenic exist at depth within the aquifer and provide a mostly local source of arsenic. It also appears that groundwater flow encountering sediments of different ages can account for some of the observed spatial patterns of groundwater heterogeneity. Compiled with observations that reducing groundwaters with both low dissolved and extractable arsenic exist in the upper deltaic region, it appears that heterogeneities in shallow floodplain arsenic can be driven by magnitude differences in sediment-labile arsenic. Finally, given that these localized packages of arsenic enrichment correspond to a particular in-filling type of facies, it appears that fluvial cycling along the delta is the underlying cause of arsenic heterogeneity in the shallow floodplain.