More than 100 million people living on the floodplains of the Ganges-Brahmaputra-Meghna, Mekong and Red River, all draining the Himalayas, are consuming arsenic contaminated water. Providing safe drinking water for these people requires a quantitative understanding of the processes regulating the groundwater arsenic content and this knowledge is presently not available. In PREDIAS we propose a revolutionary new approach to study these arsenic contaminated aquifers where sediments and groundwaters are considered as one reacting unit that is changing over time. The key hypothesis is that it is the aquifer sediment burial age that is the overall controlling parameter for the arsenic content. This new approach is explored by studying the groundwater chemistry as a function of sediment burial age, which is equivalent to the geological evolution over time, in part of the Red River floodplain in Vietnam. The investigations comprise delineating the sedimentological development over the last 9000 yrs as well as reconstructing hydrogeological conditions over that period. Process studies will reveal the effect of burial age on the chemical properties of the sediments and the arsenic release mechanisms. They comprise the binding and release mechanisms of arsenic to the aquifer sediment, and the reactivity of sedimentary organic carbon and iron oxides which drive the redox reactions controlling the water chemistry and arsenic mobilization. Information on the sedimentological and hydrogeological development over time as well as a quantification of the geochemical processes will be incorporated in a 3-D reactive transport model which over the last 9000 years, in steps of about 1000 years, can predict the evolution of the arsenic content over space and time in the groundwater of the studied area. The model can be extended in a more conceptual form to larger parts of the Red River delta and Bangladesh using satellite imaging to reveal the geological development in those areas.
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