Retardation and mobilization of arsenic at redox fronts under advective flow conditions - a concerted multidisciplinary approach (AdvectAs)

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Elevated levels of arsenic (As) in groundwater are a health problem affecting over 100 million people worldwide, particularly in the densely populated river deltas of South and Southeast Asia. Aquifers containing low and high As levels are characterized by highly contrasting redox conditions that are often separated by Fe-dominated transition zones. Such redox fronts play a crucial role with regard to As advection and retention, consequently preventing safe aquifers from contamination with As. However, because of the constantly growing water demand and increasing groundwater abstraction, aquifers being currently not affected by As are at risk of becoming As-polluted in the future.Despite more than a decade of research, it remains largely unknown to which extent sorption of As at Fe-dominated redox fronts delays the contamination of low-As aquifers under enhanced advective flow conditions. Without addressing this key issue in a comprehensive, multidisciplinary manner, it is not possible to make reliable and robust predictions about the time scale over which low-As aquifers are likely to become contaminated by incursion of water from adjacent As-bearing aquifers.We hypothesize that the stability and persistence of the redox transition zones in space and time are largely controlled by the mutual interaction of (a) transport processes, (b) microbial activity and (c) the stability of As host mineral phases (mainly Fe-bearing). We postulate that the abundance and type of Fe phases as well as of As species vary across the transition zones as a result of the availability of electron donors and acceptors, the activity of specific microbial communities, and the overall water exchange and solute transport. Furthermore, we expect that external sources of dissolved organic carbon, e.g. by vertical aquifer-aquitard exchange, foster As mobilization and enrichment in groundwater.The overarching goal of this proposed multidisciplinary research project is to assess potential future As contamination of currently 'safe' groundwaters by understanding and predicting the long-term mobility of As under enhanced hydraulic forcing across Fe-dominated redox transition zones in aquifer systems.The simultaneous characterization of these key processes on As-dynamics and their interactions will be carried out in a test field in Vietnam, which has previously been characterized by our research consortium and is particularly suitable to reach our research goals. Data obtained will be integrated in an advanced reactive transport model that couples physical transport and exchange with the key bio-geochemical reactions. This integrated model will allow to analyze the future evolution of the relevant redox fronts in time and space and the fate of As. To our knowledge, such a comprehensive approach involving all key disciplines has not been carried out up to date and will, thus, significantly enhance our understanding and our ability to predict As mobility in groundwaters.