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

地下水中砷（As）含量升高是一个影响全球1亿多人的健康问题，特别是在南亚和东南亚人口稠密的河流三角洲。含水层含有低和高的水平，其特点是高度对比的氧化还原条件，往往是分开的铁为主的过渡区。这样的氧化还原方面发挥了至关重要的作用，平流和保留，从而防止安全含水层的污染与As。然而，由于不断增长的水需求和地下水抽取量的增加，目前不受As影响的含水层有可能在未来成为As污染的，尽管有十多年的研究，它仍然在很大程度上是未知的程度上吸附在Fe为主的氧化还原锋延迟低As含水层的污染下增强平流条件。如果不以综合、多学科的方式解决这一关键问题，就不可能对低砷含水层可能被邻近含砷含水层的水侵入污染的时间尺度做出可靠和可靠的预测。我们假设氧化还原过渡带在空间和时间上的稳定性和持久性在很大程度上受以下因素的相互作用控制：（a）运输过程，（B）微生物活性和（c）As主矿物相（主要是含Fe）的稳定性。我们假设，丰富和类型的Fe相，以及作为物种的过渡区的电子供体和受体的可用性，特定的微生物群落的活动，和整体的水交换和溶质运输的结果而有所不同。此外，我们预计溶解有机碳的外部来源，例如通过垂直含水层-弱透水层交换，这个拟议的多学科研究项目的总体目标是通过了解和预测在加强水力强迫下As的长期流动性，评估目前“安全”地下水未来潜在的As污染。主导的氧化还原过渡带含水层系统。这些关键过程的动态及其相互作用的同时表征将在越南的一个试验场进行，该试验场先前已由我们的研究联盟表征，特别适合于实现我们的研究目标。所获得的数据将纳入一个先进的反应性迁移模型，该模型将物理迁移和交换与关键的生物地球化学反应相结合。这个综合模型将允许分析未来的演变，在时间和空间的相关氧化还原前沿和命运的As。据我们所知，这样一个全面的方法，涉及所有的关键学科尚未进行到最新的，因此，将大大提高我们的理解和我们的能力来预测地下水中的流动性。

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.