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

地下水中砷水平的升高是一个健康问题，影响着全球超过1亿人，特别是在南亚和东南亚人口稠密的河流三角洲。含低砷和高砷的含水层的特点是具有强烈对比的氧化还原条件，这些条件往往被以铁为主的过渡带隔开。这种氧化还原锋面对As的平流和滞留起着至关重要的作用，从而防止安全含水层受到As的污染。然而，由于不断增长的水需求和不断增加的地下水抽取，目前不受As影响的含水层在未来面临被As污染的风险。尽管进行了十多年的研究，但在增强平流流动条件下，铁为主的氧化还原前沿对As的吸附在多大程度上延迟了低As含水层的污染，目前仍不清楚。如果不以全面、多学科的方式解决这一关键问题，就不可能对低As含水层可能受到邻近含As含水层的水入侵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控于(A)迁移过程、(B)微生物活动和(C)As宿主矿物相(主要是含铁)的相互作用。我们推测，由于电子供体和受体的可用性，特定微生物群落的活动，以及整个水交换和溶质运输，过渡带中Fe相和As物种的丰度和类型有所不同。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-含水层交换，将促进As在地下水中的动员和浓缩。这项拟议的多学科研究项目的总体目标是通过了解和预测在含水层系统中以铁为主的氧化还原过渡带增强的水力强迫下As的长期流动性，评估潜在的未来作为当前“安全”地下水的污染。将在越南的试验田同时描述As动力学及其相互作用的关键过程，这是我们的研究联盟以前所描述的，特别适合于实现我们的研究目标。所获得的数据将被整合到一个先进的反应传输模型中，该模型将物理传输和交换与关键的生物地球化学反应相结合。这一综合模型将使我们能够分析相关氧化还原前沿在时间和空间上的未来演变以及AS的命运。据我们所知，这种涉及所有关键学科的综合办法迄今尚未付诸实施，因此将大大提高我们的理解和我们预测地下水流动性的能力。

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.