School of Environmental Studies, China University of Geosciences, 430074 Wuhan, China; Earth System Science Dept., Stanford University, Stanford, CA 94305, United States.
Earth System Science Dept., Stanford University, Stanford, CA 94305, United States; Environmental Science Dept., University of California-Riverside, 900 University Ave, Riverside, CA 92521, United States.
Sci Total Environ. 2019 Feb 1;649:629-639. doi: 10.1016/j.scitotenv.2018.08.205. Epub 2018 Aug 17.
The consumption of arsenic (As) contaminated groundwater affects the health of almost 20 million people in China. Unlike the preponderance of observations within the deltas of South and Southeast Asia, groundwater As concentrations in the central Yangtze River Basin, China, vary by up to an order of magnitude seasonally. In order to decipher the cause of seasonal release and retention of As between sediments and groundwater, we conducted batch sediment incubations under varying (imposed) redox conditions. Incubations were conducted under both N and O gas purges to simulate conditions observed within the field. In all cases, anoxic conditions resulted in As release to solution while As was removed from solution under oxic conditions. These experiments confirm that anoxia is a prerequisite for As mobilization into groundwater from Yangtze River Basin sediments. Alternating redox conditions resulted in Fe minerals dissolution, transformation, crystallization, and precipitation, and subsequent As release and retention in the system. More importantly, aquifer sediments at depths >15 m release As through multiple redox cycles without an exogenous electron donor (carbon source), organic matter in the sediments is sufficiently reactive to support microbial reduction of As(V) and Fe(III). These results provide direct evidence for previously described mechanisms explaining the observed seasonal variation of groundwater As concentrations in the central Yangtze River Basin, where seasonal changes in surface and groundwater levels drive changes in redox conditions and thus As concentrations.
砷（As）污染地下水的消耗影响了中国近 2000 万人的健康。与南亚和东南亚三角洲的大多数观察结果不同，中国长江中游流域地下水的砷浓度随季节变化可达一个数量级。为了解释沉积物与地下水之间砷季节性释放和保留的原因，我们在不同（强制）氧化还原条件下进行了批量沉积物培养实验。在氮气和氧气吹扫下进行培养，以模拟现场观察到的条件。在所有情况下，缺氧条件导致砷释放到溶液中，而在有氧条件下，砷从溶液中被去除。这些实验证实，缺氧是从长江流域沉积物向地下水迁移砷的先决条件。交替的氧化还原条件导致 Fe 矿物的溶解、转化、结晶和沉淀，以及随后系统中砷的释放和保留。更重要的是，深度大于 15 米的含水层沉积物在没有外源电子供体（碳源）的情况下通过多个氧化还原循环释放砷，沉积物中的有机物具有足够的反应活性，足以支持微生物还原砷（V）和铁（III）。这些结果为先前描述的解释长江中游流域地下水砷浓度季节性变化的机制提供了直接证据，其中地表和地下水水位的季节性变化导致氧化还原条件变化，从而导致砷浓度变化。
