Ziegler Brady A, Schreiber Madeline E, Cozzarelli Isabelle M, Crystal Ng G-H
Virginia Tech, Department of Geosciences, 926 W. Campus Dr., Blacksburg, VA 24061, USA.
U.S. Geological Survey, 12201 Sunrise Valley Dr., Reston, VA 20192, USA.
Environ Pollut. 2017 Dec;231(Pt 2):1351-1361. doi: 10.1016/j.envpol.2017.08.110. Epub 2017 Sep 22.
Natural attenuation of organic contaminants in groundwater can give rise to a series of complex biogeochemical reactions that release secondary contaminants to groundwater. In a crude oil contaminated aquifer, biodegradation of petroleum hydrocarbons is coupled with the reduction of ferric iron (Fe(III)) hydroxides in aquifer sediments. As a result, naturally occurring arsenic (As) adsorbed to Fe(III) hydroxides in the aquifer sediment is mobilized from sediment into groundwater. However, Fe(III) in sediment of other zones of the aquifer has the capacity to attenuate dissolved As via resorption. In order to better evaluate how long-term biodegradation coupled with Fe-reduction and As mobilization can redistribute As mass in contaminated aquifer, we quantified mass partitioning of Fe and As in the aquifer based on field observation data. Results show that Fe and As are spatially correlated in both groundwater and aquifer sediments. Mass partitioning calculations demonstrate that 99.9% of Fe and 99.5% of As are associated with aquifer sediment. The sediments act as both sources and sinks for As, depending on the redox conditions in the aquifer. Calculations reveal that at least 78% of the original As in sediment near the oil has been mobilized into groundwater over the 35-year lifespan of the plume. However, the calculations also show that only a small percentage of As (∼0.5%) remains in groundwater, due to resorption onto sediment. At the leading edge of the plume, where groundwater is suboxic, sediments sequester Fe and As, causing As to accumulate to concentrations 5.6 times greater than background concentrations. Current As sinks can serve as future sources of As as the plume evolves over time. The mass balance approach used in this study can be applied to As cycling in other aquifers where groundwater As results from biodegradation of an organic carbon point source coupled with Fe reduction.
地下水中有机污染物的自然衰减会引发一系列复杂的生物地球化学反应，这些反应会向地下水中释放次生污染物。在一个受原油污染的含水层中，石油烃的生物降解与含水层沉积物中氢氧化铁（Fe(III)）的还原作用相互关联。结果，吸附在含水层沉积物中氢氧化铁上的天然砷（As）从沉积物中释放出来进入地下水。然而，含水层其他区域沉积物中的Fe(III)具有通过再吸附作用来降低溶解态As浓度的能力。为了更好地评估长期生物降解与铁还原和砷迁移相结合如何在受污染含水层中重新分配砷的质量，我们基于现场观测数据对含水层中铁和砷的质量分配进行了量化。结果表明，铁和砷在地下水和含水层沉积物中均存在空间相关性。质量分配计算表明，99.9%的铁和99.5%的砷与含水层沉积物相关联。根据含水层中的氧化还原条件，沉积物既是砷的源也是汇。计算结果显示，在羽流35年的生命周期内，靠近油区沉积物中至少78%的原始砷已迁移到地下水中。然而，计算结果还表明，由于被沉积物再吸附，只有一小部分砷（约0.5%）残留在地下水中。在羽流前沿，地下水处于缺氧状态，沉积物会螯合铁和砷，导致砷的浓度累积至背景浓度的5.6倍。随着羽流随时间演变，当前的砷汇可能会成为未来的砷源。本研究中使用的质量平衡方法可应用于其他含水层中的砷循环，在这些含水层中，地下水中的砷是由有机碳点源的生物降解与铁还原作用共同导致的。
