Spatial and Temporal Changes in Arsenic, Iron, and Sulfur Speciation in a Shallow Aquifer

[40920]佩吉·a·奥戴沉积物和含水层孔隙水中砷的化学形态是决定溶解的砷是否积聚到潜在毒性水平的关键因素。虽然我们对砷的生物地球化学循环有了大致的了解，但在转化机制、铁和硫的作用以及在环境中调动砷的自然过程的动力学方面，我们的定量理解仍然存在很大差距。我们缺乏详细的测量方法，使我们能够量化季节对地下砷物种形成的影响，以及物种形成响应地表降雨入渗和地下水位垂直波动的速率。结合现场、实验室和建模工作，采用分子和宏观方法，我们将在旧金山湾附近的一个地点作为模型系统研究砷的迁移率和自然衰减。这些研究将解决以下问题：(i)作为对风暴事件的响应，地下水位的季节变化和变化如何影响地下砷的地球化学，从而影响砷吸收或释放的机制和速率？（ii）亚稳、混合价铁相、铁（ii、III）氢氧化物（绿锈）的形成是否在控制砷的衰减和释放中起重要作用？风暴事件和潮汐洪水如何影响不稳定铁相的形成和稳定性？（iii）通过基于现场和实验室观测和数据的分子和宏观模型，我们能否预测当地溶解性砷和铁浓度随地下水位和风暴事件的季节性变化的变化？我们将采用x射线吸收光谱方法、互补的体积和空间分辨地球化学表征、水文观测以及分子和宏观建模来解决这些问题。智力价值：我们的工作将通过识别和理解砷的化学形态来解决砷在环境中被稀释或动员的机制。这项研究还将开始解决更困难的问题，即砷在野外的转化和动员速度，并将时间变化与在分子和微观尺度上确定的机制联系起来。这种综合方法将促进对砷形态和流动性的基本了解，特别是对鲜为人知的季节波动和田间吸收和释放率的了解。更广泛的影响：在世界范围内，地下水中砷的浓度升高，一种已知的致癌物和诱变剂，有可能对9000万人产生不利影响，其中包括美国的1300万人。本研究将寻求砷在环境中迁移的机械、热力学和动力学理解。我们的方法旨在将结果从分子扩展到宏观，因此，我们在该模型研究地点的观察结果通常可转移到其他地下环境。私家侦探最近搬到了加州大学默塞德分校，这是加州大学35年来建造的第一个校区。这项研究计划将有助于促进我们初具规模的校园的研究生和本科教育，旨在为农业圣华金河谷的弱势群体提供服务。此外，我们的合作者来自工业界和国家实验室，为本科生提供暑期实习项目和与研究生合作的机会。

0409203Peggy A. O'DayThe chemical speciation of arsenic in sediments and porewaters of aquifers is the criticalfactor that determines whether dissolved arsenic accumulates to potentially toxic levels.Although we have a general understanding of the biogeochemical cycle of arsenic, there remainsa large gap in our quantitative understanding of the mechanisms of transformations, the role ofiron and sulfur, and the dynamics of natural processes that mobilize arsenic in the environment.We lack detailed measurements that allow us to quantify seasonal impacts on subsurface arsenicspeciation, and the rates at which speciation changes in response to surface infiltration fromrainfall and vertical fluctuations in water table depth. Using a combination of field, laboratory,and modeling efforts that employ both molecular and macroscopic methods, we will studyarsenic mobility and natural attenuation at a site near San Francisco Bay as a model system.These studies will address the following: (i) How do seasonal variations and changes in watertable as a response to storm events influence subsurface arsenic geochemistry, and thus themechanisms and rates of arsenic uptake or release? (ii) Does the formation of metastable, mixedvalent iron phases, Fe(II,III) hydroxides (green rust) play an important role in controllingarsenic attenuation and release? How is the formation and stability of labile iron phasesinfluenced by storm events and tidal flooding? (iii) Through molecular and macroscopicmodeling based on field and laboratory observations and data, can we predict local changes indissolved arsenic and iron concentrations in response to seasonal changes in water table and tostorm events? We will employ X-ray absorption spectroscopic methods, complementary bulkand spatially resolved geochemical characterizations, hydrologic observations, and molecularand macroscopic modeling to address these questions at different scales.Intellectual Merit: Our work will address the mechanisms by which arsenic is attenuated ormobilized in the environment through identification and understanding of its chemicalspeciation. This study will also begin to address the more difficult question of the rates ofarsenic transformation and mobilization in the field, and link temporal changes to mechanismsidentified at molecular and microscopic scales. This comprehensive approach will advancefundamental understanding of arsenic speciation and mobility, particularly with respect to littleknown seasonal fluctuations and field rates of uptake and release.Broader Impacts: Worldwide, elevated concentrations of arsenic in groundwater, a knowncarcinogen and mutagen, has the potential to adversely impact on the order of 90 million people,including 13 million in the US. This study will seek mechanistic, thermodynamic, and kineticunderstanding of arsenic mobility in the environment. Our approach seeks to scale results frommolecular to macroscopic and thus, our observations at this model study site will be generallytransferable to other subsurface settings.The P.I. has recently moved to the University of California at Merced, the first UCcampus to be built in over 35 years. This research proposal will serve to advance both graduateand undergraduate education at our fledging campus, which seeks to serve underrepresentedgroups in the agricultural San Joaquin Valley. In addition, our collaborators for this projectcome from industry and a national laboratory, providing opportunities for undergraduate summerinternship programs and collaborative work with graduate students.