Hydrogeochemical Evolution of Arsenic Concentrations and Speciation Along Groundwater Flow Paths: Linking Aqueous and Solid Phase Arsenic Speciation in Sedimentary Aquifers

导致人类砷中毒的主要媒介是饮用水的消费，主要来自地下水源。这种中毒目前影响到恒河三角洲和东南亚其他地区的数百万人。由于地下水也是美国饮用水的主要来源，了解导致含水层AS流动的因素至关重要。在组装预测模型之前，首先需要量化随着条件(即pH、氧化还原条件、溶液成分、含水层矿物表面位置)在空间和时间尺度上的变化和演变而沿地下水流动路径的行为。许多以前的研究，包括那些关于恒河三角洲的研究，在试图确定负责AS动员的过程方面都遇到了困难。这类研究的失败在一定程度上反映了缺乏沿水流路径收集的地下水和含水层固相砷的形态数据。此外，由于氧化还原反应和循环反应受到微生物的强烈影响，微生物过程可能在砷的流动性中发挥重要作用。中心假设是，伴随着化学风化和微生物促进的氧化还原反应而沿地下水流动路径发生的变化，通过吸附/解吸、共沉淀和/或矿物溶解反应，催化从含水层基质中动员并被含水层基质捕获。为了验证这一假说，我们将：(1)在两个经过充分研究的含水层(德克萨斯州的Carrizo Sand和马里兰州的Aquia含水层)中，量化DIC和DOC沿流动路径的浓度和形态[即As(III)，As(V)，有机砷化合物，硫代亚砷酸盐物种]和辅助的地球化学参数(例如，pH，温度，主要溶质，碱度，磷酸盐，溶解的二氧化硅，铁的浓度/形态，溶解的O2，Eh，H2S，DOC，d34S，d13C)；(2)确定含水层固相的形态[As(III)，As(V)]，强调As的不稳定和不稳定的库；和(3)在野外和实验室研究的基础上，建立As沿流动路径的迁移性和形态的概念/半定量模型。微生物代谢对AS动员的重要性将通过一系列批量培养研究来间接检验。此外，计划与一位国际公认的微生物学家合作，该微生物学家对AS的微生物循环进行了开创性研究，对含水层沉积物的光谱调查也是如此。该项目将显著提高我们对地下水流动系统中砷的生物地球化学的理解，并将导致建立含水层中砷的动员和生物地球化学循环的概念模型。预计项目成果将为今后的研究提供基础信息(例如，更详细的微生物研究、表面络合模型、反应传输模型)。

0510697Johannesson The principal vector responsible for arsenic (As) poisoning in human populations is consumption of drinking water, chiefly from groundwater sources. Such poisoning currently affects millions of people in the Ganges delta and other regions of Southeast Asia. Because groundwater is also the chief source of drinking water in the United States, understanding the factors responsible for As mobilization in aquifers is critical. Before predictive models can be assembled, it is first necessary to quantify As behavior along groundwater flow paths as conditions (i.e., pH, redox conditions, solution compositions, aquifer mineral surface sites) change and evolve over spatial and temporal scales. Many previous studies, including those for the Ganges delta, have suffered in their attempts to identify processes responsible for As mobilization. The failure of such studies reflects, in part, the lack of As speciation data for groundwater and aquifer solid phases collected along flow paths. Furthermore, because redox reactions, and hence As cycling, are strongly influenced by microbes, microbial processes likely play an important role in As mobility. The central hypothesis is that changes occurring along groundwater flow paths that accompany chemical weathering and microbial facilitated oxidation-reduction reactions catalyze As mobilization from, and capture by, the aquifer substrate via adsorption/desorption, co-precipitation, and/or mineral dissolution reactions. To investigate the hypothesis, we will: (1) quantify As concentrations and speciation [i.e., As(III), As(V), organoarsenicals, thioarsenite species] and ancillary geochemical parameters (e.g., pH, temperature, major solutes, alkalinity, phosphate, dissolved silica, Fe concentration/speciation, dissolved O2, Eh, H2S, DOC, d34S, d13C of DIC and DOC) along flow paths in two well studied aquifers (Carrizo Sand, Texas; Aquia aquifer, Maryland); (2) determine As speciation [As(III), As(V)] of aquifer solid phases, emphasizing the labile and nonlabile pools of solid phase As; and (3) assemble a conceptual/semi-quantitative model of As mobility and speciation along flow paths based on the field and laboratory studies. The importance of microbial metabolism on As mobilization will be indirectly examined via a series of batch incubation studies. In addition, collaboration with an internationally recognized microbiologist who has conducted seminal studies on microbial cycling of As is planned, as are spectroscopic investigations of aquifer sediments. The project will significantly improve our understanding of As biogeochemistry in groundwater flow systems, and will lead to a conceptual model of As mobilization and biogeochemical cycling in aquifers. It is expected that project results will provide fundamental information that will seed future research (e.g., more detailed microbial studies, surface complexation models, reactive transport modeling).