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).
