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

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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.