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

University Of California - Merced, Merced CA

Investigators

Abstract

0409203 Peggy A. O'Day The chemical speciation of arsenic in sediments and porewaters of aquifers is the critical factor that determines whether dissolved arsenic accumulates to potentially toxic levels. Although we have a general understanding of the biogeochemical cycle of arsenic, there remains a large gap in our quantitative understanding of the mechanisms of transformations, the role of iron 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 arsenic speciation, and the rates at which speciation changes in response to surface infiltration from rainfall 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 study arsenic 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 water table as a response to storm events influence subsurface arsenic geochemistry, and thus the mechanisms and rates of arsenic uptake or release? (ii) Does the formation of metastable, mixed valent iron phases, Fe(II,III) hydroxides (green rust) play an important role in controlling arsenic attenuation and release? How is the formation and stability of labile iron phases influenced by storm events and tidal flooding? (iii) Through molecular and macroscopic modeling based on field and laboratory observations and data, can we predict local changes in dissolved arsenic and iron concentrations in response to seasonal changes in water table and to storm events? We will employ X-ray absorption spectroscopic methods, complementary bulk and spatially resolved geochemical characterizations, hydrologic observations, and molecular and macroscopic modeling to address these questions at different scales. Intellectual Merit: Our work will address the mechanisms by which arsenic is attenuated or mobilized in the environment through identification and understanding of its chemical speciation. This study will also begin to address the more difficult question of the rates of arsenic transformation and mobilization in the field, and link temporal changes to mechanisms identified at molecular and microscopic scales. This comprehensive approach will advance fundamental understanding of arsenic speciation and mobility, particularly with respect to little known seasonal fluctuations and field rates of uptake and release. Broader Impacts: Worldwide, elevated concentrations of arsenic in groundwater, a known carcinogen 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 kinetic understanding of arsenic mobility in the environment. Our approach seeks to scale results from molecular to macroscopic and thus, our observations at this model study site will be generally transferable to other subsurface settings. The P.I. has recently moved to the University of California at Merced, the first UC campus to be built in over 35 years. This research proposal will serve to advance both graduate and undergraduate education at our fledging campus, which seeks to serve underrepresented groups in the agricultural San Joaquin Valley. In addition, our collaborators for this project come from industry and a national laboratory, providing opportunities for undergraduate summer internship programs and collaborative work with graduate students.