Date of Award:

2015

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Civil and Environmental Engineering

Advisor/Chair:

Joan E. McLean

Abstract

The basin-fill aquifers of the American Southwest host elevated concentrations of arsenic in groundwater due to the local geology. Limited information is available on arsenic dynamics in semi-arid and arid regions of the world. This study describes arsenic biogeochemistry and mechanisms of arsenic solubilization for a soil profile collected from the surface to the depth of groundwater in the Cache Valley Basin, Northern Utah.

The first objective was to delineate mechanisms of arsenic solubilization from sediments collected at the study site. Microcosms containing site groundwater and siteoxidized and site-reduced sediments, were monitored over time to observe changes in the solubilization and oxidation state of arsenic and changes in mineral phases of arsenic and iron. The observed solubilization of arsenic was decoupled from iron reduction in the site-oxidized sediments in the presence of native organic carbon, which disagreed with the widely accepted hypothesis that arsenic solubilization is derived from microbial driven reductive dissolution of iron oxides. Carbonate minerals were defined as the mineral phase associated with arsenic that contributed to the arsenic measured in solution.

The second objective was to determine how altering redox and water conditions down a profile affects arsenic geochemistry and hence solubility. Redox stratification was delineated in two sediment cores based on chemical analyses and visual observation of redox-sensitive parameters. The vadose zone released a considerable amount of arsenic, while the next zone, the carbonate enrichment zone, released the highest concentration of arsenic. Soluble arsenic was exclusively As(V) in the redox transition zone, where As was primarily associated with iron oxides. Solubilization of arsenic was limited in the deeply reduced depletion zone due to the formation of sulfide minerals.

Lateral resolution of oxidation state and elemental association of arsenic at the micron scale were delineated using synchrotron-based X-ray absorption spectroscopy under Objective 3. The presence of unaltered arsenic sulfides was revealed in the vadose zone, suggesting that arsenic was inputted continuously to the ground surface. From the water table to the deeply reduced depletion zone sediments, arsenic mineral association was dominated by manganese-bearing carbonate minerals and amorphous iron oxides, which are vulnerable to groundwater fluctuation and redox-cycling.

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