Combining Fission-Track Radiography and Scanning Electron Microscopy to Elucidate Uranium Mobility Controls

Abstract Residual solid-phase uranium from former mill tailings leachate can contribute to persistent concentrations of uranium in groundwater that exceed regulatory levels. Microscale characterization of uranium-contaminated sediment samples is lacking due to the challenges of detecting uranium at the parts-per-million level and identifying its associations with co-occurring elements. An emerging methodology, fission-track radiography, was applied to detect low-level solid-phase uranium. Scanning electron microscopy and energy dispersive x-ray spectroscopy were used to elucidate uranium associations with co-occurring aluminum, iron, and phosphorous. Uranium-contaminated sediments were collected from the upgradient source zone and downgradient plume zone aquifer sediments at Riverton, Wyoming, USA. The combined microscopic analyses showed that the uranium primarily co-occurred with amorphous aluminum hydroxide and ferric hydroxide coatings in the source zone as opposed to proximal crystalline Fe-rich grains. In the plume zone, uranium primarily co-occurred with apatite as opposed to proximal iron sulfides. The unique geochemical associations of solid-phase uranium with co-occurring aluminum hydroxide, ferric hydroxide, and apatite, as opposed to other proximal minerals, suggested that a select suite of equilibrium and kinetic reactions controls its persistence in groundwater. The combined methodology applied in this study pinpointed the potential suite of uranium reactions that can be used to inform geochemical models for further mechanistic insight and forward simulations of the fate and transport of uranium at contaminated sites.