Geochemical and mineralogical investigation of uranium in multi-element contaminated, organic-rich subsurface sediment

• Published in: Applied geochemistry Vol. 42; pp. 77 - 85
• Main Authors: Qafoku, Nikolla P., Gartman, Brandy N., Kukkadapu, Ravi K., Arey, Bruce W., Williams, Kenneth H., Mouser, Paula J., Heald, Steve M., Bargar, John R., Janot, Noémie, Yabusaki, Steve, Long, Philip E.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01-03-2014; Elsevier
• Subjects: Earth Sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Geochemistry; Mineralogy; Pollution, environment geology; Sciences of the Universe; siderite; biomass; time variations; oxides; ground water; oxidation; concentration; subsurface; carbonates; sorption; clay; contamination; natural attenuation; sedimentary rocks; aquifers; solid phase; uranium; reduction; organic carbon; stability; clastic rocks; sulfides
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2013.12.001

•Subsurface naturally reduced zones (NRZ) contain U and other potential co-contaminants.•The NRZ has a remarkable assortment of chemically complex, potential U hosts.•Micron-scale, multi-contaminant areas were discovered in NRZ.•U(IV) occurs as biogenic UO2 (82%), or biomass – bound monomeric U(IV) (18%).•NRZs may exhibit contaminant sink-source complex behavior.
Subsurface regions of alluvial sediments characterized by an abundance of refractory or lignitic organic carbon compounds and reduced Fe and S bearing minerals, which are referred to as naturally reduced zones (NRZ), are present at the Integrated Field Research Challenge site in Rifle, CO (a former U mill site), and other contaminated subsurface sites. A study was conducted to demonstrate that the NRZ contains a variety of contaminants and unique minerals and potential contaminant hosts, investigate micron-scale spatial association of U with other co-contaminants, and determine solid phase-bounded U valence state and phase identity. The NRZ sediment had significant solid phase concentrations of U and other co-contaminants suggesting competing sorption reactions and complex temporal variations in dissolved contaminant concentrations in response to transient redox conditions, compared to single contaminant systems. The NRZ sediment had a remarkable assortment of potential contaminant hosts, such as Fe oxides, siderite, Fe(II) bearing clays, rare solids such as ZnS framboids and CuSe, and, potentially, chemically complex sulfides. Micron-scale inspections of the solid phase showed that U was spatially associated with other co-contaminants. High concentration, multi-contaminant, micron size (ca. 5–30μm) areas of mainly U(IV) (53–100%) which occurred as biogenic UO2 (82%), or biomass – bound monomeric U(IV) (18%), were discovered within the sediment matrix confirming that biotically induced reduction and subsequent sequestration of contaminant U(VI) via natural attenuation occurred in this NRZ. A combination of assorted solid phase species and an abundance of redox-sensitive constituents may slow U(IV) oxidation rates, effectively enhancing the stability of U(IV) sequestered via natural attenuation, impeding rapid U flushing, and turning NRZs into sinks and long-term, slow-release sources of U contamination to groundwater.