Mineralogical and solid phase physical and geochemical controls on U mobility in the sediments from Rifle, CO

An understanding of the subsurface mineralogical, physical and solid phase geochemical controls on contaminant U behavior and fate is of primordial importance in designing remediation strategies. Systematic studies using a variety of techniques, such as XRD, μ-XRD, Mössbauer spectroscopy, SEM/EDS, SEM/FIB, TEM/SAED, EMPA, XRF, EXAFS, and XANES were conducted with a variety of subsurface sediments from the Integrated Field Research Challenge site at Rifle, CO, to study U interaction with different subsurface minerals. The cores represented a cross section of sediment conditions that ranged from typical aquifer sediment (minimally reduced) to highly naturally bioreduced sediment from the saturated zone. Some of the subsurface minerals are unique, e.g., the framboidal pyrites of the naturally reduced zone (NRZ) of the Winchester gallery, or the sulfide minerals of different elements and co-contaminants (e.g., ZnS framboids) that were present in the NRZ of the La Quinta experimental plot. All these and other more common minerals, such as Fe oxides and phyllosilicate minerals have a pronounced effect on U fate and behavior in the Rifle subsurface. The solid phase physical and geochemical properties revealed a number of interesting trends. All particle size frequency distributions of the sediments from well CD-08 of the Plot C experimental plot were polymodal, implying complex pore size distribution and water retention functions. Surface area analysis suggests the presence of significant surface area in Rifle IFRC sediment, even on fine-gravel sediment. The wet chemical extractions and microwave digestion analyses showed that substantial amounts of co-contaminants, such as V, Zn, As, Se and Cu (some of them are redox sensitive elements which may compete for the available electrons), were present in the subsurface sediments. NRZs contain elevated concentrations of natural organic matter, Fe(II) and reduced inorganic sulfides. All of these constituents can scavenge oxidants introduced following seasonal incursion of (sub)oxic groundwater or the sustained release or accumulation of endogenous oxidants (e.g., MnO2), promoting retention of U(IV).