URANIUM TRANSPORT IN WEATHERED BEDROCK: APPLICATION OF ENVIRONMENTAL PETROLOGY

Processes that control the transport of U have been studied intensively over the past fifty years. However, in order to ensure a sound scientific basis for the design and implementation of remediation strategies associated with U contaminated sites there remains a need for new data from representative natural geologic systems. Of particular concern is how these U transport mechanisms vary over geologic time scales within inhomogeneous rock-soil-groundwater systems. One of the most unique places in the world to study such phenomena is the Coles Hill U deposit located in Virginia. Due to its geologic, geochemical and mineralogic nature as well as its physiographic setting and location, this site is unparalleled for studying U dispersal in a dynamic natural system typical of the eastern US. The Coles Hill deposit is hosted within fractures that cross-cut quartzo-feldspathic augen gneisses along the north-west margin of the Danville Triassic Basin. It is estimated that approximately 5 million kilograms of elemental U has been made available for near surface transport through the formation of the weathered zone over the deposit. The primary ore assemblage (bulk U~1,000 ppm) consists of U4+ minerals coffinite and pitchblende associated with fluor-apatite. The oxidized, weathered rock and groundwater-saturated saprolite overlying the ore zones contain 800–1300 ppm U associated with abundant U6+ phosphates of the autunite group. Above this zone no discrete U6+ minerals have been detected; however, the bulk U concentrations remain greater than 200 ppm. The U in this unsaturated zone is associated with phosphate adsorbed onto iron hydroxide mineral coatings. These data suggest that the key interval of U mobilization is within the oxidized soil column at the dynamic interface between the saturated and unsaturated zones. Furthermore, these observations suggests that the Coles Hill vadose zone has reached a steady state with respect to U adsorption and thus records the finite limit of U that can be fixed within this system given ambient physical and chemical conditions.