Speaker: Philip E. Long, Pacific Northwest National Laboratory
Philip Long is a Staff Scientist at Pacific Northwest National Laboratory and the Principal Investigator for the Rifle Integrated Research Challenge Project at Rifle, Colorado.
Program Description
The Integrated Field Research Challenge site (IFRC) at Rifle, Colorado, is a shallow alluvial aquifer adjacent to the Colorado River that is contaminated with uranium from a former milling operation. The Rifle Integrated Research Challenge Project seeks to identify new approaches and strategies to help resolve questions about the movement of subsurface and contaminants.
The Rifle Field Study involves examining the stimulation of subsurface microorganisms aimed at reducing and immobilizing uranium in the subsurface. The site is ideal for electron donor amendment experiments at the field scale in which soluble uranium(VI) can be nearly completely reduced to insoluble uranium(IV) by Geobacter sp. stimulated by the electron donor (dilute acetate). Reduction and removal of uranium from groundwater is achieved even in a flowing aquifer system that is continuously challenged by up-gradient uranium. Reoxidation of biogenic uranium(IV) remains an issue for application of bioremediation of uranium-contaminated aquifers. However, recent data from field experiments at the Rifle IFRC indicate that reoxidation of biogenic uraninite is much slower than expected, suggesting that kinetic controls on reoxidation of uranium(IV) may be important at sites with low nitrate and dissolved oxygen. In spite of these complexities, a 3-D, variably-saturated flow and biogeochemical reactive transport model that includes physically and chemically heterogeneous media, successfully describes observed system behaviors during biostimulation. The underlying biogeochemical mechanisms appear to be captured, suggesting that outcomes of bioremediation and natural attenuation of uranium should be predictable.
In addition to biostimulation of aquifers as a remediation technology, the U.S. Department of Energy is also interested significant improvement in our ability predict natural attenuation of uranium plumes in groundwater. Monitoring of the Rifle site over the last decade shows differing patterns of uranium(VI) concentration and has revealed the presence of naturally occurring uranium(IV). In some wells, uranium concentration varies on approximately an annual cycle (~ 0.4 to 0.8 uM), whereas more recent detailed monitoring in other wells shows stable uranium concentration, but significant differences in V, As, and Se associated with rise or fall of the water table during spring runoff. An increase in dissolved oxygen near the water table and its entrainment into the aquifer and subsequent consumption during falling water levels indicate the importance of biogeochemical cycling associated with the annual runoff cycle in the Colorado River.
The natural occurrence of uranium(IV) is likely common in alluvial aquifers containing significant amounts of detrital plant matter, giving rise to dissolved organic carbon in groundwater that sustains microbial communities capable of reducing uranium(VI) to uranium(IV). However, the presence of uranium(IV) complicates prediction of natural attenuation of uranium which is challenging enough in sites like the 300 Area at Hanford where uranium(VI) is the dominant if not the only redox status of uranium. Light source spectroscopy is crucial to assessing uranium redox status and bonding at contaminates with uranium(IV). Moreover, planned new detectors with ~ an order of magnitude higher sensitivity are crucial, since very low concentrations of uranium on solids still result in groundwater concentrations that exceed applicable standards, and we need to understand uranium biogeochemistry at those concentrations in order to achieve a mechanistic prediction of long-term behavior of uranium groundwater plumes.
