U.S. DEPARTMENT OF ENERGY (DOE) OFFICE OF LEGACY MANAGEMENT (LM): LONG-TERM SURVEILLANCE AND MAINTENANCE AT FORMER DOE DEFENSE SITES (Invited Presentation)

The U.S. Department of Energy (DOE) Office of Legacy Management (LM) conducts long-term surveillance and maintenance at more than 100 former DOE defense sites that have been remediated, but where residual contamination remains and groundwater cleanup is ongoing. Uranium (U) contamination, often in shallow alluvial aquifers at, and as a result of, operations at former U ore processing facilities, persists.

Originally, LM thought that advective transport could flush U out of these aquifers and dilute concentrations or use pump and treat technologies to reduce U concentrations and meet regulatory standards within 100 years. However, collaborative, interdisciplinary research by national laboratories, universities, tribal nations, and other state and federal agencies (and frequently involving students) at LM sites is elucidating what had been poorly understood biochemical mechanisms controlling U fate and transport in the environment. Those sites included Rifle, Colorado; Riverton, Wyoming; and Shiprock, New Mexico, among others.

Complexities include U in the unsaturated zone that can be temporarily resolubilized with changes in the water table, relatively immobilized U in fine-grained layers in otherwise saturated sediments. Where such “persistent secondary sources” of U occur, the original mass of U was clearly underestimated.

Other challenges are where U from naturally occurring radioactive material (NORM) in regions where U in groundwater is a naturally occurring constituent. Although uncertainties remain, results of this research, including some presented by others in this session, is helping DOE better evaluate and manage human health risks, assess whether plumes can migrate to other water sources, evaluate ways of removing or stabilizing U in place, as well as address concerns of stakeholders about U at these sites.