Lanthanide Field Tracers Demonstrate Enhanced Transport of Transuranic Radionuclides by Natural Organic Matter

Transuranic (TRU) radionuclides buried 25 years ago in shallow unlined disposal trenches in a fractured shale saprolite had been detected in groundwater from downgradient monitoring wells and in surface water seeps. Field observations had suggested the actinide radionuclides were mobilized by natural organic matter (NOM) and rapidly transported with little retardation. A 73-day natural gradient tracer experiment injected trivalent lanthanides (Nd and Eu) as analogues to determine the mechanisms and rates of actinide transport at the field scale. Adsorption isotherms for 241 Am and Eu with saprolite from the site confirmed a very high affinity for adsorption ( R > 50 000) in the absence of NOM. However, reactive and nonreactive tracers arrived at approximately the same time along a 10-m long deep flow path, and anion-exchange chromatog raphy and filtration suggested that the mobile lanthanides in groundwater were a NOM complex. Although flow through a shallow flow path was intermittent, reflecting transient recharge events, large storms resulted in coincident peaks of both reactive and nonreactive tracers, suggesting that they migrated at similar rates over distances of 78 m. We conclude that NOM facilitated the almost-unretarded transport of lanthanide tracers and, by analogy, that NOM is facilitating the mobilization and rapid migration of the TRU radionuclides.