Lanthanide field tracers demonstrate enhanced transport of transuranic radionuclides by natural organic matter

Transuranic (TRU) radionuclides buried 25 years ago in shallow un lined dis posal trench es in a fractured shale saprolite had been detected in groundw ater from downgradient monitoring wells and in surface water seeps. Field o bservations had suggested the actinide radionuclides were mobilized by natu ral organic matter(NOM) and rapidly transported with little retardation. A 73-day natural gradient tracer experiment injected trivalent lanthanides (N d and Eu) as analogues to determine the mechanisms and rates of actinide tr ansport at the field scale. Adsorption isotherms for Am-241 and Eu with sap rolite from the site confirmed a very high affinity for adsorption (R > 50 000) in the absence of NOM. However, reactive and nonreactive tracers arriv ed at approximately the same time along a 10-m long deep flow path, and ani on-exchange chromatography and filtration suggested that the mobile lanthan ides in groundwater were a NOM complex. Although flow through a shallow flo w path was intermittent, reflecting transient recharge events, large storms resulted in coincident peaks of both reactive and nonreactive tracers, sug gesting that they migrated at similar rates over distances of 78 m. We conc lude that NOM facilitated the almost-unretarded transport of lanthanide tra cers and, by analogy, that NOM is facilitating the mobilization and rapid m igration of the TRU radionuclides.