MOBILIZATION OF TRANSURANIC RADIONUCLIDES FROM DISPOSAL TRENCHES BY NATURAL ORGANIC-MATTER

Transuranic (TRU) radionuclides in groundwater at the Oak Ridge Nation al Laboratory migrate rapidly and with little retardation of the radio nuclides over distances of 80 m. Several interacting hydrogeochemical processes contribute to the observed releases of actinides (Cm-244 and Am-241) from the shallow unlined disposal trenches, through the highl y weathered, fractured shale (saprolite) and to the surface-water seep s at White Oak Creek. Major releases are promoted when seasonal fluctu ations in the water table permit groundwater to contact actinide-conta minated waste. Local recharge of stormwater into the trenches appears to permit minor releases, perhaps due to transient saturation within t he trenches but above the local water table. Although the hydrogeology of the site permits contact of the TRU waste with the groundwater, th e expected inorganic species of the actinides should strongly adsorb t o the layer silicates and mineral oxides of the shale saprolite, Yet t he timing of the actinide releases relative to when rising groundwater intercepts the trenches suggests that actinide transport is rapid, an d the relative magnitude of peak actinide levels in wells near the tre nches and at downgradient seeps suggests that then is very limited ret ention of the actinides by the formation. Based on anion exchange chro matography of the groundwater and geochemical modeling, the mobilizati on and transport of the actinides is demonstrated to result from compl exation of the actinides by natural organic matter (NOM). Storm events contribute to mobilization by promoting hydrologic links between the TRU waste and groundwater, and by increasing the concentration of NOM in the mobile soil and groundwater. This study demonstrates that even in formations characterized by abundant mineral phases known to strong ly adsorb actinides, the actinides can be transported essentially cons ervatively as NOM complexes. (C) 1998 Elsevier Science B.V.