Effects of humic substances on the Am-241 migration in a sandy aquifer: column experiments with Gorleben groundwater/sediment systems

Migration experiments were performed to study the influence of aquatic humi c substances on the transport behavior of Am-241(III). Four groundwaters wi th different humic substance concentrations (DOC: 1 to 80 mg/l) were sample d together with Pleistocene aeolian quartz sand from the Gorleben site. San d, groundwaters and humic substances were characterized by different analyt ical methods (e.g., ICP-MS, X-ray diffraction, X-ray fluorescence analysis, ultrafiltration). The sand was equilibrated with each groundwater under in ert gas atmosphere with 1% CO2 for a period of at least 3 months. As confir med by ultrafiltration, the size distribution of humic colloids remained un changed during equilibration. The hydraulic properties of sand columns were characterized with tritiated water as an inert tracer. Column and batch ex periments were carried out with each groundwater as a function of the react ion period and flow velocity. In addition, the influence of the equilibrati on period of Am with groundwater was investigated prior to the injection in to a column. The results revealed that increasing humic substance concentra tion reduced the Am sorption onto sand and enhanced the transport as colloi d-borne Am species. The migration of colloid-borne Am was slightly faster t han the groundwater flow velocity. Furthermore, the mi,oration behavior of Am was found to depend on kinetically controlled interaction of humic collo id-bound Am with the sand surface. The application of the laboratory data t o natural conditions was examined. The results were found applicable for th e assessment of humic colloid facilitated radionuclide migration in natural aquifers. (C) 1998 Published by Elsevier Science B.V. All rights reserved.