241Am migration in a sandy aquifer studied by long-term column experiments

The migration behavior of 241Am(III) in a sandy aquifer was studied under near-natural conditions by long-term column experiments of more than 1 year duration. Columns with 50 cm length and 5 cm in diameter were packed with aeolian quartz sand and equilibrated with two different groundwaters having an original dissolved organic carbon concentration (DOC) of 1.1 and 7.2 mg.dm -3 , respectively, from the Gorleben site (Lower Saxony, Germany). In each experiment, 1 cm 3 of Am-spiked groundwater ([Am] = 0.2 to 2μmol.dm -3 ) was injected into the column. The flow rate of the groundwater was adjusted to 0.28 m.d -1 . A small colloid-borne Am fraction was found to elute together with tritiated water. After 414 and 559 days, respectively, the experiments were terminated. Whereas the nonsorbing tracer of tritiated water would have covered a distance of about 350 m in that time period, the maximum of the Am activity was detected between 32 and 40 mm column length. Applying selective dissolution analysis to the sand surface, Am was found to be preferentially bound to iron hydroxide/oxide sites. From this Am distribution, a retardation factor R of about 10 4 was determined and compared to static batch experiments. The Am breakthrough was calculated for the conditions of the column experiment.