Model of colloid-influenced contaminant transport applied to Eu-humate migration experiments

The transport code TRAPIC (transport of pollutants influenced by colloids) has been applied to simulate the Eu migration in column experiments with humic-rich groundwater. In the model, Eu is considered as dissolved, sorbed to the surface of the aquifer material, and sorbed to mobile and immobile colloids. The mass exchange between the corresponding phases is modelled by kinetically controlled sorption and filtration processes. Eu-humate column experiments with a similar groundwater sediment system, performed with different darcy-velocities using a column length of 50 cm, have been simulated within a previous study. The total pathway length of the migration experiment, considered in this paper, is 10 m. The migration experiments have the following characteristics in common: One part of Eu migrates without retention through the column, although a significant retardation had to be expected from the results of batch experiments, and the recovery is reduced for increased transport time and increased transport distances respectively. Also a significant tailing of the breakthrough curves has been observed. Good agreement is obtained by simulating the migration experiments, using sorption parameters according to the simulation parameters used in the previous study. For both of the experiments the following sorption and filtration processes are identified: Desorption of Eu from the colloids followed by sorption to the matrix, and filtration of the colloidal contaminants by the matrix. The rates of desorption, obtained by the simulations, increase exponentially with increased transport times and vary in a consistent way for the two considered experiments. (orig.)