Application of a geochemical transport model to predict heavy metal retention (Pb) by clay liners

PHREEQC, a geochemical transport model, is used to simulate diffusive transport of Pb through a 10-cm-thick clay liner. The results are compared to those of Roehl and Czurda [Applied Clay Science 12 (1998) 387] who studied Pb migration by diffusion in a carefully monitored laboratory experiment. The computer simulation accounts for effects due to adsorption by ion exchange, changes in CEC, variable ion selectivity, and porosity or compacted density. It facilitates evaluation of changes in the diffusion coefficient and solution input parameters. The effective Pb diffusion coefficient determined for the simulation is 3×10−10 m2 s−1 and for the 520-day experiment of Roehl and Czurda it is 2.3×10−10 m2 s−1. Differences in the retardation factors (23.6 and 503, respectively) indicate that the model does not account for all of the adsorption mechanisms suggested by the experimental investigation. Thus, less Pb is retained and the liner is predicted to fail more rapidly than the actual results indicate. Models have great flexibility, but need to be verified by field data before they can be applied to specific waste site conditions.