A comparative analysis of contaminant migration models using barrier material data

Waste containment facilities are often composed of barriers such as liners, grout curtains, and slurry walls. The primary design objective for such sy stems is to mitigate against the release and transport of contaminants. It is often necessary to quantify barrier effectiveness in order to conduct ri sk and exposure assessments. The extent to which a barrier material is effe ctive can be assessed using analytical methods, laboratory testing and fiel d monitoring. Obviously, there is a great deal of time and expense associat ed with both laboratory and field monitoring, making modeling an attractive first alternative. There are, however, numerous solutions to the well-know n advection-dispersion equation that vary in accuracy and applicability, de pending on initial and boundary conditions. Moreover, most of the equations formulated for transport through porous media were developed for use in aq uifer rather than barrier material. Prudent model selection involves matchi ng the conditions to be analyzed with the appropriate mathematical descript ion. In this article, five transport equations are analyzed and compared with la boratory results and projected field conditions for the migration of Pb2+ t hrough soil-bentonite. After 30 days of continuous source injection, measur able concentrations of lead were only detected in the first 0.5 cm of a col umn of soil-bentonite. All five solutions predicted approximately the same level of penetration for the column tests; however, significant differences emerged after extrapolation to field conditions. For barrier design purpos es, the only equations recommended are Equation 3 (the complete solution fr om Ogata and Banks [1961]) and Equation 6 (Crank's [1956] solution to Fick' s Second Law).