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 systems is to mitigate against the release and transport of contaminants. It is often necessary to quantify barrier effectiveness in order to conduct risk and exposure assessments. The extent to which a barrier material is effective can be assessed using analytical methods, laboratory testing, and field monitoring. Obviously, there is a great deal of time and expense associated with both laboratory and field monitoring, making modeling an attractive first alternative. There are, however, numerous solutions to the well-known advection-dispersion equation that vary in accuracy and applicability, depending on initial and boundary conditions. Moreover, most of the equations formulated for transport through porous media were developed for use in aquifer rather than barrier material. Prudent model selection involves matching the conditions to be analyzed with the appropriate mathematical description. In this article, five transport equations are analyzed and compared with laboratory results and projected field conditions for the migration of Pb2+ through soil-bentonite. After 30 days of continuous source injection, measurable concentrations of lead were only detected in the first 0.5 cm of a column 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 purposes, the only equations recommended are Equation 3 (the complete solution from Ogata and Banks [1961]) and Equation 6 (Crank's [1956] solution to Fick's Second Law).