Fundamental considerations for column testing of engineered, clay-based barriers

Abstract

The purpose of column testing is to determine the transport and fate properties, typically the hydrodynamic dispersion coefficient (Dh) and retardation factor (Rd), of aqueous miscible chemical species (solutes) with respect to a given porous medium. This paper presents fundamental considerations required for column testing of engineered, lowpermeability, clay-based barriers, such as compacted clay liners, geosynthetic clay liners, and soil-bentonite vertical cutoff walls, used to contain chemicals in a variety of solid and liquid waste containment and remediation applications. The presentation is limited to column tests conducted with a constant solute source concentration, and includes descriptions of the general procedures for conducting column tests as well as the expected behavior of the solute (effluent) breakthrough curve (BTC) based on several considerations. Significant information can be gained simply from the shape of the solute BTC. For example, a sigmoidal symmetric BTC implies advective-dominated solute transport with linear, instantaneous, and reversible sorption, whereas a sigmoidal asymmetric BTC can result from diffusion-dominated transport, nonlinear sorption, and/or nonequilibrium (kinetic) sorption. Also, because clay-based barriers are susceptible to hydraulic incompatibility when permeated with chemical solutions, significant hydraulic incompatibility can lead to a significant change in the seepage velocity (vs) under the constant hydraulic head (gradient) condition, which invalidates the use of analytical transport models to determine Dh and Rd via fitting of the measured solute BTCs, as well as the use of dimensionless time or pore volumes of flow. For this reason, constant flow hydraulic control is recommended for conducting column tests with engineered, low-permeability, clay-based barriers, because significant changes in hydraulic conductivity do not significantly affect vs. Finally, the potential significance of diffusion on solute transport through clay-based barriers can complicate interpretation of the existence of an effective porosity and determination of the correct, mass-based definition of Rd from the BTCs emanating from traditional concentration-based column tests. However, these issues are readily discerned when cumulative mass column testing is performed. Example results from both traditional, concentration-based column testing and alternative, cumulative mass column testing of clay-based barriers are provided to illustrate application of the concepts presented.