Membrane Behavior in Clay Barriers for Geoenvironmental Containment:
Background, Significance and Relevance

By: Charles D. Shackelford, Professor, Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, CO 80523-1372, USA; P: 970-491-5051: E: shackel@engr.colostate.edu; F: 970-491-7727.

ABSTRACT:

The containment function of clay barriers (e.g., compacted clay liners (CCLs), vertical cutoff walls, etc.) used for geoenvironmental applications (e.g., landfills, waste impoundments, contaminant plumes, etc.) can be enhanced if the barriers are able to exhibit membrane behavior, or the ability of the clay to restrict the migration of solutes (contaminants). In this regard, the potential existence of membrane behavior in such clay barriers has been evaluated through extensive research conducted at Colorado State University over the past decade. The results indicate that membrane behavior may be significant in clay barriers containing a significant percentage of high swelling smectite (e.g., montmorillonite), such as bentonite. Such clay barriers include traditional geosynthetic clay liners (GCLs), soil-bentonite (SB) vertical cutoff walls, and compacted soil-bentonite liners. Accordingly, this presentation will summarize the results of this research focused on evaluating the potential existence and magnitude of membrane behavior in several different types of clay barriers, including GCLs, SB backfills for vertical cutoff walls, and a natural clay suitable for use as a CCL on the basis of low hydraulic conductivity.

The presentation will include a brief background on the mechanisms associated with membrane behavior in clays, including solute restriction, chemico-osmosis, and hyperfiltration, and the methods for measuring membrane behavior in clay barriers. This background information will be followed by the results of experimental research illustrating the significance of membrane behavior in clay barriers, including the factors affecting membrane behavior. The presentation then will focus on the relevance of membrane behavior in clay barriers for geoenvironmental containment applications, including the relevance of clay membrane behavior with respect to contaminant levels expected in field applications, and the need to account for membrane behavior in terms of seepage and solute diffusion through clay barriers. Finally, ongoing research currently focused on evaluating the potential relevance of membrane behavior in unsaturated clay barriers and the potential use of polymer modified bentonites known as bentonite polymer nanocomposites (BPNs) as more robust substitutes for traditional bentonites used in clay barriers will be described.