The Role of Coupled Flow Phenomena in the Performance Assessment of Bentonite-Based Contaminant Barriers

Bentonite-based barriers are able to act as semipermeable membranes that partially inhibit the passage of charged solutes (i.e., ions). This behavior also induces chemico-osmosis, whereby water migrates from a location of lower solute concentration to a location of higher solute concentration. This paper explores the potential significance of chemico-osmosis on solute transport through bentonite-based barriers using a phenomenological transport approach combined with a physical model relating the macroscale transport properties, such as the reflection coefficient, w, and the hydraulic conductivity, k, to the microscale physicochemical and fabric properties of the bentonite. The practical relevance of chemico-osmosis is illustrated through a calculation example involving a geosynthetic clay liner used as a single barrier or coupled to a geomembrane to form a composite barrier. The results indicate that the influence of chemico-osmosis is dependent upon the leachate concentration and the bentonite void ratio. Based on the available experimental data, for solute concentrations lower than 100 mM, the counterflow due to chemical osmosis is prevalent over the flow generated by the hydraulic gradient and the solute mass flow is reduced by a factor in the range from 1 to 10.