Bentonite (Bent) is a favorable buffermaterial forwaste containment barriers, although its properties may be affected
by the pore fluid chemistry. Hence, this study investigates the role of different organic pollutants including
methanol, acetone, acetic acid, and citric acid on the macro and microstructure responses of Na+-Bent. The
chemicals at various concentrations (0 to 80% by the volume of distilled water) were separately added to the
soil sampleswhichwere then subjected to a set of tests. The results indicate that an increase in the concentration
of organic permeants by decreasing the dielectric constant (ε) reduces the plasticity index, compressibility and
swelling potential of soil; whereas, the permeability increases. Such changes are ascribed to a tighter packing
of the clay particles, as clearly confirmed by the SEMmicrographs and XRD patterns. The decrease in the absolute
value of soil zeta potential also supports the development of coagulated structure in the artificially contaminated
samples upon reducing the ε of solution. However, the results reveal that the sensitivity of soil properties to the
variations in the ε value is pronounced at the intermediate stages of concentration (i.e. the organic fluid/water
ratio b 60%) when the ε is higher than 50. In addition, at relatively low concentration, the acidic permeants
dramatically alter the soil fabric due to having lower pH, causing remarkable effects on the hydro-mechanical parameters
as compared to the other chemicals. Overall, the organic pollutants, depending on their type and concentration,
exert negative impact on the barrier performance of Na+-Bent which are not precisely comparable
only with reference to the changes in the dielectric constant of pore fluid, and thereby may not be completely explained
by the diffuse double layer (DDL) theory.