Abstract: Remediation and risk control of groundwater contaminated by emerging pollutants such as perfluoroalkyl and polyfluoroalkyl substances (PFAS) have received attention from global researchers. However, very limited studies have addressed whether exposure to PFAS can alter hydraulic conductivity (k) of bentonite and why this change is occurred. Accordingly, this research aims to fill the gap. A series of macroscopic tests are conducted to evaluate effects of exposure to PFAS on soil pH, free swell index, liquid limit, and k of sodium-activated calcium bentonite. The k of the bentonite filter cakes are evaluated via modified fluid loss test. Three types of PFAS are selected including perfluorobutane sulfonic acid (PFBS with short chain), perfluorohexanoic acid (PFHxA with short chain), and perfluorooctanoic acid (PFOA with long chain). Deionized water is tested as the control. In addition, a set of multi-scale, in terms of nanoscopic, microscopic, and mesoscopic scale tests are carried out to reveal the underneath mechanisms of interactions between PFAS and bentonite. These tests include dielectric constant measurement, Zeta potential measurement, ultraviolet absorption spectroscopy test, X-ray diffraction test, Fourier transform infrared spectroscopy test, and scanning electron microscopy tests. The results show that pH values of the PFBS, PFHxA and PFOA solutions, and deionized water are 3.6, 3.6, 3.8, and 6.8, respectively. Free swell index values of bentonite tested in the PFBS, PFHxA and PFOA solutions decrease by 3.1%, 1.6% and 5.5% respectively, as compared to that tested in the deionized water. Exposure to PFBS, PFHxA, and PFOA yields a decrease in liquid limit by 2.9%, 2.3% and 3.0%, respectively, as compared to exposure to deionized water. Under the same void ratio, k of the bentonite filter cake to PFBS, PFHxA, and PFOA increases by 1.6~3.1 times, 1.5~3.0 times and 1.2~1.8 times, respectively. As compared to k to deionized water, the reasons why k of bentonite filter cake to three PFAS solutions are higher than that to deionized water are attributed to the synergistic effects of three factors as follows: (1) pH and relative dielectric constant of PFBS, PFHxA and PFOA solutions are higher than those of deionized water; (2) Total amount of hydrogen bonds formed in bentonite is lower in three PFAS testing solutions; and (3) Zeta potential of bentonite particles is less negative in three PFAS solutions as compared to that in deionized water.