TCE and PCE plume persistence based on different clay types

This study was performed to understand dense non-aqueous phase liquid (DNAPL) persistence by the back diffusion from the three types of clay using one-dimensional analytical solutions. The conceptual model was designed with 5 m thickness of an aquifer underlain by 0.7 m thickness of an aquitard. The aquitard was considered to be a finite domain boundary at the bottom of bentonite (B), kaolinite (K), and montmorillonite (M) layers. The tortuosity of each clay was assumed to be 0.95 (B), 0.55 (K), and 0.05 (M). A diffusion model scenario assumed a step change in concentration boundary condition representing complete removal of trichloroethylene (TCE) and tetrachloroethylene (PCE) at the source zone, after 10 years of source loading. Total accumulated mass in the aquitard during the forward diffusion showed that PCE was 57.3 (B), 44.3 (K), 13.3 (M) g/m², and TCE was 329.2 (B), 256.2 (K), 76.8 (M) g/m². The calculated tailing concentration of PCE at the aquifer during the back diffusion maintained higher concentrations than the maximum contaminant level (MCL, PCE = 5 μg/L) for 22 (B), 16 (K), and 11 (M) years, respectively, in the same order as the tortuosity of clays. The calculated tailing concentration of TCE above MCL (TCE = 5 μg/L) was maintained for 38 (B), 43 (K), and 19 (M) years. These results showed that the total accumulated mass of TCE was higher in the bentonite layer than the kaolinite layer, but the TCE tailing concentrations caused by back diffusion from the kaolinite layer maintained 5 years longer above MCL than those caused by back diffusion from the bentonite layer due to different values of tortuosity for bentonite and kaolinite. The results of this study indicate that the tortuosity of clays has a significant influence on plume persistence caused by back diffusion.