Diesel breakthrough resistance of soil-bentonite backfills via capillary barrier effect

Diesel contamination at petrochemical sites poses a critical threat to groundwater security and public health. Soil-bentonite backfills are widely used as vertical barriers in contaminated sites, yet their long-term containment performance against diesel is strongly influenced by the pore structure and the resultant capillary resistance. This study evaluated the diesel retention capacity of sand-bentonite (S_dB) versus silt-bentonite (S_tB) through long-term flexible-wall permeation tests under incremental permeation pressures (10, 20, and 30 kPa). Diesel breakthrough behavior and capillary entry pressure threshold were analyzed. The underlying mechanisms were further elucidated. The results revealed three key findings: (1) S_tB exhibited excellent retardation capacity under all tested pressures (10-30 kPa), maintaining extremely low daily flow rates (< 0.5 mL/d). In contrast, S_dB experienced rapid diesel breakthrough at 20 and 30 kPa, with daily flow rates exceeding 25 mL/d within a short period; (2) The capillary entry pressure threshold of S_dB was less than 10 kPa, while that of S_tB ranged from 20 to 30 kPa, indicating a significantly higher capillary resistance for S_tB; (3) S_tB had much finer pores concentrated at 0.05-0.39 μm (0.28-2.13 μm for S_dB), corresponding to the denser pore structure. Consistent with the Young-Laplace equation, smaller pore throats produced higher capillary entry pressures. Consequently, S_tB had higher capillary entry pressure and superior long-term diesel containment. These findings underscored the promising application of fines-rich soil‑bentonite backfills against organic non-aqueous phase liquids at petrochemical sites, and provide practical guidance for the design of geoenvironmental containment systems.