Multifactorial Drivers for Vertical BTEX Transport in Saturated Porous Media

Abstract Benzene, Toluene, Ethylbenzene, and Xylenes (BTEX), as typical volatile organic pollutants, pose a serious threat to groundwater security through their migration in saturated porous media. This is particularly critical in coastal estuaries, where the vertical migration of BTEX is complexly regulated by various environmental factors; however, the coupled multi-factor driving mechanisms remain unclear. In this study, laboratory-based one-dimensional dynamic column experiments were conducted to systematically investigate the effects of pH, salinity, organic matter, and nutrients (nitrate) on the vertical migration of nine BTEX compounds in sediments from the Bailang River estuary of Laizhou Bay. The results showed that the overall migration of BTEX decreased with increasing soil depth, being most significant in the 0–4 cm layer. Neutral conditions (pH=7) were most conducive to BTEX migration, with the migration extent following the order: neutral > alkaline > acidic. Increased organic matter content significantly enhanced the adsorption and retention capacity of the media for BTEX, resulting in migration levels in the order: low > medium > high organic matter. Elevated salinity compressed the electrical double layer and promoted colloid aggregation, thereby inhibiting migration, with the pattern observed as: pure water > low salinity > high salinity. The addition of nitrate increased ionic strength, inducing a salting-out effect and potentially altering colloid stability, which consequently suppressed the downward migration of BTEX. This study reveals the migration patterns and mechanisms of BTEX in saturated media under the synergistic influence of multiple factors, providing a scientific basis for groundwater pollution risk assessment and source control in coastal areas.