Multi-factor driving mechanism of vertical migration of BTEX in saturated porous media

As typical volatile organic pollutants, the migration behavior of BTEX in saturated porous media poses a serious threat to the safety of groundwater environments. This threat is particularly pronounced in coastal estuarine zones, where significant variations in environmental factors may strongly influence the vertical migration of BTEX; however, the relevant mechanisms and governing laws remain poorly understood. In this study, laboratory 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 collected from the Bailang River estuary in Laizhou Bay. The results show that BTEX migration generally decreases with increasing soil depth, with the most pronounced retention occurring in the 0–4 cm layer. Neutral conditions (pH = 7) were most favorable for BTEX migration, with the overall order being: neutral > alkaline > acidic. Increasing organic matter content significantly enhanced the adsorption and retention capacity of the porous medium for BTEX, and the migration amounts followed the order: low organic matter > medium organic matter > high organic matter. Elevated salinity promoted colloidal aggregation by compressing the electrical double layer, thereby inhibiting migration, with the order: pure water > low salinity > high salinity. The addition of nitrate triggered a salting-out effect by increasing ionic strength and potentially altering colloidal stability, thus suppressing the downward migration of BTEX. This study reveals the effects and underlying mechanisms of key environmental factors on the vertical migration of BTEX in saturated porous media, providing experimental evidence for the risk identification and control of groundwater contamination in coastal zones.