• • PS exhibits pronounced vertical stratification in porous aquifers: under slow-release conditions, concentration at column bottom can be two orders of magnitude higher than at top, whereas Br− shows minimal stratification (decay rate 0.009 d−1), indicating density-driven migration of PS that may bypass shallow LNAPL zones.
• • PS vertical migration is primarily density-driven with diffusion as secondary, while Br− migration is diffusion-dominated (effective diffusion coefficient 2.2×10−9 m2·s−1); this mechanistic difference explains why PS sinks while LNAPLs remain shallow, reducing contact efficiency.
• • Slow-release PS formulations increase persistence: average PS decay rate under slow-release (0.072 d−1) is slightly higher than non-slow-release (0.059 d−1), but slow-release maintains higher concentrations over time, potentially enhancing oxidant availability but also exacerbating stratification.
• • BTX (LNAPLs) remain concentrated in shallow aquifer layers, similar to Br−, and their contact with PS is limited when aquifer thickness is large; this stratification effect directly increases remediation cost and difficulty, as oxidant must be delivered to deeper zones to react with contaminants.