Capillary fringe microbial hotspots driven by spatial encounters reshape multi-field coupling and control petroleum...

Biodegradation in the unsaturated zone critically retards the downward migration of petroleum contaminants, yet the coupled bio-hydro-geochemical mechanisms that govern this process remain poorly defined. In particular, it is unclear whether the capillary fringe functions as a critical biogeochemical barrier or merely a passive conduit for contaminant transport. Here, we demonstrate that the capillary fringe serves as a microbial hotspot that effectively impedes contaminant transport through coupled biological, hydrological, and geochemical feedbacks. In sand column experiments, biofilm accumulation in the capillary fringe significantly exceeded that in the overlying vadose zone and underlying saturated zone, leading to pore clogging and enhanced diesel retention. Acting as a stress-inducing carbon source, retained diesel stimulated microbial growth and subsequent EPS secretion, which further reduced pore connectivity and promoted preferential flow pathways. This positive feedback drove pronounced microbial enrichment in the capillary fringe, with biomass increasing from 7.58 mg/g to a peak of 13.36 mg/g. The peak biomass was 3.55 and 2.34 times greater than that observed in the overlying vadose zone and underlying saturated zone, respectively. As diesel migration gradually stabilized, the dominant microbial control mechanism transitioned from indirect contaminant retention via pore structure modification in the early phase to direct biodegradation in the late phase. Concurrently, continuous microbial metabolism reshaped the local chemical environment, evidenced by declining pH and oxidation-reduction potential and rising electrical conductivity, while elevated electron transport system activity (ETSA) and ATP levels indicated efficient microbial adaptation to these dynamic conditions. Ultimately, the capillary fringe margin achieved a diesel removal efficiency of 71 % (primarily driven by biological processes), highlighting its central role as a bioactive zone and providing a mechanistic basis for enhanced bioremediation strategies targeting this critical region.