High-resolution mapping decouples hydrodynamic sorting and sediment transport as primary drivers of PAH distribution in the East China Sea inner shelf

Based on high-resolution spatial analysis of 463 surface sediment samples from the East China Sea (ECS) inner shelf, this study reveals that hydrodynamic sorting and sediment transport dynamics, rather than source strength alone, are the primary drivers governing polycyclic aromatic hydrocarbons (PAHs) distribution and compositional patterns, challenging emission-centric paradigms. Sedimentary PAHs exhibit extreme spatial heterogeneity, ranging from 19.38 to 10,047 ng/g. Notably, semi-enclosed bays (e.g., Sanmen Bay) serve as significant pollution hotspots, contrasting sharply with lower levels found in adjacent open-shelf areas. We identified a novel "congener fractionation" phenomenon along the estuary-shelf continuum: low-molecular-weight PAHs (LMW; 2-3 rings) dominate proximal nearshore zones due to preferential deposition within coarse sediment fractions under strong tidal regimes. Conversely, high-molecular-weight PAHs (HMW; 4-6 rings) exhibit a stronger affinity for fine-grained particles and undergo offshore transport via prevailing currents (e.g., Zhejiang-Fujian Coastal Current-ZFCC), leading to significant enrichment within distal mud belts (HMW PAHs constituting up to 90 %). This fractionation stems from molecular-weight-dependent variations in adsorption capacity, transport potential, and susceptibility to degradation, creating compositional gradients that mimic weathering signatures and confound conventional source diagnostics. Source apportionment identified combustion sources (contributing 75-92 % of the total PAHs) as dominant across most study areas, except within Xiangshan Bay where petrogenic sources were predominant (80.8 %). Hydrodynamic constraints and topographic bottlenecks (e.g. semi-enclosed bays) significantly amplify legacy accumulation by sequestering contaminated fine sediments. Seasonal currents (e.g., monsoon-driven flows) effectively facilitate the along-shelf transport of particle-bound HMW PAHs. Ecological risk assessment, based on calculated benzo[a]pyrene toxic equivalent quotients (TEQ: 0.84-2063.60 ng/g), correlates with sediment texture, highlighting fine-grained depocenters (especially semi-enclosed bays) as primary high-risk sink zones. Consequently, effective pollution mitigation strategies should integrate hydrodynamic controls and sediment management alongside targeted source reduction measures.