Effect of organic carbon enrichment on halogenated organic pollutants in wetland sediments of the Yellow River Estuary and Jiaozhou Bay, China.

Coastal wetlands, vital ecosystems, are threatened by anthropogenic activities and act as significant sinks for halogenated organic pollutants (HOPs). Despite the persistent toxicity and ecological risks associated with HOPs, their spatial distribution and environmental behavior in coastal sediments, particularly the role of total organic carbon (TOC) in modulating their fate, remain poorly understood. We investigated the contamination characteristics of 25 halogenated polycyclic aromatic hydrocarbons (HPAHs) in the surface sediments of two coastal wetlands, the Yellow River estuary and Jiaozhou Bay, with different environmental backgrounds. HPAH concentrations were higher in Jiaozhou Bay (23.81-121.78 ng g, mean 50.58 ± 28.85 ng g) than in the Yellow River estuary (14.69-30.12 ng g, mean 23.48 ± 5.42 ng g), influenced by hydrodynamic conditions and anthropogenic activities. While TOC showed a weak correlation with HPAHs under the low TOC levels and dynamic sedimentary conditions of the Yellow River estuary, it showed significant positive correlations with most HPAHs in Jiaozhou Bay, where limited water exchange enhanced pollutant accumulation. Risk assessment based on the risk quotient (RQ) indicated that the overall RQ was lower in Jiaozhou Bay, despite the higher mean HPAH concentration (2.2 times higher than in the Yellow River estuary), which may be due to the higher TOC content that enhances adsorption and reduces bioaccessibility. Fish and Daphnia in both wetlands were almost always at minimal risk levels, although localized elevated risks may occur in areas with higher pollution loads. These findings highlight the importance of integrating TOC and hydrodynamic factors in coastal pollution management.