Source tracing of heavy metals in estuaries: How hydro-morphological evolution governs contaminant flux and fate under human pressure

The sink-source dynamics of hazardous heavy metals in anthropogenically perturbed estuarine sediments remain elusive, given the complex interplay between anthropogenic inputs and hydro-morphological controls. This study investigates the spatial heterogeneity, environmental fate, and source quantification of seven heavy metals in the sediments of the Pearl River Estuary (PRE; China). The concentrations of heavy metals exhibited a decreasing trend from the northeast to the southwest, with the east shoal being identified as a key hotspot. This distinct pattern can be ascribed to the combined effects of fine-particle accumulation and the proximity to coastal industrial emission sources. Notably, a "source-sink decoupling" phenomenon was further revealed in the west trough. Despite the favorable sedimentary conditions, the low level of heavy metal enrichment is due to the spatial disconnection from major anthropogenic inputs. This finding challenges the traditional "grain-size control" paradigm, underscoring the crucial role of the spatial coupling between hydrodynamics, sedimentary environments, and pollution sources. Furthermore, the roles of organic matter and grain-size parameters are quantified using structural equation modeling, revealing that organic matter acts as the dominant biogeochemical carrier for metal accumulation. The outcomes of positive matrix factorization analysis suggest that anthropogenic inputs contribute over 89% of the total heavy metal load, with industrial and agricultural sources being the dominant contributors. Geo-accumulation and ecological risks have clearly identified Cu, Cd, and Hg as the principal elements driving environmental risks and ecological hazards. Therefore, this study provides a critical scientific basis for formulating targeted strategies to alleviate heavy metal hazards and associated ecological risks in dynamic estuarine systems.