Comprehensive assessment and source apportionment of heavy metals in a typical estuarine-bay system: a case study of Guanghai Bay, South China Sea

Coastal areas have become industrialized, increasing the risk of heavy metal pollution. Industrial, agricultural, and domestic wastewater carrying heavy metal pollution enters Guanghai Bay via rivers or direct discharge, posing significant environmental dangers. These risks are detrimental to regional economic development and the bay’s ecosystem. Heavy metal concentrations in 29 seawater, 16 sediment, and 16 marine organism samples from Guanghai Bay were analyzed using atomic absorption spectroscopy and multiple pollution indices, such as the geo-accumulation index, potential ecological risk index, single factor pollution index, water quality index, single pollution index, bioaccumulation factor, and principal component analysis. The results showed that sediment and seawater heavy metals generally met Class I criteria, although high variation coefficients suggested notable external influences. Spatially, Cu, Pb, Zn, and Cd concentrations in sediments were highest in the northwestern estuary-influenced coastal waters, while Hg exhibited a distinct high-concentration zone between the Chuan River and Chuanshan Archipelago. In seawater, Cd and As hotspots were located near northwestern estuaries and Yutang Port, whereas Cu and Pb showed unique distribution patterns with significant spatial heterogeneity. Cd is the most damaging element in sediments, posing moderate to high ecological hazards in some areas. Mercury and lead showed moderate pollution in the seawater at certain locations, although the overall water quality remained clean. Marine organisms meet biological quality criteria but show strong bioaccumulation of certain metals. The principal component analysis identified terrestrial inputs, riverine transport, industrial/agricultural discharge, and traffic emissions as major causes of seawater pollution, whereas sediment metals were derived from both natural and anthropogenic sources. This study provides a scientific foundation for managing bay ecology in the face of rapid urbanization and economic growth. The novelty of this work lies in its integrated approach, combining multiple pollution indices and spatial analysis to identify pollutant sources and ecological risks in a typical estuarine-bay-archipelago system—a rarely studied ecosystem in the northern South China Sea. The study addresses the critical scientific problem of distinguishing between natural and anthropogenic contributions to heavy metal pollution, which is essential for targeted environmental management.