Distribution characteristics and pollution assessment of heavy metals in mangrove sediments in Shantou

To systematically investigate the contamination status of heavy metals within mangrove sediments in Shantou, this study set up sampling sites at typical mangrove stands across five research zones, namely Yifengxi, Lianyang River and Waisha River belonging to the Hanjiang River Basin, as well as Sandoumen and Niutianyang located in the Rongjiang River Basin. The concentrations of eight heavy metal elements (Zn, Cr, Ni, Cu, Cd, Pb, As and Hg) extracted from sediment samples collected at a depth range of 0~20 cm were quantitatively measured in laboratory analyses. Two widely recognized evaluation approaches, the geo-accumulation index and potential ecological risk index, were adopted to comprehensively characterize the pollution gradients and corresponding ecological hazards induced by heavy metal accumulation. Furthermore, principal component analysis (PCA) was performed to identify and discriminate the potential anthropogenic sources contributing to the enrichment of different heavy metal species. The analytical outputs revealed that the average concentrations of all eight target heavy metals in mangrove sediments throughout the study area exceeded the baseline soil background values established for Guangdong Province. Statistically, the mean concentrations of Cu, Zn, Pb, Cd, Cr, Ni, As and Hg were 2.60, 2.64, 1.65, 6.25, 1.53, 1.74, 1.38 and 1.67 times the corresponding regional background values, respectively. In terms of pollution grading, Cu contamination in sediments spanned from slight to moderate-partial levels, while Zn exhibited pollution ranging from slight to moderate. Cadmium presented the broadest pollution spectrum, covering slight to severe-partial pollution. In contrast, Pb, Cr, Ni, As and Hg only displayed zero to slight pollution across all sampling zones. The integrated potential ecological risk of mangrove sediments in the whole research region was categorized as relatively high. Among all sampling zones, Sandoumen and Niutianyang bore the most prominent ecological risks, with Cd and Hg identified as the primary heavy metals triggering severe ecological hazards to coastal mangrove wetland ecosystems. Source apportionment results illustrated that the enrichment of Zn, Cr, Ni, Cu and Cd was predominantly derived from industrial wastewater and solid waste discharges stemming from smelting, chemical manufacturing, plastic processing, electroplating, leather tanning, printing and dyeing, and textile production. The accumulation of Pb and As showed strong correlations with agricultural practices including poultry and livestock breeding as well as vegetable and fruit cultivation. Mercury enrichment in sediments, however, was primarily attributed to pollutant effluents generated by aquaculture activities such as shrimp, crab and fish farming. In light of the above research conclusions, competent environmental authorities are recommended to strengthen full-spectrum supervision and strict control over diverse pollution sources. Supplementary restoration strategies represented by phytoremediation can be implemented to mitigate the environmental stress caused by excessive heavy metal accumulation in sediments. Such integrated management measures are expected to facilitate balanced and sustainable development that harmonizes local economic growth with the conservation of coastal ecological environments.