Pollution characteristics and risk analysis of heavy metals in surface sediments in the Kaozhouyang Mangrove Zone

Based on the surface sediment samples from the Kaozhouyang mangrove zone in April 2024, this paper analyzes the content and pollution characteristics of seven heavy metal elements, including cadmium (Cd), mercury (Hg), lead (Pb), chromium (Cr), arsenic (As), zinc (Zn), and copper (Cu), and evaluates their potential ecological risks to the mangrove environment. This study also discusses the impacts of organic matter and pH on the metals. The results show that the concentration values of heavy metals follow the order: Zn > Pb > Cr > Cu > As > Cd > Hg, with their average values of 66.19, 37.10, 35.77, 11.28, 10.38, 0.06, and 0.06 mg·kg−1, respectively. Regionally, the heavy metal concentrations are higher in the northern and eastern parts of Kaozhouyang than in the southern and western parts. Compared with heavy metal levels in sediments from other bays, the pollution level of heavy metals in Kaozhouyang is relatively low, with Pb, Zn, and Hg being the main pollutants, which are closely related to the economic development status of the bay. The organic matter is significantly positively correlated with heavy metals, among which TOC (0.15%—1.25%) is significantly correlated with Cu, Zn, As, and Pb, and TON (0.02%—0.13%) is significantly correlated with Cr, Cu, Zn, As, Pb, and Cd. This indicates that organic matter affects the distribution characteristics of these heavy metals and promotes the enrichment of heavy metals in sediments. The results of correlation analysis and principal component analysis indicate that the sources of heavy metals in the area mainly include mixed pollution sources from industries, domestic sewage and release of organic matter decomposition. These findings provide crucial support for the protection of the mangrove forests in the Kaozhouyang.