Spatial Distribution and Ecological Risk Assessment of Heavy Metals in the Sediment of a Tropical Mangrove Wetland on Hainan Island, China

Mangroves have a high ecological service value and play an important role in achieving carbon neutrality. However, mangrove wetland soil quality is constantly being affected, and the ecological services provided are gradually declining due to the threat of various pollutants, especially heavy metal pollution. Exploring the sources and ecological risks of heavy metals in mangrove sediments will be helpful in improving mangrove protection. In 2020, sediments were collected from terrestrial and aquatic areas of Dongzhai Harbor mangrove wetland in Hainan, China, and were analyzed for the concentrations of nine heavy metals (As, Cd, Cr, Cu, Hg, Pb, Ni, Zn, Co). The results showed that there were obvious spatial distributions of heavy metals in sediments. The high concentrations of heavy metals occurred largely in terrestrial areas and in 0-20 cm of the sediment surface layer. Correlation analysis and cluster analysis indicated that As mainly originated from ships and aquaculture in the harbor waters, Cd and Hg from agriculture, Cr, Cu, Ni, Zn and Co from the weathering of parent rocks, and Pb from soot emitted from metal smelters and automobile exhaust. The individual potential ecological risk index (E-r(i)) indicated that contaminating elements were mainly Cd and Hg. The potential ecological risk index (RI) and multiple probable effect concentrations quality (mPECQs) indicated that the areas with high heavy metal concentration and the 0-20 cm range of sediment surface layer were more susceptible to heavy metal contamination. Although there were no obvious ecological risks in the area, these results could facilitate the understanding of the distribution of heavy metal pollution in mangroves and provide information to achieve sustainable development of mangroves.