Geochemical Speciation, Risk Assessment, and Sources Identification of Heavy Metals in Mangrove Surface Sediments from the Nanliu River Estuary of the Beibu Gulf, China

To better understand heavy metal pollution and the potential ecological risk of mangrove sediments in the Nanliu River estuary, the speciation and distribution characteristics of heavy metals Fe, Mn, Zn, Co, Ni, Cd, Cr, Cu, and Pb in 13 surface sediments in the study area were determined and analyzed using a modified four-step BCR extraction method, and the ecological risk of heavy metals was assessed using the Geo-accumulation Index ( I geo ), Potential Ecological Risk Index ( RI ), Risk Assessment Code ( RAC ), Pollution Load Index ( PLI ), Individual contamination factors ( ICF ) and Global contamination factor ( GCF ) methods, and source analyses were performed using correlation analysis and cluster analysis. The results showed that the heavy metal was in the order of Fe > Mn > Cu > Zn > Cr > Pb > Co > Ni > Cd. Except for Fe, Zn, Ni, Cr, Pb, and Co, the average heavy metal content of Mn, Cd, and Cu all exceeded the environmental background value; the Fe, Zn, Co, Ni, Cr, Cu, and Pd are mainly in the residual speciation, while Mn and Cd are mainly weak acid extraction and oxidation, respectively, both of which are predominantly in unstable speciation and are easily released into the environment. Mn and Cd pose a substantial ecological risk, while Cu and Pb present a moderate risk and require precaution. The source analysis results indicate that Fe, Mn, Zn, Ni, Cr, and Pb are most likely to originate from natural sources and the transportation industry, Co and Cu are likely to be mainly from ship manufacturing industrial activities, and Cd is likely to be mainly from agriculture and aquaculture. The GCF and PLI results show that places with high heavy metal enrichment and ecological risk are primarily located in areas with high industrial, agricultural, or human activity impacts.