Geochemical Distribution, Enrichment, and Potential Toxicity of Trace Metals in the Surface Sediments of Okinawa Mangrove, Southwest Japan

Surface sediments along the coast of Okinawa Island were analyzed to determine their geochemical compositions, and to assess sediment quality and the potential for ecological harm, based on comparison with established international sediment quality guidelines. The Contamination factor (CF) and the geoaccumualtion index (Igeo) of As, Pb, Zn, Cu, Ni, and Cr were computed to determine the pollution status of the mangrove and foreshore sediments. The lowest effect level (LEL) and the severe effect level (SEL) established by the New York State Department of Environmental Conservation (NYSDEC), and the threshold effect level (TEL) value and the probable effect level (PEL) developed by the Canadian Council of Ministers of the Environment (CCME) were the benchmarks applied to assess the potential for ecological harm. Among the sampling set, the highest average concentration of Pb (22 mg/kg), Zn (82 mg/kg), Ni (26 mg/kg), and Cr (81 mg/kg) occurred in the Ohura mangrove sediments, that of As (17 mg/kg) in the Gesashi mangrove, and that of Cu (22 mg/kg) in the suspended solids. The loss on ignition values of the foreshore sediments (20.32%) are nearly two and three times greater than that of the Gesashi mangrove and the Ohura mangrove, respectively. On average, the CFs of As, Pb, Zn, Cu, Ni, and Cr in the suspended solids, those of Pb, Zn, Cu, Ni, and Cr in the Gesashi mangrove and foreshore sediments, and those of Ni and Cr in the Ohura mangrove display low contamination (CF≤1), whereas the average CF of As in the three sampling areas show moderate enrichment (CF:1-3). The average Igeo values indicate that among the selected trace metals, only As in the Gesashi and Ohura mangroves show significant values, but even these are rated as unpolluted to moderately polluted (Igeo:0-1). The average concentrations of As in the Gesashi and Ohura mangroves and the foreshore sediments exceed both the LEL and TEL, but fall below the SEL and PEL, suggesting that this metal may moderately impact biota health. This is also the case for Cu and Ni in the Ohura mangrove and the suspended solids, and for Cr in the Ohura mangrove.