Mechanisms Influencing the Spatial Distribution of Trace Metals in Surficial Sediments off the South-Western Taiwan

Surficial sediments collected from the south-western coast off Taiwan were determined for the grain size, organic carbon contents (OC), carbonate and leachable trace metals (Co, Cu, Fe, Mn, Ni, Pb and Zn). The sediments in the study area were mostly the very fine sands (0.125–0.0625 mm, φ=3–4). However, muds (0.0625–0.0156 mm, φ=4–6) were observed in the trough area located in the south of the Waisanding Barrier Island. Concentration ranges of organic carbon, carbonate and leachable trace metals in the sediments were: OC, 0.06–0.49% (mean, 0.2%); carbonate, 1.48.6–3.68% (2.23%); Fe, 0.41–1.52% (0.85%); Co, 4.6–18.2 mg/kg (8.9 mg/kg); Cu, 0.4–16.7 mg/kg (4.39 mg/kg); Mn, 186–625 mg/kg (338 mg/kg); Ni, 2.1–10.2 mg/kg; Pb, 0.73–21.8 mg/kg (6.8 mg/kg) and Zn, 3.6–56.4 mg/kg (22.1 mg/kg). Normalization technique and comparison with the results obtained in other marine environment around Taiwan are used to assess the contaminated status of the study area. The results suggest that the sediment of the south-western coast off Taiwan has not been contaminated by trace metals. The distribution patterns of trace metals and OC are fairly similar, and oppositely correlated with that of sediment mean grain-size. That is the finer the sediment, the higher the concentrations of trace metals and OC. Moreover, higher concentrations of trace metals and OC were found in the trough located in the south of the Waisanding Barrier Island while lower values were in the near-shore area. The sediment grain size is probably the most crucial parameter which controls the spatial distribution of trace metals, except Mn, in the south-western coast off Taiwan. Although, the maximum contour of Mn also occurs in the trough, the concentration of Mn does not correlate well with the sediment grain size. In contrast, concentration of Mn significantly correlates with the carbonate, which may suggest that Mn is adsorbed or co-precipitated with the calcium carbonate and forms rhodochrosite-calcite (MnCO3–CaCO3) in the water column.