The aim of this work was to determine the concentration of trace elements (Zn, As, Cd and Pb) in mangrove sediments using the European community bureau of reference (BCR) sequential extraction method. The sampling was conducted in August 2016 at 8 stations of Songkhla Lake in southern Thailand. Trace element concentrations were determined using the inductively coupled plasma mass spectrophotometer (ICPMS). Zn (37.20-64.42%) and Cd (63.44-77.50%) were found dominant in the acid-soluble fraction (F1). Pb (62.01-89.48%) was detected dominantly in reducible fraction (F2). As (49.92-73.45%) was found dominant in F2 (at stations 2, 4, 6, and 8) and in oxidizable fraction (F3) with the concentrations of 48.71-68.02% (at stations 1, 3, 5, and 7). All trace elements (Zn, As, Cd, and Pb) were bound to the bioavailable fractions (first three fractions) and were low in residual fraction (F4). Calculation of ecological risk assessment of Cd revealed that the individual concentration factor (ICF) >6, the global concentration factor (GCF) >24 and the risk assessment code (RAC) >50% in all stations were classified as high concentration and very high risk for aquatic organisms.

Mangrove sediment and mangrove plant Soneratia caseolaris in Songkhla Lake, Thailand have been increasingly threatened by trace metal pollution. Mangroves receive trace metal pollution from upstream areas and the sea. However, little is known about the mangrove plant's capacity to uptake and store trace metals. In this study, the concentrations of As, Cd, Pb and Zn in mangrove plant parts (leaf, root, and bark) of Sonneratia caseolaris and sediments were determined. Sediment and plant samples were digested using total metal concentration procedure and ICPMS techniques. It was found that the maximum values of total concentrations of As in mangrove sediments were classified as heavily polluted according to USEPA. The highest concentrations of As and Zn were found to be in bark whereas Cd and Pb in the root. The fractions of all the metals were measured using BCR techniques and their results revealed that the bioavailability fraction was greater than the residual fraction. The order of the amounts of trace metals present in the fractions was as follows; As: Oxidizable > Reducible > Residual > Exchangeable and the order of Cd: Exchangeable > Reducible > Oxidizable > Residual. Both BCF and TF values showed limited accumulation of these elements in their aboveground parts and thus presented a low food chain hazard except at Phawong and U-Taphao canals.