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Studies on heavy metals (cadmium, lead, copper and zinc) and total organic matter in sediment were carried out by collecting sediment samples from 40 stations in the Inner Gulf of Thailand. The field surveys were conducted two times in April and December 2017 as a representative of pre southwest monsoon and post southwest monsoon, respectively. The result showed that the concentrations of lead, copper and zinc in sediment ranged between 9.8-27.1, 2.4-41.1 and 5.5-91.2 ppm, respectively. The result demonstrated that some of the study areas had copper concentration in sediment over the standard level of coastal sediment quality which was issued by Pollution Control Department of Thailand, while the cadmium concentration was lower than method detection limit. Most of the heavy metals in sediment had a high concentration in the coastal area and decreased toward the offshore area. According to the total organic matter in the sediment, it ranged between 1.9-18.2 %. The result from statistical analysis revealed that lead, copper and zinc in sediment had a significant difference among time. All concentrations of studied heavy metals trended to decrease from pre southwest monsoon to post southwest monsoon. Moreover, the heavy metals in sediment (except cadmium) had significantly correlated with total organic matter. The result from the geo-accumulation (Igeo) index indicated that the concentration of heavy metals in sediment in the Inner Gulf of Thailand was still unpolluted status.
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