Contamination of mercury surface sediment in the Gulf of Thailand

Abstract

Surface sediment samples were collected from 39 and 51 stations in the Upper and Lower Gulf of Thailand (GOT), respectively, during an expedition of R.V. Suk of the Royal Thai Navy in October 2003. The sediment samples were analyzed for their sedimentological characteristics using wet sieving and laser diffraction techniques. Organic carbon and carbonate contents in sediments were determined by wet oxidation and acid-base titration techniques, respectively. Total mercury contamination in the sediments was determined by cold-vapor atomic absorption techniques after extraction with aqua regia. The results indicated that grain size fraction of sediment from the Gulf was silt and sand. Average organic carbon in the upper and lower GOT were 0.93 ± 0.61 and 0.68 ± 0.37 percent, respectively. Carbonate contents in the upper and lower GOT were 15.1 ± 7.9 and 16.3 ± 8.6 percent, respectively. Total mercury in the upper GOT sediment was ranged from < 2 to 92.4 with an average of 33.9 ± 27.4 µg/kg dry weight, while total mercury in the lower GOT sediments was less than the upper GOT. Total mercury in the lower GOT was ranged from 6.5 to 50.1 with an average of 24.4 ± 9.0 µg/kg dry weight. However the median of total mercury in the both parts is similar. This indicated the contribution of mercury from land based sources carrying to the GOT through four major rivers that empty to the upper GOT. In general, the mercury concentration in most places was direct related to the percentage of fine grain particles (< 63 µm) presented in the sediments. Although, the level of mercury in the lower GOT sediments was lower than the upper GOT, a petroleum exploration in the lower GOT was shown somewhat related to an elevation of mercury content in some area.