4.4. Mercury contamination
Total mercury in seawater and sediment of the Gulf
of Thailand is shown in Table 6. Considering the data
obtained from several surveys, it can be found that the
mercury concentration in seawater during the period
1974–1980 is comparable to natural level as suggested by
Kothny (1973), i.e. in the range of 0.01–0.38 ppb. High
mercury concentrations (44.7–847 ppb) nevertheless
were reported during 1983–1987. The levels were even
higher than those detected in Minamata Bay, Japan
(1.6–3.6 ppb). Whether these reported data are valid or
not, there is a need for clarification both on sample
collection and analytical methods. Most mercury concentrations
in the sediments were still within the acceptable
limit of 0.3 ppm (Ministry of Transport, Japan,
1976), except certain locations such as the Chao Phraya River estuary and the east coast of the Gulf. Higher
mercury concentrations in such areas might be due to
the contamination from urban and industrial areas.
Total mercury concentration in biota of the Gulf of
Thailand are shown in Table 7. In the coastal area, almost
all mercury concentration in fish were lower than
0.2 lg/g wet. These concentrations could be regarded as
a natural background of mercury in fish in general.
Nevertheless fishes in the off shore area, in the vicinity of
natural gas platforms, exhibited higher mercury concentrations.
These fishes were caught and analyzed recently
(ARRI, 1998). Between 5% and 10% of fish at
Erawan and Funan platforms had mercury concentrations
higher than 0.5 lg/g. This concentration is the
maximum permissible concentration in fish set by the
FAO. The biological magnification of mercury was
mentioned in several reports. Fish of higher trophic
levels bore higher residue than those in the lower trophic
levels. This suggests that mercury might be concentrated
in the same manner as organic compounds such as organochlorine
compounds, i.e. passed through and amplified
along the food chain.