Since mercury is known to be a naturally occurring component of geologically derived hydrocarbons in the Gulf of Thailand, the Bongkot Operation Marine Environmental Monitoring concentrated on the measurement of mercury content in commercial species of demersal fishes in the vicinity of the Bongkot Operation Platform. The main species studied were groupers Epinephelus areolatus, red snappers Lutjanus sebae, sharptooth jobfish Pristipomoides typus. But in the two most recent surveys, sharptooth jobfish was not available so it had to be replaced with other species e.g. painted sweetlips Diagramma pictum.

Before late in 1997, treated produced water from the production platform was discharged into the sea. This water contained certain amount of mercury. Since then, the treated produced water was re-injected back into the depleted gas wells. And less and less direct discharge was made until in the year 2002, the volume of produced water re-injected was over 99.9%. Hence, the seawater surrounding the Bongkot Platform showed only the background level of mercury in natural seawater and fish should not be contaminated with mercury. From the results of the fish monitoring in 1997, 1998, 2000, 2002, 2004, the mercury content in fishes appear to be lower, within the natural range of values among each individual fish of the same species. The exception is painted sweetlips which showed a slightly higher average in 2004 compared to the concentration found in 2002, although the number of this species used in the analysis was much smaller than other species. Equally large numbers should be used to ascertain this issue in the next monitoring survey.

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