Seasonal dispersion of petroleum contaminants in the Gulf of Thailand

Six oceanographic moorings were maintained in 1993 and 1994 and provided data on the water circulation and the flushing characteristics of the Gulf of Thailand. The concentration of dissolved and dispersed petroleum hydrocarbons (DDPH) was measured at 78 sites in 1994 and 1995 in coastal waters of the Gulf. The water circulation was sluggish and the Gulf was poorly flushed; the mean currents were generally < 0.07 ms(-1). Under the influence of the South China Sea, an anticyclonic gyre existed in the southeast monsoon, a cyclonic gyre in the northwest monsoon, and sluggish currents the rest of the time. Even in the dry season brackish water was found inshore; this suggests that freshwater, that arrived in the Gulf in the previous wet season, was trapped along the coast and that little mixing occurred between offshore and coastal waters. In coastal waters of the Inner Gulf and the Eastern Sea Board there were occasional acute pollution events (DDPH > 40 mu g l(-1)), superimposed 25% of the time upon chronic pollution (DDPH approximate to 4 mu g l(-1)) due to limited flushing of the Inner Gulf and the Eastern Sea Board, and the presence of slightly contaminated water elsewhere (DDPH < 1.2 mu g l(-1)) 75% of the time. Only the Outer Gulf seems relatively uncontaminated (DDPH approximate to 0.01-0.1 mu g l(-1)). The seasonal distribution of the DDPH appeared to be controlled by the water circulation; indeed the highest DDPH values in the Inner Gulf occurred in November-December because of the net currents were weak and variable. The highest DDPH values on the Eastern Sea Board occurred in April-August when the region simultaneously received contaminated coastal water from the Inner Gulf. The smallest DDPH values on the Eastern Sea Board occurred in September-November because strong westward currents prevailed which flushed the contaminants. The observed currents and DDPH data were used to drive an oil spill model which predicted that acute contamination occurs at least once a year everywhere in the Inner Gulf. (C) 1998 Elsevier Science Ltd. All rights reserved.