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Aspects of the long-term fate of petroleum hydrocarbons in the marine environment Green, David Robin


The longterm fate of petroleum in four marine environments was Investigated: The fate of petroleum on the surface of the ocean was elucidated by undertaking a detailed study of petroleum residues polluting the Pacific Ocean. First, the extent of contamination of the Pacific by petroleum residues was assessed by measuring the amounts of tar in 2092 neuston tows over a nine-year period (1967-1975). The South Pacific was found to be free of tar; the Northeast Pacific was slightly polluted, with an average of 0.03 mg/m² . The Northwest Pacific, particularly the Kuroshio current system, was the most severely polluted area: all 55 tows between 25° and 40°N in the Northwest Pacific were contaminated. The average concentration in that area 2 was 2.1 mg/m² , representing a standing stock of about 25,000 metric tons of tar. Chemical analyses of the tar as well as its distribution pattern strongly imply that it originates primarily from tanker traffic, and from tanker sludge in particular. The pollutants appear to be discharged by tanker on the very large Middle East to Japan tanker route, then become entrained in the Kuroshio current and create a plume of contamination which extends downstream for 7000 kilometers across the Pacific. Initially evaporation is the most important weathering mechanism acting on the tar, removing component up to the volatility of pentadecane over a period of days or perhaps weeks. Thereafter, microbial degradation is dominant, probably acting for over a year on many particles. Both of these processes increase the density of the residues, and this effect, combined with the overburden of fouling growth that develops, eventually results in the slow sinking of the tar into the depths of the ocean. The fate of petroleum in the intertidal environment was studied by following the natural degradation of the oil after a small (200 ton) oil spill of #5 fuel oil. The most important weathering process was microbial degradation. Evaporation played only a minor role, while photo-oxidation and dissolution had no apparent effect. The microbal attack took approximately one year to complete the degradation of the n-paraffin fraction of the spilled oil, leaving a thin asphaltic residue on the beach. The combined effect of microbial degradation and abrasive weathering removed roughly 95% of the oil from the beach over the period of a year. The fate of oil in the benthic environment was studied by treating 500 ml quantities of crude oil with a commercial sinkant, then placing the oil on soft sediments in about 6 meters of water. Again in the benthic environment, microbial action was the process responsible for the degradation of the oil. Evaporation had no opportunity to act, dissolution was ineffective, and photo-oxidation did not occur because of the low energy and intensity of the light reaching the sediment. The benthic petroleum samples were slow to degrade: the oil remained unchanged in chemical composition for at least 6 months, and after 16 months the n-paraffins were only partly degraded. Oil dissolved in the water column was investigated by adding a spike of #2 fuel oil to an enclosed column of water 2 m in diameter by 15 m deep, and monitoring its fate by fluorescence spectroscopy. For water a meter or two in depth, exchange with the atmosphere played the dominant role in removing the hydrocarbons from the water column, but at 7 m and below, microbial degradation and sedimentation were the more important processes. The disappearance of the oil approximately followed an exponential decay curve. The half life for a large dissolved oil spike was about 3 days (less for a smaller spike) so that 95% removal occurred within 2 weeks.

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