Table 1. Comparison of Estimates for Petroleum Hydrocarbons Annually 

 Entering the Ocean, Circa 1969-1971 



*PHC input from recreational boating assumed to be incorporated in the river 



runoff value. 

 fBased upon assumed 10 percent return from the atmosphere. 



Source: Reproduced from Petroleum in the Marine Environment, 1 975, with 

 the permission of the National Academy of Sciences, Washington DC. 



periods of geological time. Petroleum consists of the remains of the microscopic 

 plants and animals that lived in coastal seas. During the millions of years since this 

 once-living material was deposited in sediment, the biogenic components have 

 undergone chemical reactions that have produced the complex mixture we call pe- 

 troleum. The compositions of petroleums vary greatly with source, but the qualitative 

 composition is surprisingly similar. These common compositional features are 

 thought to reflect the common origin and similar chemical history of petroleum. In 

 every sense, then, petroleum is a natural substance. But it is a natural substance that 

 has been buried for millions of years, during which time it has generated chemical 

 components that do not exist in the normal food web of ecosystems. Since some of 

 the compounds produced during petroleum generation are toxic to biota, it is appro- 

 priate to consider briefly and generally the composition of petroleum. 



Petroleum is composed of compounds of carbon and hydrogen (hydrocarbons) 

 and minor amounts of organic molecules that also contain nitrogen, sulfur, or oxy- 

 gen (NSO compounds). The hydrocarbons present in petroleum belong to five 

 classes: 



• normal alkanes 



• branched chain alkanes 

 cycloalkanes or naphthenes 

 aromatics 

 naphtheno-aromatics 





The NSO compounds are diverse in structure, not easily classified but important 

 from an environmental viewpoint based on their toxic properties. Structural for- 

 mulas of typical compounds from each hydrocarbon class plus NSO compounds are 

 shown in Figure I. Petroleums are characterized by the relative -amounts of these 

 molecular types that they contain and by the molecular weight distribution of each 

 type. The molecular weight distribution as reflected in boiling range is the property 

 that defines the various fractions of petroleum produced by a refinery. The boiling 

 range fractions such as gasoline, kerosene, and fuel oil are those most well known. 

 Despite the fact that millions of barrels of crude oil are used every day, most people 

 have not had direct experience with it. Figure 2 graphically illustrates the molecular 



97 



