minor elements nitrogen, sulfur, and oxygen make up the remaining 3 percent (NRC, 1985). Crude oil sometimes contains mineral salts, as well as trace metals such as nickel, vanadium, and chromium. In general, the hydrocarbon compounds found in crude oil are characterized by their structure (see Speight, 1991 for greater discussion of the classification of petroleum related compounds). These compounds include the saturates, olefins, aromatics, and polar compounds. Understanding these different compounds and their structures is important for understanding the fate and effect of releases of crude oil or products derived from it.
The saturate group of compounds in various crude oils consists primarily of alkanes, which are composed of hydrogen and carbon with the maximum number of hydrogen atoms around each carbon (Speight, 1991). Thus, the term “saturate” is used because the carbons are saturated with hydrogen. The saturate group also includes cycloalkanes, which are compounds made up of the same carbon and hydrogen constituents, but with the carbon atoms bonded to each other in rings. Higher-molecular-weight saturate compounds are often referred to as “waxes.”
Olefins, or unsaturated compounds, are those that contain fewer hydrogen atoms than the maximum possible. Olefins have at least one carbon-to-carbon double bond, which displaces two hydrogen atoms. Significant amounts of olefins are found only in refined products (NRC, 1985; Speight, 1991).
Aromatic compounds include at least one benzene ring. Benzene rings are very stable, and therefore persistent in the environment, and can have toxic effects on organisms. The more volatile monoaromatic (single-ring) compounds found in crude oil are often referred to as BTEX, or benzene, toluene, ethylbenzene, and xylene (NRC, 1985; Speight, 1991).
Aromatic hydrocarbons may account for about 1 to 20 percent of the total hydrocarbons in crude oil. Benzene and alkyl benzenes with one or two methyl or ethyl groups (toluene, xylenes, ethylbenzene), the BTEX compounds, may be present at a concentration of several percent in light crude oil, but more typically are present at concentrations of 1,000 to 10,000 mg/kg (Speight, 1991). Usually, toluene is the most abundant of the BTEX compounds, followed by benzene or one of the three xylene isomers. More highly alkylated benzenes usually are present at low concentrations in crude oils.
Polycyclic aromatic (multiple-ring) hydrocarbons (PAH, also called polynuclear aromatic hydrocarbons, PNA) consist of at least two benzene rings. A typical crude oil may contain 0.2 percent to more than 7 percent total PAH.
Some related aromatic compounds (not technically hydrocarbons because they may contain within their structure many elements such as sulfur, nitrogen, and oxygen) are detected with the same analytical techniques and often occur with true polycyclic aromatic hydrocarbons. Thus, these compounds are often grouped with, and discussed as, PAH. PAH includes those compounds that have the most serious environmental effects of the compounds in crude oil. PAH in the environment are derived largely from combustion of oil and coal, but are also produced by the burning of wood, forest fires, and a variety of other combustion sources.
The abundance of aromatic hydrocarbons in petroleum usually decreases with increasing molecular weight. In most cases, one-ring (benzene) through three-ring (phenanthrene) aromatic hydrocarbons and related heterocyclic aromatic hydrocarbons, such as dibenzothiophene, account for at least 90 percent of the aromatic hydrocarbons that can be resolved in crude petroleum by conventional analytical methods (Neff, 1990). Four- through six-ring PAH (pyrene/fluoranthene through coronene), some of which are known mammalian carcinogens, usually are present at low or trace concentrations in crude oils (Kerr et al., 1999). The PAH in petroleum often contain one or more methyl, ethyl, or occasionally higher alkyl substituents on one or more aromatic carbons. As a general rule, these alkylated PAH are more abundant than the parent compounds in petroleum (Sporsol et al., 1983).
Of the hydrocarbon compounds common in petroleum, PAH appear to pose the greatest toxicity to the environment (see Chapter 5 for greater discussion). Most of the PAH compounds in petroleum are not as toxic as those produced by certain combustion processes, but most groups are significant components of runoff from paved surfaces.
Polar compounds are those that have a significant molecular charge as a result of bonding with elements such as sulfur, nitrogen, or oxygen. The polarity of the molecule results in behavior that differs from that of unpolarized compounds under some circumstances. In the petroleum industry, the smallest polar compounds are known as resins. The larger polar compounds are called asphaltenes and often make up the greatest percentage of the asphalt commonly used for road construction. Asphaltenes often are very large molecules, and if abundant in a specific volume of oil, they have a significant effect on oil behavior.
The properties of liquid petroleum, including crude oil or refined products, that are most important in understanding the behavior and fate of spills or other releases are viscosity, density, and solubility (see Chapter 4 for greater detail).
Viscosity is the resistance to flow in a liquid. The lower the viscosity, the more readily the liquid flows. The viscosity of oil or petroleum products is determined largely by the proportion of lighter and heavier fractions that it contains. The greater the percentage of light components such as saturates and the lesser the amount of asphaltenes, the lower is the viscosity. Highly viscous oils tend to weather more slowly because they do not spread into thin slicks. Instead, they form tarballs, which can be transported long distances and accumulate in thick deposits on shorelines that can persist for decades.