Density is the mass of a given volume of oil or petroleum product and is typically expressed in grams per cubic centimeter.1 It is the property used by the petroleum industry to define light or heavy crude oils. Density is also important because it indicates whether a particular oil will float or sink in water. The density of pure water is 1.0 g/cm3 (at 15ºC) and the density of most oils ranges from 0.7 to 0.99 g/cm3 (at 15ºC), thus most oils will float on water. Since the density of seawater is 1.03 g/cm3 (at 15ºC), thus even heavier oils will usually float on it. Density is often used as a surrogate for predicting the relative rate of natural weathering when crude oil or other petroleum products are released to the environment. Light oils contain petroleum hydrocarbons that are readily lost via evaporation and microbial degradation. Heavy oils contain a greater percentage of the higher-molecular-weight petroleum hydrocarbons that are more resistant to weathering.

Solubility in water is the measure of the amount of an oil or petroleum product that will dissolve in the water column on a molecular basis. Because the amount of dissolved oil is always small, this is not as significant a loss mechanism as evaporation. In fact, the solubility of oil in water is generally less than 100 parts per million (ppm). However, solubility is an important process because the water-soluble fractions of the oil are sometimes toxic to aquatic life. Thus, although solubilization represents a minor loss process, the concentration of toxic compounds dissolved in water from oil may be sufficient to have impacts on marine organisms.


Oil or petroleum products spilled on water undergo a series of changes in physical and chemical properties that, in combination, are termed “weathering.” Weathering processes occur at very different rates but begin immediately after oil is released into the environment. Weathering rates are not consistent and are usually highest immediately after the release. Both weathering processes and the rates at which they occur depend more on the type of oil than on environmental conditions. Most weathering processes are highly temperature dependent, however, and will often slow to insignificant rates as the temperature approaches zero. Table 2-1 is a summary of the processes that affect the fate of petroleum hydrocarbons from seven major input categories. Each input is ranked using a scale of high, medium, and low that indicates the relative importance of each process. The table is intended only to convey variability and is based on many assumptions. Nevertheless, it does provide a general idea of the relative importance of these processes. Clearly one of the biggest problems in developing such a table is that the importance of a particular process will depend on the details of the spill event or release. Table 2-1 attempts to account for this to a limited extent in the case of accidental spills by including subcategories for various oil types (see Chapter 4). This table emphasizes the role various environmental processes can play in spills of widely varying types. This in turn underscores how just one facet of the complex set of variables may vary from spill to spill, making each spill a unique event. Thus, the chemical and physical character of crude oils or refined products greatly influence how these compounds behave in the environment as well as the degree and duration of the environmental effects of their release.

Relating Size of Release to Impact on Organisms

This report attempts to compile and estimate total release (or loadings) of petroleum hydrocarbons to the marine environment from a variety of sources. These loading rates, in units of mass per unit time, are useful to compare the relative importance of various types of loadings and to explore the spatial distribution of loadings. Obviously, sources of petroleum that release significant amounts (whether through spills or chronic discharges) represent areas where policymakers, scientists, and engineers may want to focus greater attention. Attributing specific environmental responses to loadings calculated at worldwide or regional scales, however, is currently not possible.

As discussed earlier, petroleum is a complex group of mixtures, and each group may contain widely varying relative amounts of hundreds (or more) compounds. Although many of the compounds are apparently benign, many other, such as some types of PAH, are known to cause toxic effects in some marine organisms. To further complicate this picture, marine organisms (even in the same taxa) vary greatly in their sensitivity to the same compound. Predicting the environmental response to a specific release of a known quantity of a refined petroleum product (which contains far fewer compounds than crude oil) requires much site-specific information about the nature of the receiving water body. Thus, the estimated loadings reported later in this chapter or in Chapter 3, are best used as a guide for future policymaking. In addition to identifying potential sources of concern, these estimates may have some value as performance metrics. Specifically, in those cases where reasonable comparisons can be made to estimates developed in earlier studies, they have value as a measure of the effectiveness of already implemented policies designed to reduce petroleum pollution.

Much of what is known about the impacts of petroleum hydrocarbons comes from studies of catastrophic oil spills and chronic seeps. These two aspects of petroleum pollution (loading and impact) are distinct, and it is not possible to


The oil and gas industry, especially in the United States, often uses specific gravity instead of density. Specific gravity is used by the American Petroleum Institute (API) to classify various “weights” of oil. The density of a crude or refined product is thus measured as API gravity (ºAPI), which equals (141.5/specific gravity)—131.5.

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