control petroleum transport (movement) in surface waters are reasonably well understood, and conceptual models exist to build deterministic models for specific loadings in a specific area for periods of time (less than one week).
Figure 4-1 shows the interrelationships among the physical, chemical, and biological processes that crude oil undergoes when introduced into the marine environment, subsequently weathers, and is then transported away from the source. Processes involved in the weathering of crude oil include evaporation, emulsification, and dissolution, whereas chemical processes focus on oxidation, particularly photooxidation. The principal biological process that affects crude oil in the marine environment is microbial oxidation. As crude oil weathers, it may also undergo various transport processes including advection and spreading, dispersion and entrainment, sinking and sedimentation, partitioning and bioavailability, and stranding which leads in some cases to tarball formation. These processes are all discussed briefly, along with special considerations of oil and ice, and oil from deepwater releases. This chapter concludes with a discussion of conceptual and computer models and a summary of fates of oil inputs to the ocean from seeps, surface spills, deepwater releases, and diffuse sources such as the atmosphere, land run off, and recreation.
Following an oil spill or any other event that releases crude oil or crude oil products into the marine environment, weathering processes begin immediately to transform the materials into substances with physical and chemical characteristics that differ from the original source material.
In many oil spills, evaporation is the most important process in terms of mass balance. Within a few days following a spill, light crude oils can lose up to 75 percent of their initial volume and medium crudes up to 40 percent. In contrast, heavy or residual oils will lose no more than 10 percent of their volume in the first few days following a spill. Most oil spill behavior models include evaporation as a process and as a factor in the output of the model.
Despite the importance of the process, relatively little work has been conducted on the basic physics and chemistry of oil spill evaporation (Fingas, 1995). The particular difficulty with oil evaporation is that oil is a mixture of hundreds of compounds, and this mixture varies from source to source and over time. Much of the work described in the literature focuses on “calibrating” equations developed for water evaporation (Fingas, 1995). Initial prediction of oil evaporation was carried our by using water evaporation equations such as the one developed by Sutton (1934).
Later work of Mackay and colleagues (Mackay and Matsugu, 1973; Stiver and Mackay, 1984) was applied to describe the evaporation of crude oil through the use of mass-transfer coefficients as a function of wind speed and spill area. Stiver and Mackay (1984) further developed relationships between evaporative molar flux, mass transfer coefficient at prevailing wind speed, area of spill, vapor pressure of the bulk liquid, gas constant, and temperature.
In all of this previous work, boundary-layer regulation was assumed to be the primary mechanism for petroleum evaporation. This assumption was never tested by experimentation. Subsequently, Fingas (1995) showed that boundary regulation is slight for petroleum evaporation in the thin layers typically found on surface oil slicks, and a simple equation can be used to model evaporation:
Percentage evaporated = C (T)ln (t), (1)
where C is a constant that can be empirically-determined or predicted on the basis of distillation data, T is temperature, and t is time. Empirical equations for many oils have been determined, and the equation parameters found experimentally for the evaporation of oils can be related to commonly available distillation data for the oil (Fingas, 1999). For example,
Percentage evaporated = 0.165 (percent D)ln(t), (2)
where percent D is the percentage (by weight) distilled at 180ºC and t is time in minutes, can be used for oil evaporation prediction. Figure 4-2 shows typical evaporation rates of different oils, the values of which were obtained from experiments under controlled conditions.
Emulsification is the process of formation of various states of water in oil, often called “chocolate mousse” or “mousse” among oil spill workers. These emulsions significantly change the properties and characteristics of spilled oil. Stable emulsions contain between 60 and 85 percent water thus expanding the volume by three to five times the original volume of spilled material. The density of the resulting emulsion can be as great as 1.03 g/mL compared to a starting density ranging from about 0.95 g/mL to as low as 0.80 g/mL. Most significantly, the viscosity of the oil typically changes from a few hundred to a few hundred thousand milli Pascal-seconds, a typical increase of three orders of magnitude. This increase in viscosity can change a liquid petroleum product into a heavy, semi-solid material. Emulsification, if it occurs, has a great effect on the behavior of oil spills at sea. As a result of emulsification, evaporation slows spreading by orders of magnitude, and the oil rides