Schematic of some important oil spill processes.
Understanding the fate of oil in the marine environment is of widespread interest, including scientists, regulatory agencies, and industry. Concerns include oil spill mitigation and remediation, resource exploitation planning, and transportation planning. Yet, many aspects remain poorly understood and quantified. This is in part due to the complexity of its interactions, the unpredictability of accidental spills, and the difficulty of obtaining permission for planned release experiments. Also, planned releases are by their nature short-term, while oil spills from an undersea pipeline or leaking tanker can continue for a longer time period.
The COP seep field is one of the largest known in the world, and produces perennial oil slicks that are often visible stretching to tens of kilometers, while the highest concentration of tar along the Pacific coast washes up at Coal Oil Point. As a result, the COP seep field provides an ideal natural laboratory for studying oil slick processes.
Oil rises from the seabed on bubbles and forms oil slicks at the sea surface. The rising oily bubbles can appear from clear to dark black. However, many apparently clear looking bubbles burst and produce a small, light gray oily slick. Very oily, black bubbles often do not burst immediately upon surfacing, but can float for many tens of seconds, bursting many meters away.
Depending upon the chemical composition of the oil and its thickness, the oil slick will have a wide range of appearances, from light gray sheen (thin), to rainbow colored, to brown and black (thick). As volatiles are removed, the appearance can become wazy, and if compressed or acted upon by waves, the oil will form brown mousse. These appearances will change also as the oil evolves chemically due to evaporation and other processes, or if the source oil has a different composition. Finally, tar (or more accurately, asphaltum), a form of heavily aged oil, can be found floating in the slicks.
Research efforts use several innovative tools and approaches. For slick tracking we use hollow microspheres. For oil sampling, we use CATDRUMS, a catamaran drum sampler. We also map spatial distribution of tar at the Coal Oil Point beaches, measure local meteorological conditions with a boat based station, and use a numerical oil slick track model, GNOME.
Characterize the effect of wind and currents on oil slick advection Measure volatilization rates in the real world. Identify sources of tar on the beach. Better estimate surface dispersion rates Identify sources of variability in beach tar
To better understand the evolution and advection of oil in a complex coastal environment|
Schematic of some important oil spill processes. |
Oil Spill Processes
Our focus has been on processes on a time scale 0.1 to 10 hours. relevant processes including volatilization of the lighter oil components (evaporation), dissolution, spreading, mixing, flocculation, dispersion, and diffusion, advection, and photolysis. With regard to the oil slick volume, the primary process (in calm seas) is volatilization, which can reduce slick volume by up to 70% and up to 40% for light crude and refined oils, respectively. (Fingas, 1995).
Advection
Both winds and currents cause advection of the oil slick. However, since the oil slick is at the interface, it is the vector sum of the wind and current at this boundary between the sea and air that determines the direction and speed of the oil. Wind is conventionally measured at 10 meters; however, the wind at the sea surface is much less. Consider wind over land. Because the land is stationary, the wind must drop to zero at the surface, with the profile determined by momentum transfer from the wind to the surface. The situation is more complex over the ocean because the water surface is mobile and moreover, the roughness elements (waves) are moving (with different velocities). The rule of thumb is that 3% of the wind speed at 10 meters yields the interface drift. However, our research suggests that this is not universally appropriate. This value of 3% assumes a certain logarithmic vertical wind profile. However, the logarithmic wind profile is determined by many parameters, including atmospheric stability. Warm air over cold water is more stable, while cold air over warm water is less stable.
 Two example wind profiles for more and less atmospheric stability and a uniform current profile. |
Just as there is a logarithmic wind profile within the air side of the air-sea boundary, there is also a logarithmic profile near in the water side of the air-sea boundary. Because of the much greater density of water than air, the water profile in the current is much sharper. Unlike for the atmosphere, there is no "standard" depth for water current measurements.. Different methods (e.g., drift bouys, CODAR, and other flow tracers) probe different water depths. For example, toss an orange, a small pumpkin and the tiny glass balloons in the water and they all move at different velocities because the forces on them from the wind and water are different as they integrate over a different portion of the current and wind profiles.
