The release of crude oil or refined petroleum products into the marine environment initiates “weathering,” a collective term for the physical, chemical, and biological changes that immediately alter the oil’s composition and physical properties. Spilled oil rapidly changes form, density, and toxicity as it is subjected to the dynamic forces of the ocean, air, and sun. The speed and extent of this transformation depend heavily on the type of oil spilled and prevailing environmental factors like temperature, wave action, and wind.
Initial Physical Changes on the Surface
The moment oil breaches the water surface, it begins a rapid horizontal movement known as spreading, driven initially by gravity and its own surface tension. This process quickly transforms a concentrated mass into an expanding slick that can cover a significant area, often thinning to a mere fraction of a millimeter. The spreading continues until the slick reaches a thickness of approximately 0.1 millimeters, where the forces of surface tension and viscosity achieve equilibrium.
The entire oil slick is simultaneously driven across the ocean by advection, a physical transport mechanism governed by winds, currents, and tides. Within the first hours to days, this movement can distribute the oil far from its source, creating long, narrow bands of oil residue. As the oil moves, it is also rapidly losing mass through evaporation, where the lighter, more volatile hydrocarbon components transition into the atmosphere.
Evaporation is the most significant process for mass loss immediately following a spill, especially for lighter crude oils and refined products like gasoline. Low molecular weight compounds, which have boiling points lower than approximately 280 degrees Celsius, volatilize quickly. A light crude oil can lose up to 75 percent of its initial volume within the first few days, though heavier oils may lose less than 10 percent. The rate of evaporation depends highly on ambient temperature, the oil’s initial composition, and wind speed.
Transformation into Oil and Water Mixtures
As the lighter components evaporate, the remaining oil becomes denser and more viscous, concentrating the heavier, stickier compounds. A small fraction of the oil’s water-soluble components dissolves directly into the water column. This process, known as dissolution, occurs rapidly in the first hours and is a major pathway for the immediate introduction of potentially toxic compounds into the marine water.
The mechanical energy from wave action and turbulence begins to churn the remaining weathered oil with the surrounding seawater. This mixing leads to the formation of a stable mixture called a water-in-oil emulsion, commonly referred to by spill responders as “mousse” due to its brown, foamy appearance. Mousse can incorporate between 60 to 80 percent water in the form of tiny droplets dispersed throughout the continuous oil phase.
Emulsification dramatically changes the physical properties of the oil, increasing its viscosity by a thousand-fold or more. The stability of this emulsion is maintained by the concentration of natural surfactants, such as asphaltenes and resins, in the remaining oil residue. These compounds form a mechanical film around the microscopic water droplets, preventing them from separating from the oil. The resulting highly viscous material is extremely resistant to natural breakdown and significantly complicates mechanical cleanup efforts.
Final Degradation and Removal
The long-term breakdown and removal of the remaining weathered oil residue is dominated by a combination of biological, chemical, and physical processes. Biodegradation, carried out by naturally occurring marine microorganisms like bacteria and fungi, is the primary mechanism for the ultimate removal of hydrocarbons from the environment. These microbes utilize the oil’s hydrocarbons as a source of energy and carbon, breaking the complex molecules down into simpler, less harmful compounds, such as carbon dioxide and water.
The efficacy of microbial degradation is highly dependent on the presence of oxygen, making the process much faster in surface waters under aerobic conditions. The process is often limited by the availability of other nutrients in the seawater, particularly nitrogen and phosphorus, which the oil does not provide. When a spill occurs, the existing population of hydrocarbon-degrading microbes often experiences a “bloom,” but the lack of sufficient nutrients can slow the overall rate of cleanup.
Another significant process transforming the oil on the surface is photo-oxidation, where compounds in the oil react chemically with oxygen under the influence of sunlight’s ultraviolet radiation. This process is most effective on thin slicks and can occur rapidly, creating new oxygenated compounds that are often more polar and sometimes more persistent. Photo-oxidation contributes to the formation of residual materials like tar, which can persist in the environment for decades.
For the heaviest, most weathered residues, the final removal process is often sedimentation and sinking. Oil that has lost its lighter fractions and undergone emulsification can mix with suspended particulate matter, sand, or clay, forming aggregates. These aggregates, sometimes referred to as “marine oil snow,” become dense enough to lose buoyancy and settle to the seafloor. Once settled in the cold, low-oxygen conditions of the deep ocean or coastal sediments, the rate of biodegradation slows dramatically, allowing the oil residue to persist largely intact for many years.