Fate of oils in the ocean:
Oil spills are one of the significant reasons of pollution of the marine ecology. In earlier articles, we have seen environmental impacts of oils. Although not a substantial cause of marine pollution (as compared to urban and agricultural runoffs), I will still like to dwell more on this topic today, because of two reasons. One being, effects of oil spills can be devastating and long lasting, sometimes ranging even up to few decades. And two, nobody exactly knows the quantities of oils entering the oceans.
In the present article, we will see characteristics and fate of oil when spilled into oceans.
When many of us think of oil spills, we might think of say an oil tanker or a cargo ship running aground and spilling its contents into the oceans. Accidental or deliberate discharges from ships, especially tankers, offshore platforms, and pipelines, are the most visible cause of oil pollution of the marine environment. However, there are several other ways crude or refined oil may reach the marine environment. And all of those spills are substantial and add up too.
In a report published in 2002 by the National Research Council (NRC) of the U.S. National Academy of Sciences, the average total worldwide annual release of petroleum (oils) from all known sources to the sea has been estimated to a possible maximum of 8.4 million tonnes per year. According to the report, the main categories of sources that contribute to the total input are as follows:
- Natural seeps: 46% – Oil exists in many environments and may be naturally spilled from the sea floor into the marine environment due to climatic conditions, disturbance, or eroding of sedimentary rocks.
- Spills from consumption of oils: 37% – These include urban runoffs from, vehicular spills, (also from roadside garages) and accidental oil spills on the roads and deliberate discharges from ships (say bilge), boats and recreational yachts.
- Accidental spills from vessels: 12% – These are major maritime accidents when a tanker or a cargo ship, say, runs aground or collides with other ship.
- Extraction of oil: 3% – These are spills occurring during oil extraction from sea-beds. Off Santa Barbara in California, for example, around 25 tons of oil flows from seafloor cracks daily.
As soon as the oil is spilled into the marine environment, it becomes subject to many natural processes, known together as ‘weathering.’ Weathering processes quickly and progressively change the character of the oil and redistribute much of it into other parts of the environment. The importance of each of these processes, on the fate of the oil, depends on where the spill occurs (i.e., the environmental conditions) and chemical and physical properties of the spilled oil. The future of the oil dramatically influences the mechanism and scale of ecological effects. Various processes involved in weathering are as follows:
Evaporation – Most oils contain a proportion of low molecular weight hydrocarbons. When released into the sea, evaporation of these hydrocarbons into the atmosphere will begin immediately. This process increases the viscosity of the spilled oil but also reduces the volume and acute toxicity of the remaining oil.
Spreading and movement – On the sea surface, spilled oil will start spreading immediately and will continue for a few weeks, depending on oil’s pour point and viscosity. Light oils will spread very quickly, but heavy oils will spread more slowly and remain thicker for longer. Any surface life or animals that need to come to the surface to breathe will be impacted.
Dissolution – While most hydrocarbons are almost insoluble in water, some of the smaller aromatic hydrocarbons, such as benzene and toluene, are relatively soluble. Thus, during a spill, a small proportion dissolves. This water-soluble fraction has a disproportionate impact on marine organisms, being more bioavailable and often more acutely toxic.
Dispersion – Agitation of the oil in the water, leads to the formation of oil droplets that become mixed into the water column; the higher the agitation, the higher the mixing potential. The extent and depth of mixing depend on wave action and water currents. This process can potentially lead to subsurface marine life being exposed to contamination.
Emulsification – Larger droplets of dispersed oil will quickly resurface and can trap seawater droplets to form a water-in-oil emulsion (or ‘chocolate mousse’ formation). The higher the mixing effect (turbulent conditions), the more water is incorporated into the emulsion. Hence the volume of the emulsion can increase even up to 5 times of the amount of oil spilled initially.
Sedimentation – Sea water contains typically various amounts of suspended solids (fine sediments and other particles) in the water column. Dispersed oil droplets can bind to suspended solids and change their physical characteristics. Deposition of these suspended solids together with oil droplets to the seabed can occur, endangering life at the sea bottom.
Sinking – Sinking occurs if the spilled oil is denser than seawater, and can result in very persistent accumulations that lie on the seabed and sometimes become buried. The impacted area of the seabed is typically smaller than that affected by sedimentation of dispersed oil, but sunken oil can cause long-term smothering and loss of habitat.
Shoreline stranding – Most moderate or large oil spills result in shoreline oiling, which may then impact the full range of habitats and species present below the high tide level, and sometimes above it.
Photo-oxidation – Hydrocarbons exposed to ultraviolet (UV) light can be photochemically oxidized to form other compounds. This process is often a minor component of the weathering process. However, laboratory studies of some compounds have found that the resulting products can be more toxic than the parent compounds, primarily because they are more soluble in water.
Biodegradation – And finally and most importantly, nature has its mechanism of self-defence. Marine bacteria have evolved to produce enzymes that allow them to utilize hydrocarbons from crude oil as a food source. By metabolizing hydrocarbons they grow and multiply, and in turn become a food source for other organisms. It is through this natural process that the majority of the oil from a spill is ultimately biodegraded, and the energy and materials contained within it are returned to the food chain.
In next article, we will see how oil spill clean-up is done and how useful it can be.
- Office of Response and Restoration, National Oceanic and Atmospheric Administration
- Global Marine Oil Pollution Information Gateway
- Impacts of oil spills on marine ecology – The global oil and gas industry association for environmental and social issues