MARINEBIOTECH.ORG | Crude Realities

Annual Oil Input to the Oceans

The majority of the estimated 3.2 million tons of oil annually released into the world's oceans is from small-volume chronic sources like tanker operations (0.7 million tons/year) and municipal waste (0.7 million tons/year).

Fortunately, these small-volume spills are for the most part rapidly removed by a number of natural processes like evaporation, dissolution, biodegradation, emulsification and sedimentation.

The large accidental spills that garner most of the headlines only account for 13% (0.42 million tons/year) of the total input (Portier 2002). But, it is these large spills that have the potential to swamp the system, overwhelming the ocean's natural ability to cleanse itself. In such situations, remnants from large oils spills can remain for several months to several years or more.

Chronology of an Oil Spill

After an oil spill has occurred, a predictable series of natural 'weathering processes' immediately begins to occur. The general steps in this progression are depicted in the accompanying figure, and briefly considered in the subsections below. Click on subsection headings to examine componnent weathering processes in more detail.

Immediately after a spill, the oil spreads out onto the surface of the ocean as a thin slick. This is important because it exposes a maximal amount of surface area to each of the subsequent weathering processes. Just 1 g of spilled oil can form a slick of up to 10 m2, and a 1 ton spill can form a slick of up to 12 km2.

In the first hours and says of the spill, evaporation at the surface of the slick is the dominant weathering process. If the spill consists of a lightweight, highly refined product like gasoline, evaporation can very effectively remove nearly all of the spill contamination in as little as 24 hours. For spills of most medium-weight crudes the removal is less complete but substantial nevertheless. Typically, 10-30% of the material from these spills can be removed through evaporation in the first 24 hours.

Other factors effecting the evaporation of a spill include the amount of the spill exposed at the surface of the slick, wind and sea surface conditions, air temperature, and insolation intensity. Another factor is emulsification of the slick, which significantly retards the rate of evaporation.

As a slick breaks apart the oil is dispersed over an area much larger than the original localized impact site. Strong winds, currents, and turbulent seas facilitate the process of dispersion.

Still more of the oil slick is removed as the water-soluble portion of the petroleum hydrocarbons are dissolved into the surrounding seawater. Although this reduces the size of the slick it presents an environmental problem since the water-soluble spill components and breakdown products are those that are most toxic to marine life. Small aromatic hydrocarbons like benzene,and toluene, and somewhat larger polycyclic aromatic hydrocarbons (PAHs) like naphthalene, are among the water-soluble petroleum components known to have toxic effects.

Emulsification is the formation of a mixture of two distinct liquids, seawater and oil in the case of a marine spill. Fine oil droplets are suspended within (but not dissolved into) the water and the emulsification formed occupies a volume that can be up to four times that of the oil it formed from. Moreover, the viscous emulsion is considerably more long-lived within the environment than the source oil, and its formation slows subsequent weathering processes.

Emulsification tends to occur under conditions of strong winds and/or waves and generally not until an oil spill has persisted on the water for at least several hours. A persistent, partially emulsified mixture of water in oil is sometimes referred to as a "mousse." Mousse is resistant to biodegradation, the important final weathering stage, and in shallow marsh environments it can persist within sediments for years to decades.

Very few crudes are dense enough to sink on their own in seawater, and few of them weather fully enough to yield a residue dense enough to do so either (unless the oil is ignited, in which case sufficiently dense residues may be formed). Yet oil spill components and residues do find their way into marine sediments. This is because adhesion of suspended silt or particulate organic matter to the oil over time increases the effective density of the aggregate that is formed allowing it to sink.

Coastal habitats are particularly vulnerable to input of sinking oil contaminants because the suspended solids load in these areas is conducive to the formation of sinking oil/silt aggregates. Moreover, oil washing up on beaches and marshes and given ample opportunity to mix with sand and mud is likely to be of sinking density if it gets washed back offshore.

Chemical oxidation of the spilled oil also occurs, and this process is facilitated by exposure of the oil to sunlight. Oxidation contributes to the total water-soluble fraction of oil components. Less complete oxidation also contributes to the formation of persistent petroleum compounds called tars. The overall contribution of photooxidation to oil spill removal is small. Even exposed to strong sunlight, photooxidation only breaks down about a tenth of a percent (0.1%) of an exposed slick in a day.

Microbial oil degradation is a critical late-stage step in the natural weathering of petroleum spills, as it is the stage that gradually removes the last of the petroleum pollutants from the marine environment.

Microbial breakdown of petroleum compounds occurs most rapidly via the oxidative metabolic pathways of the degrading organisms. As such, biodegradation is predicted to occur fastest in environments with ample oxygen as well as a diverse and healthy oil-degrading flora. Conversely, oxygen-starved marine sediments that are often sites of petroleum contamination are among the habitats where aerobic metabolism is severely limited and microbial oil breakdown must therefore proceed via slower anaerobic pathways. Even though breakdown within these sites is slow, it may still have a substantial cumulative impact over time.