Every now and then, Goan beaches offer an offending sight: black streaks of darkish oily material strewn across the beach. An example is given in Figure 1. On a closer look, the black streaks turn out to be consisting of oily darkish ball-like material with diameters ranging from couple of centimeters to about 20 centimetres. Walking through them leaves an oily film on feet. The ugly sight of this darkish material lying on the white sand for which Goan beaches are well known, leaves the citizen puzzled and concerned: where did the mess come from? Can the beach be cleaned of it? Will it leave a long term impact? This article has been put together to provide answers to questions such as these for the lay-person. The darkish material is made of “tarballs”. NIO’s researchers first came across them way back in 1970s when the institute was in its early formative stages. Widespread concern about them, not only in Goa but elsewhere too in coastal areas of India, prompted the institute to find out more about them. By mid-seventies it became clear that tarballs appear along the entire coast of India, and their appearance on the west coast was usually seasonal, with high likelihood during May-October
Figure 1: Dark streaks made of tarballs lying on a south Goa beach during the last week of July 2005.
Tarballs along the coast of Goa arise whenever an oil-slick occurs in the vicinity of the coastline. One common cause for such a slick is dumping oil overboard by a passing ship. But then, why would any ship dump oil overboard? A scenario resulting in such dumping is the following. Consider an oil tanker that routinely transports oil (say diesel) from Mumbai, where it picks up the oil, to Kochi, where it is offloaded. Let us assume that such a tanker operating in coastal areas of India travels along a route that has depth of about 20 m. Along the west coast of India, this would be at a distance of about 20 km from the coastline. After offloading the oil at Kochi, the ship returns, and while on the way, Captain of the ship decides that one of the tanks needs to be cleaned. Though aware that such cleaning would violate internationally accepted norms for preventing oil pollution, he decides to go ahead, since he is aware that enforcement of the norms is not effective along the Indian coast. The cleaning gets done by spraying the tank with jets of heated sea water and then flushing overboard the water now having lot of sticky diesel that was coating the tank. In essence, this operation leads to an “oil-spill”, albeit a minor one, at a distance of about 20 km from the coast. During the first few hours of a spill, the oil spreads into a thin slick. Winds and waves tear the slick into smaller patches that are scattered over a much wider area. Various physical, chemical, and biological processes change appearance of the oil. These processes are generally called “weathering.”
There are other ways by which such spills, minor or major, can occur. A collision between two ships can rupture the hull of a ship, leading to leakage from a tank filled with oil used to run the ship’s engines; accidental grounding of a ship can damage a tank leading to spillage of oil, etc. It appears, however, that the most common cause of minor spills along the Goan coast is similar to the one outlined above: unscrupulous ships violating international rules regarding when, where, and how to clean tanks. Such behaviour can lead to appearance of tarballs on Goan beaches. As seen below, whether it will lead to appearance of tarballs on a coast in Goa depends on a number of factors including characteristics of weathering of spilled oil, and nature of winds at the time of the spill.
After a spill, initially, the lighter components of the oil evaporate much like a small gasoline spill. In the cases of heavier types of oil, such as crude oil, much of the oil remains behind. At the same time, some crude oils mix with water to form an emulsion that often looks like chocolate pudding. This emulsion is much thicker and stickier than the original oil. Winds and waves continue to stretch and tear the oil patches into smaller pieces, or tarballs. While some tarballs may be as large as 30 cm in diameter, most are coin-sized or a bit bigger.
Weathering processes eventually create a tarball that is hard and crusty on the outside and soft and gooey on the inside. Turbulence in the water or beach activity from people or animals may break open tarballs, exposing their softer, more fluid centers. Temperature has an important effect on the stickiness of tarballs. As air and water temperatures increase, tarballs become more fluid and, therefore, sticky-similar to an asphalt road warmed by the summer sun. Another factor influencing stickiness is the amount of particulates and sediments present in the water or on the shoreline, which can adhere to tarballs. The more sand and debris attached to a tarball, the more difficult it is to break the tarball open. These factors make it extremely difficult to predict how long a tarball will remain sticky. In case density of the tarballs happens to be more than density of the ambient waters, then the tarballs sink to the sea floor and can remain there for a long time. More worrisome are the tarballs whose density is less than that of ambient water. These float and can be carried to other places, including to beaches along a coast.
NIO scientists had collected samples of typically sized tarballs that made the black streaks on the beach shown in Figure 1. There tarballs are shown in Figure 2.
Figure 2: Typical tarballs collected from beaches of south Goa during the last week of July 2005. The scales shown at the lower left corner and at the upper right hand corner are each 10 cm long. Tarballs like the ones shown above formed the dark streaks shown in Figure 1.
The direction of movement of floating tarballs is very much dependent on winds, waves on the surface of a sea, and currents. Wind pushes a slick (including tarballs) in the direction of wind-flow. Stronger and more persistent the wind, faster is the movement of a tarball. Waves on sea surface can also push tarballs in the direction in which the waves go. Stronger the waves, more effective they are in transporting tarballs. Currents carry the tarballs with them. As noted earlier, studies by NIO researchers during 1970s showed that chance of occurrence of tarballs along the beaches of the west coast of India is high during May-October. Why should this be? Erring ships that violate code or conduct regarding oil pollution at sea would be passing by the west coast throughout the year. Why then is the occurrence of tarballs higher during May-October?
One needs to understand a bit of oceanography and meteorology of the western coastal region of India to answer this question. Along this region, the most important of the three causes (winds, wind-waves, and currents) of movement of tarballs is the wind. Winds along the coast have a well defined annual cycle, the characteristic of which can be seen from Figure 3. Normally winds start blowing towards the coast in May. They keep getting stronger as the monsoon sets and are strongest in July-August. In September and October the winds weaken, and by November they are no more oriented towards the coast. The strength of waves on the ocean surface along the west coast exhibits a pattern that is pretty much the same as the winds: waves get stronger in May, and continue to increase in strength as the monsoon picks up. As a result, the waves are bigger, and the sea much rougher, during the monsoon. The waves along the coast are usually oriented towards the coast. Stronger the waves are, more effective is the transport of tarballs by them.
Figure 3: Nature of winds over the north Indian Ocean. Direction of the arrow points in the direction of winds. Length of the arrow is proportional to strength of the wind. The arrow drawn in the upper right hand corner has a strength of 10 metres per second and can be used to estimate the speed associated with each of the arrows.
It is under the influence of winds, and to a lesser extent that of waves, that an oil slick, or tar balls formed from an oil slick, get transported towards the coast during May-October. How quickly the oil slick reaches the coast depends on how close to the coastline the slick occurred, strength of the winds, and orientation of the winds during movement of the slick. If the slick occurs close to the coast, and the winds are strong enough, then the slick could land on the coast before the slick turned into tar balls. In this case the slick could reach the coast as chocolate coloured emulsion. This is what seems to have happened around 20th May on the Ashvem beach in Goa where newspapers reported arrival of oily chocolate coloured deposits.
What happens to an oil-slick that occurs off the coast of Goa during November-April when the winds may not be able to carry the remnants of the slick (such as tarballs) towards the coast? We don’t really know the answer to this question. Presumably, there are two ways for the slick to evolve. The first applies to the case when the composition of the spilled oil is such that its weathering leads to formation of tarballs with density higher than that of ambient water. In this case the tarballs are expected to sink and remain on the sea floor, where they spend a very long time. The other possibility is that if weathering leads to formation of tarballs that float (i.e. their density is less than that of ambient water) then, with winds favourably oriented to take them away from the coast, the tarballs would get dispersed over the vast open sea where they spend a long time. In either case, the long term fate of the tarballs remains unknown.
For most people, an occasional brief contact with a small amount of oil, while not recommended, will do no harm. However, some people are especially sensitive to chemicals, including the hydrocarbons found in crude oil and petroleum products. They may have an allergic reaction or develop rashes even from brief contact with oil. In general, contact with oil should be avoided. If contact occurs, the area should be washed with soap and water, baby oil, or some cleaning compound recommended by a doctor as safe for skin. Avoid using solvents, gasoline, kerosene, diesel fuel, or similar products on the skin. These products, when applied to skin, present a greater health hazard than the smeared tarball itself. To clean beaches after tarballs arrive there, the tarballs may be picked manually. If the concentration of tarballs is severe, the top layer of sand containing the tarballs may be removed and replaced with clean sand. This might become necessary if freshly formed tar balls, which are soft and brownish in colour, get deposited on the beach. The soft tar balls have tendency to melt with the heat of the sun and the petroleum materials present get absorbed in the sand. Such tainted sand should be removed so that the pretroleum does not pose threat to the sand dwelling intertidal organisms such as bivalves and worms.
If corrective measures, like the ones given above are taken promptly, a beach can recover without long-term impact. In July, 2005, after the tar ball contamination episode at Mobor beach, Dr. Classy D’Silva analysed the total petroleum hydrocarbons (TPH) in water and sediments along Mobor beach. She repeated the observations in November, 2005 at the same site. She noticed that the levels of TPH in water and sediments had recovered to normal levels.
In summary, according to our present understanding, the tarballs that reach the Goan beaches results from errant ships carrying out illegal operations at sea. These lead to oil slicks that are small enough to go unnoticed in the open sea. Weathered products of the slick lead to formation of tarballs. If winds blow towards the coast, then the tarballs can travel to the coast of Goa, producing ugly black streaks on reaching the beaches there. This usually happens during May-October because the winds are suitably oriented at this time. Prompt cleaning of beaches is desirable when tarballs land on beaches to avoid long-term impacts.