An unidentified ship which dumped bilge water at a point close to the coast is the most likely reason for the oil slick, which hit the beach along Uthandi and Kanathur around 3 p.m. on Friday, according to government officials.

Bilge water is a mixture of fresh water, oil, sludge and chemicals primarily collected from a ship engine’s heat exchange system.

“The whole coast was black and there was knee-deep black sludge along a three-km stretch,” said M. Arumugam, a local fisherman, on Saturday. The slick had started disappearing due to wave action.

According to a Fisheries Department official, it is against international regulations to dump bilge water, which is generated when water used as engine coolant mixes with other fluids, anywhere close to the coast.

“It is usually done to reduce the draft of the ship as the waste water adds to the weight of the vessel.”

MARPOL (Marine Pollution Convention) regulations to which India is a signatory states that oil and petroleum products must not be discharged anywhere within the Exclusive Economic Zone (200 nautical miles).

M.Rajendiran, Executive Engineer, Pondicherry Port, who had been sent to investigate if the slick could spread towards the Union Territory, said the standard operating procedure followed by ports is to collect the bilge water so that it would not be discharged into the sea.