Environmental campaign group Greenpeace is blaming the discovery of a huge area of polluted “black water” off the Chinese coast on waste water from local dyeing and finishing factories.

The organisation said it had found toxic chemicals including hormone disruptor nonylphenol and heavy metal in an area of polluted water the size of 50 Olympic swimming pools off the coast at Eastern Shishi City.

It claimed the pipe belonged to the Haitian Environmental Engineering Co, leading from a communal waste water treatment plant for 19 dyeing and finishing factories in Shishi City’s Wubao Dyeing Industrial Zone.

Shishi is one of China’s biggest children’s wear manufacturing centres and exports more than 70% of its products, Greenpeace added.

“This is not an isolated case. Among 435 registered discharge points, more than two-thirds were caught violating environment standards in 2012, and one in every four of them never met the standards at all,” said Lee Chih An, toxics campaigner, Greenpeace East Asia.

“In this case, the communal waste water discharge pipe is 2.4km long and buried deep down the sea – that’s how far polluters are willing to go to hide their dirty secrets.”

Greepeace has submitted its findings to the Chinese government, and is calling for action.

It says a policy on hazardous chemicals that came into effect last March requires factories to register the discharge and transfer information of the hazardous chemicals they use by 31 January 2014.

Contrary to this requirement, however, no update has been disclosed to the public so far, the activists say.