Dongguan authorities have admitted for the first time that water from the city's section of the Dongjiang, or East River, contains excessive levels of heavy metals and industrial pollutants and is not suitable for drinking for at least six months of the year.

The Dongjiang, one of the world's most polluted rivers, supplies drinking water to Hong Kong and a dozen Pearl River Delta cities including Shenzhen, Guangzhou, Dongguan and Huizhou . The water used by Hong Kong does not come from the Dongguan section but from further upstream.

A study conducted by Dongguan's water resources bureau found that the quality of water in the river had worsened steadily in recent years, with most of the pollutants being heavy metals from untreated sewage discharged directly into the river, reported.

The authorities said levels of carcinogenic nickel had increased dramatically.

In 2010, Dongguan authorities found that only 54 per cent of 114 water samples taken from the Dongjiang met the mainland's drinking standard.

In May, laboratory tests showed that many water samples from the river contained too much ammonia nitrogen and were not even suitable for industrial use or swimming.

said the authorities blamed mining near the upper reaches of the river for the heavy metal pollution. They said they had never made water quality data for the Dongguan section of the river public before because they were worried it could cause a panic.

All but one of Dongguan's 124 waterworks are incapable of treating the water sufficiently to make it drinkable.

Authorities said they still used treatment methods that could not filter out industrial pollutants and agricultural chemicals, and more than seven million residents did not have access to reliable supplies of safe drinking water. At least 83 of the waterworks were caught supplying poor quality water between January and May.

This article appeared in the South China Morning Post print edition as: Dongjiang water not fit to drink