The amount of heavy metals and ammonia in raw water supplied to Hong Kong reached unacceptable levels early this year, according to water supply chiefs. Water Supplies Department assistant director Damien Ku Chi-chung said during January and February the Muk Wu pumping station at the Man Kam To border detected levels of ammonia as high as two micrograms per litre - twice the acceptable level. He said the overall level of manganese also increased during the two months, but added that there had been an improvement in recent weeks. Excessive ammonia can cause vomiting, as can heavy metals, which can also cause long-term illnesses. Mr Ku said the pollutants had gathered in December during the period when water supplies from the mainland were stopped for maintenance. When supplies resumed, the pollutants were washed down. The department increased the level of chloride to neutralise the pollutants. 'The situation has stabilised and the level of ammonia is very good now,' Mr Ku said. He appealed for calm over the quality of river water from the Dongjiang in Guangdong, citing stringent inspections by his department and improvement work by the province on pollution. 'There's no denying certain pollutants exist, but when we supply water to the tap, every drop is clean and up to World Health Organisation standards,' he said. Cross-border anti-pollution co-operation progressed yesterday when a top-level SAR-mainland working group was officially formed. The Joint Working Group on Sustainable Development and Environmental Protection held its first meeting in Guangzhou. Led by Lily Yam Kwan Pui-ying, Secretary for Environment and Food, and Yuan Zheng, Director-General of the Guangdong Environmental Protection Bureau, the group will meet twice a year to discuss measures to deal with cross-border pollution.