ĐÀ NẴNG — Poor levels of dissolved oxygen (DO) in water was blamed as the main reason for the mass fish deaths at Đà Nẵng’s 29-3 Lake – one of the biggest lakes in the park of the same name.

Director of the city’s natural resources and environment department, Lê Quang Nam, confirmed yesterday the oxygen deficiency in the water caused fish to die en masse over the past two days, Monday and Tuesday.

Nam said measurements of DO in the lake water were 50 per cent lower than usual, and pollutants were released from the city’s drainage system to the lake.

Director of the city’s drainage and waste water treatment company, Mai Mã, said it was the worst mass fish death that had occurred in the lake.

“Some fish were often found dead in the lake in hot summer weather, but it’s very serious this year. We suspect that pollution would be the main cause of the mass fish dead,” Mã said.

He said four tonnes of dead fish were collected in two days.

Mã said the company had collected four samples of water at different positions in the lake for tests.

Chairman of the city’s People’s Committee, Huỳnh Đức Thơ asked relevant agencies and departments to find out the reason of the fish deaths.

In different researches and surveys in previous years, experts warned that almost all lakes in the city were contaminated with toxic metals and other dangerous substances in wastewater, while waste water has yet to receive treatment before being released into the environment.

Deputy head of the environment faculty from Đà Nẵng Technology College, Trần Văn Quang, said pollutants from industrial parks, untreated drainage and wastewater in the city and industrial centres polluted the water surface and water sources.

He said hazardous heavy metals such as mercury, lead and copper were found in 10 major lakes in the city, and the worst polluted is the 29-3 Lake.

He had found excessive heavy metals contained in lake sludge that would kill fish en masse.

Wastewater from the city’s drainage system carries contaminants to the 11ha lake, which accumulates in sludge to contaminate the lake for a long time.

The city has invested VNĐ15 billion (over US$700,000) in wastewater treatment, but that’s still a modest amount. Meanwhile, household sewage, rubbish, dust and soil are still dumped into some lakes in the city. — VNS