All water samples taken October 16-21 have styrene levels within standards prescribed by the Ministry of Health, said Vu Dang Dinh, the spokesman of the committee.

Tap water samples will continue to be taken from Thanh Xuan, Hoang Mai, Nam Tu Liem, Cau Giay, Hoai Duc, Dan Phuong, Thach That and Quoc Oai districts until the end of October, and their results publicized in local media, Dinh said.

He also said that the city will continue to provide clean water in tankers and containers for citizens who demand it. The Vinaconex Water Supply Join Stock Company (Viwasupco) has been asked to supply water free of charge until the end of the month.

Viwasupco would also need to install automatic monitoring systems to continually monitor water quality at its sources, review its treatment process and build its own reservoir for sourced water, Dinh added.

Hoang Trung Hai, the city's Party chief, said Tuesday that Hanoi still lacks water quality monitoring systems, causing the city to react slowly to the recent crisis.

"We can't allow a situation in which no monitoring system managed to detect pollution, then when pollution is finally detected, we don't know how to deal with it," Hai said, noting how Viwasupco hesitated over whether it should cut water supply or not after discovering that its water source was contaminated.

There have been "loopholes" in the sharing of information between relevant authorities, and when this was combined with a lack of a response protocol for crises, confusion ensued. Authorities "didn’t know which data should be trusted," Hai said.

"In the future, Hanoi will formulate procedures to manage and monitor its water sources and force water supply companies to abide by set standards," he said.

Nearly nine tons of used oil had been dumped into a creek in Ky Son District, Hoa Binh Province, which eventually contaminated Hanoi's water supply, police said Saturday.

The oil was umped into a creek in Hoa Binh that feeds into the Da River, which is sourced by the Vinaconex Water Supply Join Stock Company (Viwasupco), on October 8.

Ly Dinh Vu, 37, Nguyen Chuong Dai, 25, and Hoang Van Tham, 33, have been arrested by Hoa Binh police for their involvement in the dumping. Police have said they are continuing the investigation into the roles of other people and/or organizations in the environmental crime.

Residents first detected an unpleasant, pungent odor in their tap water on October 10. Tests of the smelly water later found that the level of styrene, an organic compound that is classified as "probably" carcinogenic, was 1.3-3.6 times higher than normal.

Hanoi authorities then warned residents that they should not use the water for drinking or cooking, only for other purposes like washing clothes and bathing. They continued to maintain the advice even after water supply was restored Thursday after being suspended for day for Viwasupco to clean its pipes and tanks.