The arsenic level in the groundwater of at least nine locations in the city is high enough to call for immediate intervention, a government study has revealed. The study has dubbed "borderline" the arsenic content in the groundwater of at least 10 places.

Around 20 blocks in North 24-Parganas, 10 in South 24-Parganas and three in Howrah - all districts neighbouring Calcutta - has arsenic levels in groundwater more than the tolerance level.

The information was shared with the Assembly on Tuesday by water resources investigation and development minister Soumen Mahapatra in reply to a question by Trinamul's Nakashipara MLA Kallol Khan, who had asked what steps the government had taken to check the quality and chemical content of water.

"The study conducted by our department over the past one-and-a-half years has revealed nine places where the situation needs immediate intervention. They include Sinthee More, Prince Anwar Shah Road, Prince Ghulam Mohammad Shah Road, Vivekananda Road, Darga Road, Balia More in Garia and RK Mukherjee Road in Baranagar.

"At all these places the groundwater has arsenic content more than the tolerance level prescribed by the World Health Organisation - 0.01 mg per litre (parts per million or ppm)," the minister said.

The highest level, according to the study, has been found at Sinthee More (0.096ppm), followed by Vivekananda Road (0.049ppm) and Darga Road (0.047ppm).

There were at least 11 more locations where the arsenic content in the groundwater has been dubbed borderline. "At these places, the level is 0.01ppm and could increase any day. They include Amherst Street, Biren Roy Road, Thakurpukur, Anandapur, Mukundapur, Bidhan Sarani, Cossipore and Chitpur," he said.

The minister later told reporters that groundwater samples from 31 places in the city were tested following reports that residents of multi-storeyed buildings at these places were drawing on this source. "The residents should depend less on groundwater and install rooftop water harvesting units. Otherwise, repeated use of arsenic-contaminated water could lead to skin or kidney problems, including cancer," he said.

The government will introduce a bill in the next session of the Assembly seeking more state control over groundwater use and better water management. Mahapatra hinted that the bill would have provisions for penalties for misuse.