A third of Calcutta may sink if the drastic fall in the groundwater level is not arrested immediately.
The city’s underground water level has receded by seven to 11 metres between 1958 and 2003, a survey by the Central Groundwater Board of the Union water resources ministry has revealed.
The fall has been the steepest in the Park Street area, while some other precarious stretches are Fort William, Rajabazar, Kalighat, Camac Street and Ballygunge.
Another study, conducted by the city-based IISWBM, revealed that the water level in around 30 per cent of the city area has shrunk to the extent of triggering subsidence.
The central board’s report, submitted to the Calcutta Municipal Corporation, has attributed the rapid decline to excessive withdrawal of groundwater through deep tubewells without any effort at replenishment.
“The gap between the base of the clay cover, which lies just beneath the land surface, and the groundwater level is steadily narrowing. If the trend continues, there will be cave-ins,” said Pradip Sikdar, who teaches environment management at IISWBM.
The surface is most stable when the groundwater level enters the clay layer and remains close to the earth’s surface (see graphic). The situation is alarming — “supercritical”, according to experts — when the water level falls below the midpoint of the clay cover and remains close to the sand surface.
In the worst case — prevalent in around 30 per cent of the city area — the water level either comes very close or falls below the clay layer. “This is ideal for subsidence,” said Sikdar, who has been conducting the study for IISWBM that has identified several places where the projected subsidence ranges from 10 to 14 mm a year (see graphic).
“Signatures of subsidence have already been felt in the Rajabazar-Sealdah area,” stated a report by P.S. Ray, of the National Remote Sensing Centre, who had studied the city’s groundwater level through laser mapping.
A CMDA study in 1998 had identified 21 wards — in Sealdah-Maniktala, Ballygunge, Jodhpur Park and Beleghata — where the groundwater level was found to be in the “risk to high risk” zone.
The report by the Central Groundwater Board underlined the need to “reduce the stress on underground water” and “undertake recharging schemes”. R.C. Guha, a scientist of the board, said rainwater harvesting was needed to artificially recharge groundwater.
“The presence of the thick clay layer and metalled roads and concrete pavements prevent natural recharging. The only solution is channelling rainwater through pipes,” pointed out Guha.
The board has set up two such installations in Patuli, which are recharging groundwater at a rate of 1.95 lakh gallons a year.