heavy metals are seeping into Kolkata's groundwater from Dhapa, its waste dumping ground on the eastern fringe of the
city. A study has found traces of chromium, zinc, cadmium and copper in areas around Dhapa. Copper and cadmium above permissible limits was also
Under pressure are the East Calcutta Wetlands (ecw), the city's natural and the only sewage treatment system, located near Dhapa. "Sewage of the entire city collects at ecw along with industrial waste from tanneries, battery and jewellery factories. These wastes contain heavy metals whose load is so high that the wetlands can't filter all of it," says hydrogeologist P K Sikdar of the Indian Institute of Social Welfare and Business Management, Kolkata.
Polluted water is permeating the aquifer through a number of deep-buried paleochannels (underground channels carved out by once-active streams and rivers). These have cut into the confining layer of the aquifer under Dhapa and ecw. The aquifer lies under an approximately 40-metre deep layer of clay in this region, from where groundwater is extracted.
Measures to reduce stress on the aquifer and ecw need to be taken now. "We have to be very cautious about how and at what rate we use groundwater," says Sikdar. Construction of highrises should be banned, he says. They use up a lot of groundwater because they usually don't get surface water supply. Use of surface water should be increased and the aquifer needs to be recharged through rainwater harvesting.
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