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 recorded.

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.