"The way low-lying areas, rivers and land around the city are being polluted, groundwater could become toxic," said Prof M Mozammel Hoque, an expert of Buet's Institute of Water & Flood Management.

The experts said the risk of untreated toxic waste disposal from pharmaceutical factories and hospitals are increasing at an alarming rate and polluting the land surface.

"The toxic waste discharged on the land seeps through the ground and eventually reaches the groundwater," explained Prof Hoque.

Groundwater levels are decreasing due to over exploitation and the water of nearby polluted rivers on the city's periphery are gradually entering into the water table.

The rapid fall of groundwater level due to over extraction causes problems like water scarcity, contamination from polluted low-lying areas and rivers, increase in the cost of water lifting and the risk of land subsidence.

Rapid urbanisation has increased the demand of water day by day forcing the water supply authorities to lift huge amount of groundwater. Studies show the level of groundwater in the city is falling at a rate of up to 2 metres every year.

During the dry season the level of groundwater falls but in the rainy season the rain balances the level. But due to concrete pavements rainwater cannot reach underground.

"Our study shows that the level of groundwater in Motijheel in 2000 was 30 metres. If the groundwater falls in such a rate, you will need to go down to 92 metres in 2020 to find it. In Gulshan the level was 22 metres in 2000 and it would be 70 metres in 2020," said Prof Hoque. According to a survey jointly carried out by Department of Environment (DoE) and Canadian International Development Agency (Cida), more than 7,000 factories are located along the rivers and canals in Dhaka.

These factories, excluding the tanneries at Hazaribagh, discharge more than 60 million litres of toxic waste daily into the waters, land and air. The tanneries add to the massive pollution by pumping an additional 7.7 million litres of highly toxic liquid wastes directly into the nearby canals and the river Buriganga.

The tanneries also dump 135 tonnes of solid wastes into the river per day. Factories in Tejgaon industrial area drain wastes into the Begunbari canal that winds its way into Norai canal and the river Balu.

According to experts the most polluting industries in the city are tanneries, textile, pharmaceutical and chemical industries. As these toxic wastes accumulate on vast stretches of lands, the groundwater gets charged with contaminated water.

According to Dhaka Wasa, the annual water extraction in Dhaka city was 130 million cubic metre in 1983 while in 1999 it rose to 350m cubic metre. The private annual extraction in 1991 was 25m cubic metre while in 2004 it was 125m cubic metre.

The experts suggested that the government should take a programme to prevent city's groundwater from being polluted by toxic waste and over-extraction.

The programme may include protecting the present water bodies and increasing the use of surface water. The water from rivers like Dhaleswari and Padma can be used.

"At present the government is using some water from Shitalakhya river through the process of water treatment plant. If the government find it feasible then more rivers can be brought under the programme," suggested Prof Hoque.