The level of pollution in the Buriganga and most parts of Turag and Norai flowing around the capital is so high that no living organism can survive in the waters of these rivers, researchers say.
A three-year research finds that some invertebrates and small organisms come into being in these rivers when water flow increases during rains. But these life forms completely disappear in the dry season, they add.
The oxygen level of the Buriganga, Tongi canal of Turag and a part of Norai is less than one in per microgram. All species of local fish need 4-6 level of oxygen in per microgram to survive, says the study conducted by the water resources department, Bangladesh University of Engineering and Technology (Buet).
The researchers also conducted their study at 34 spots in 25 rivers in the northeastern region from April 2005 to April 2008. The outcome of the study also contributed to the department's preparations for a map showing quality of the country's 25 rivers.
During water quality assessment using bio-indicators method, the researchers collected samples thrice from the Chadnighat point of Buriganga but twice failed to get any single living being.
Bio-indicators respond to the quality deterioration from multiple and varied pollution sources and thus provide an overall picture of water quality.
"In this method we collect sediments from the riverbed and test the environmental health of the river by counting available aquatic creatures in those. The creatures in the sediments vary on the basis of pollution level," said Prof Dr M Fazlul Bari of the Buet department.
In the third time, the team found some leech, aquatic worms and small snails, said Shah Alam, research assistant of Prof Bari.
"Third time we collected samples from the Buriganga after rainfall. We guess some creatures came with the water from outside; otherwise there's no way of getting there," added Alam.
During the study the researchers collected samples and tested those in pre-monsoon and post-monsoon seasons.
In the post-monsoon season the oxygen level increases slightly but yet not enough, the researchers say. They found that oxygen levels were .7 and .25 in Chadnighat point, .27 and .63 in the Norai near Trimohini, and .27 and .63 in the Turag in Tongi before and after the monsoon respectively.
Besides the rivers around the city, 25 spots in Mymensingh, Sherpur, Jamalpur and Tangail, three in Gazipur, three in Narayanganj and three in Chadnighat, Tongi and Trimohini came under the research.
The selected sites represent a gradient of river pollution from less polluted and moderate polluted to severely degraded stretches of rivers.
The researchers have found the Turag, Balu, Buriganga and Shitalakkhya to be the most affected rivers.
They say the waters in these rivers are affected by industrial effluents and wastewater. The water is also affected by municipal sewage disposal faecal contamination, agro-chemicals and large amount of suspended sediments carried by upstream flow.
The level of discharging waste material into the rivers is so high that the condition of the rivers could be even worse, the researchers warned.
"Still these rivers have flowing waters, so the pollution level goes up and down. Otherwise, the situation would be even worse," said Prof Bari.
Moreover, every day 100 tonnes of solid waste including trimmings of finished leather, shaving dusts, hair, fleshing, trimming of raw hides and skins are dumped into the Buriganga, posing a serious threat to the environment.
Besides, industrial waste from a few thousands industries in Savar, Ashulia, Tejgaon and old parts of the city and its suburbs is getting directly mixed with the river waters, polluting the rivers for decades.
Lack of proper management, more than two-thirds of the city sewage is falling into the rivers garlanding the city which is another major source of water pollution.