Date:12/05/2006 URL: http://www.thehindu.com/2006/05/12/stories/2006051204520800.htm

Tamil Nadu

Chembarambakkam lake under `green attack'

Vani Doraisamy

Water hyacinth, aquatic plants now cover half the water body Water hyacinth, aquatic plants now cover half the water body

CHENNAI: The Chembarambakkam Lake, one of the city's main drinking water sources, is under "green attack." It is losing water due to the invasion by aquatic vegetation, according to a study done by the Centre for Remote Sensing and Geoinformatics, Sathyabama Deemed University. The centre is a collaborative venture of the varsity and the Indian Space Research Organisation.

The study was done by scientists A. Surendran and V.E. Nethaji Mariappan over two years starting 2004 using remote sensing data collected through Landsat, with data collected in 2000 and 2002 as reference points. Though not immediately visible, the weed was spotted just below the surface, remaining dormant as the water level has increased now but ready to surface once it recedes.

"We found the lake slowly being taken over by three types of aquatic plants, Eichornea [commonly known as water hyacinth], Prosopis juliflora and Ipomea Cornea."

Though the vegetation was on the decline between 2000-2002, it has been steadily increasing since 2004.

Aquatic cover

Though in April 2004, the vegetation was concentrated only on the central and southern part of the lake, the spread increased by December. Of the 656.43 hectares of the lake, the dense aquatic cover has taken over nearly 333 hectares in the centre," says Mr. Mariappan.

This means that the lake will lose more water through evapotranspiration (loss of water through leaves through transpiration) rather than just evaporation.

"In India, 26 per cent more water loss has been observed from water bodies through water hyacinth cover.

Evapotranspiration rates are two to eight times more than the evaporation from a free water surface and the water loss through leaf cover is around 7.76 times [with a 1.36 times margin of variation] during different seasons," the scientists say.

Apart from water loss, the proliferation of vegetation is also an indicator of aquatic pollution, especially chemical oxygen on demand (COD) and biological oxygen on demand (BOD), caused by the presence of toxic organic content and leachate of chemical nutrients such as nitrates and phosphates from agricultural fields through the adjoining Bangaru Canal.

"We found the vegetation increase was due to an increase in nutrient-rich sediment load. Also, a chain of tanks starting from Sriperumbudur drain pollutants such as heavy metals into the lake, from large scale industrial units in the belt.

This provides more COD and BOD to the existing flora and fauna which degrades the ecosystem as water loss would be enormous," according to Mr. Mariappan.