CHENNAI: The presence of water hyacinth is a strong indicator of sewage contamination in any waterbody. Most lakes and ponds in Chennai suffer from this problem.
But for the first time, one of the city's largest reservoirs, Chembarambakkam lake has been affected by this issue.
Most of the natural inlets of the reservoir are contaminated with domestic sewage from surrounding houses and even small scale industries. A visit by Express on Tuesday confirmed this.
Inlets which are meant to carry run-off water to the reservoir have become sewage carriers over the years. The effects of this are for everyone to see.
Satellite images showed that for a radius of three kilometres, hyacinth has covered the waterspread area of the reservoir right in the middle.
Raj Bhagat Palanichamy, a remote sensing expert, who constantly monitors waterbodies in Chennai found this abnormal growth of invasive weeds spread over the lake since last month.
"Usually, I keep a check on the reservoir levels and crosscheck using satellite images. This time I found a huge discrepancy between the images and data. Due to the hyacinth growth, the satellite couldn't estimate the water volume," said Raj, who works with World Resources Institute, India.
Most of these inlets enter the reservoir from the Bengaluru-Chennai highway which cuts across many residential settlements like Kannadapalayam, Kuthambakkam and Narasingapuram.
Water channels originating from smaller waterbodies in Mevalurkuppam also reach the lake, which carry untreated sewage.
"Sewage from industries and houses has all the nutrients that water hyacinth needs to survive. During summer when the lake was dry, the sewage draining into the reservoir was very visible. Now with good rains, sewage is able to penetrate further into the lake," said Raj.
Locals from the suburbs who frequent the area also said that this weed cover was spreading at an alarming rate.
"I visited the reservoir a month ago. I could see seemai karuvelam weeds and water hyacinth only near the entrance. When I visited again on Sunday, it looked like the whole reservoir was filled with this," said Nandakumar, a student from Kundrathur.
Even when the reservoir was up to the brim after the floods in 2015, such invasive weeds were not there, said Dr Saiprasath, a resident of Chitlapakkam. "Along with tall semmai karuvelam weeds, a native weed that locals call thakkai poondu also grows on top of the water. The PWD officials need to get rid of this soon," he said.
A senior Public Works Department (PWD) official said that mixing of sewage in the reservoir has been happening for quite long now.
"Water hyacinth is a strong indicator of presence of sewage. It is hard to detect where it is coming due to the vast size of the lake. But we plan to set up modular STPs near all waterbodies in Chennai soon to curb this menace," added the official.