The mismatch between rapid urbanisation and lack of sufficient sewer infrastructure has threatened the relatively cleaner stretch of the Cooum river. Residents have raised concerns over persistent sewage pollution near the Putlur check dam, located 40 km from Chennai.

Residents of Manavala Nagar, Tiruvallur, said groundwater levels in the neighbourhood had taken a hit after portions of the riverbed were dried up for bridge construction as part of the Chennai Peripheral Ring Road project, a six-lane expressway.

Sewage pollution in the upper reaches of the river can also affect Chennai’s water source, as water diverted through the Puduchatram and Korattur check dams eventually reaches the Chembarambakkam drinking water reservoir.

Pointing out that water stored near the river’s checkdams was a pivotal source of groundwater recharge, residents said this has worsened sewage issues.

D. Anandakrishnan, a resident of Manavala Nagar, said “The entry of untreated sewage directly into the river, particularly near the dry riverbed, will worsen groundwater pollution and undermine efforts to store freshwater near check dams. We have been demanding that the dysfunctional Periyakuppam sewage treatment plant be upgraded to handle growing volume of sewage generated.”

The local body, which has sunk borewells along the river, also faced scarcity after water was pumped out for the bridge construction, he said.

Suresh Ramanathan, president, Manavala Nagar Residents Welfare Association, said “Groundwater has already depleted in areas like Vengathur, Putlur, and Manavala Nagar by a depth of 20 feet after the riverbed was dried up for the work. Temporary storage structures should be built in the upstream or downstream portion of the river to store water and avert water scarcity in the localities.”

Residents said several representations had been submitted seeking steps to arrest untreated sewage flow into the river and restore waterway as a groundwater source.

Officials of the Water Resources Department said a joint inspection was recently conducted with the Municipal Administration and Water Supply Department and the Tamil Nadu Pollution Control Board. The team has suggested that the sewage treatment plant’s capacity be increased to cater to the sewage generation of the rapidly urbanising Tiruvallur municipality.

While advanced soil biotechnology is being deployed to improve quality of treated sewage discharged into the river, the plant capacity was not inadequate. A report suggesting measures to mitigate sewage pollution in the river has been submitted to the Tiruvallur local body and district administration, officials said.

Noting that the work for the Chennai Peripheral Ring Road project across the river portion would not adversely impact groundwater, an official said the contracting firm had been advised to build a temporary bund downstream of the work site.