CHENNAI: A recent field study by a Chennai-based firm recorded critically low dissolved oxygen levels in the Thenpennai river at Kelavarapalli dam near Hosur, where the river enters Tamil Nadu.
The study found dissolved oxygen (DO) levels of just 0.1 to 0.2 mg/L at the point where the river enters from Karnataka. According to the firm, BioSynk, the near-zero oxygen level indicates severe organic pollution and conditions unsuitable for aquatic life.
Healthy freshwater ecosystems generally require DO levels of around 7 to 9 mg/L, while prolonged exposure to levels below 4 mg/L can adversely affect fish and other aquatic organisms.
Kelavarapalli dam, located about 8 km from the Karnataka border and 10 km from Hosur, has been witnessing persistent formation of toxic-looking foam along several kilometres of the river. The foam is generally associated with sewage and other pollutants entering the water, while turbulence caused by water falling from a height further contributes to frothing.
Tamil Nadu Environment and Climate Change Minister VK Rajeev inspected the dam on July 7 amid concerns of pollution.
Field tests recorded total dissolved solids (TDS) levels between 485 and 491 ppm (the healthy level is 50 to 300). The team also reported elevated levels of phosphorus and nitrates in the water. According to the firm, the presence of these nutrients is contributing to excessive frothing and the extensive growth of water hyacinth in the reservoir.
The dam has a watershed area of about 3,676 hectares and is an important water source for the SIPCOT Hosur drinking water scheme and the Hosur corporation's water supply system.
The pollution of the river has also raised concerns among farmers, with the study team stating that more than 8,000 acres of agricultural land depend on the water source. Farmers are reportedly using the water for irrigation, including flower cultivation.
The team claimed that prolonged use of the polluted water could affect groundwater quality, with open wells and borewells in agricultural areas also showing signs of contamination. It further said farm workers handling the water had reported skin rashes and allergies, while livestock were unable to consume the water.
Dakshayani S Dalavai, managing director of BioSynk and a member of the expert team, said the interstate pollution issue could be addressed without relying solely on conventional sewage treatment plants (STPs), which require continuous power, manpower and maintenance.
She proposed an inline treatment system at the Karnataka-Tamil Nadu border, using the natural gravity flow of the incoming water to treat the polluted stream before it enters the reservoir.
Dalavai said similar modular, gravity-assisted systems had been deployed in river basins in northern India and Gujarat. She said a policy intervention by the Tamil Nadu and Karnataka governments could address the 15-year-old issue within one or two months.
Farmers and residents have urged both governments to take immediate measures to prevent further pollution, protect agricultural livelihoods and restore the quality of groundwater in the region.