Tamil Nadu toxic foam crisis: Bengaluru sewage chokes rivers and dams

Tamil Nadu is witnessing an alarming environmental crisis as toxic foam and green-coloured water continue to engulf major water bodies, raising fresh concerns over untreated sewage flowing downstream from Bengaluru into the state.

The latest crisis has unfolded at the KRP (Kallavarapalli) Dam in Hosur, Krishnagiri district, where a thick layer of froth has been accumulating for nearly a month following heavy rains. Officials say the contamination threatens agriculture, aquatic life and public health, while local farmers fear long-term damage to irrigation sources.

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Krishnagiri District Collector C Dinesh Kumar has confirmed that the heavy foaming is the result of massive sewage discharge originating from Bengaluru city.

Pollution trail

The environmental crisis has its roots in the South Pennar (Thenpennai) river system. Originating from the Nandi Hills near Bengaluru, the river flows into the Kallavarapalli Dam after passing through heavily polluted upstream lakes such as Bellandur Lake, Agara Lake and Varthur Lake.

Years of untreated and partially treated sewage entering these lakes have significantly degraded water quality downstream. The Central Pollution Control Board (CPCB) has classified this stretch as a Priority-I polluted river segment due to severe contamination.

The situation worsened due to heavy rainfall, which increased the volume of polluted water entering the reservoir and resulted in large-scale foaming.

Farmers alarmed

Farmers dependent on the KRP Dam for irrigation say the crisis has reached an alarming stage.

R Narayanasamy, president of the Kallavarapalli Dam Water Users Association, said the pollution is primarily caused by domestic sewage rather than industrial chemical effluents.

According to him, the contamination threatens more than 8,000 acres of agricultural land dependent on the reservoir for irrigation.

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Other farmers have urged both the Tamil Nadu and Karnataka governments to inspect industries and mills situated along the South Pennar river to identify additional pollution sources.

Oxygen collapse

Officials from the Water Resources Department have warned that dissolved oxygen levels in the affected waters have dropped below one milligram per litre, creating conditions in which fish cannot survive.

Legal intervention into the issue began in 2020 when the National Green Tribunal (NGT) took suo motu cognisance of the pollution and constituted a joint committee to investigate the matter.

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The committee concluded that Bengaluru's municipal sewage was the primary source of pollution affecting downstream water bodies in Tamil Nadu.

Sewage gap

Bengaluru generates nearly 1,329 million litres of sewage every day, but its existing sewage treatment infrastructure can process only around 830 million litres.

This leaves nearly 500 million litres of untreated sewage entering downstream rivers every day.

Although authorities are constructing 12 additional sewage treatment plants to bridge the treatment gap, the ecological damage in Tamil Nadu continues.

Pollution confirmed

Water quality tests conducted by the Tamil Nadu Pollution Control Board (TNPCB) between April 2025 and March 2026 detected exceptionally high concentrations of ammonia, nitrogen, phosphates and faecal coliform bacteria entering Tamil Nadu from the Karnataka border.

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The excess nutrients have triggered eutrophication, a process in which nutrient-rich waters promote explosive growth of algae and invasive water hyacinth.

This has given the KRP Dam its distinct green appearance while further reducing oxygen levels in the water.

Authorities are currently removing water hyacinth from the reservoir, while the Tamil Nadu government has formally urged Karnataka and the Central Pollution Control Board to implement a permanent solution to the recurring problem.

Palar concern

A similar environmental concern has now surfaced along the Palar river in Tirupathur district.

In a coconut farm near Vaniyambadi, a farmer found thick foam emerging from his irrigation well after switching on a motor pump, raising fears that groundwater may also be affected.

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District Environmental Engineer Kalaichelvi said an immediate inspection could not be conducted because officials were attending a ministerial review meeting. However, she assured that a team of engineers would inspect the affected well.

NGT findings

The latest incident resembles earlier findings submitted by the Tamil Nadu Pollution Control Board before the National Green Tribunal regarding recurring frothing beneath the Marapattu bridge.

Investigations found that nearly 7 million litres of untreated municipal sewage enter the Palar river every day from Vaniyambadi and Udayendram because of inadequate sewage treatment infrastructure.

Although more than 130 tanneries operate in the region, officials said these industries are connected to a Common Effluent Treatment Plant (CETP), and recent inspections did not find evidence of direct industrial discharge into the river.

Groundwater contamination fears

A local resident alleged that groundwater contamination has become a serious concern.

"The groundwater has been polluted. Many companies have installed borewells and are mixing sewage into them. The nearby panchayat and municipal sewage is also directly discharged into the Palar river," the resident said.

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As farming communities across Krishnagiri and Tirupathur continue to grapple with deteriorating water quality, experts say comprehensive municipal sewage treatment and stronger interstate cooperation remain critical to protecting rivers, agriculture and public health.

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