95% sewage, 5% rain: How Bengaluru’s lakes turned into wastewater pools

Sewage, not rainwater, now fills a majority of lakes downstream of the Hebbal-Nagavara Valley, a study of 44 interconnected lakes has found.

Researchers said during dry years, nearly 95 per cent of downstream lakes depend mainly on sewage, while even in wet years, about 62 per cent receive more sewage than rainwater runoff.

Many lakes that once filled during the monsoon and dried in the summer have turned perennial, sustained by a steady flow of domestic wastewater. Upstream lakes, however, receive less sewage.

"Upstream lakes, typically on the city's outskirts where land is less developed, are still largely fed by monsoon runoff. Their water levels rise and fall with the rains, making them well suited to absorb flood surges and support seasonal ecosystems. Downstream lakes, by contrast, reflect the accumulated characteristics of everything upstream — and in Bengaluru, that means a steady diet of sewage," the research said.

The study, 'Cascading Urban Lakes: How Spatial, Temporal, and Anthropogenic Characteristics Control Lake Behaviour', was conducted by WELL Labs researchers Rashmi Kulranjan and Veena Srinivasan, and published in the journal 'Frontiers in Water'.

Rashmi said treated sewage inflow may not be entirely harmful if managed carefully.

"Entry of untreated sewage is not acceptable. But pumping in treated sewage water may help keep the lakes full and replenish groundwater. However, it is important to determine which lakes are suitable for it. Lakes filled with sewage water cannot act as flood mitigation zones and hence, it is crucial to ensure that sewage flow is controlled at lakes that are vital in preventing floods," she said.

GBA measures do not suit all lakes

From diversion channels to sluice gates, the Greater Bengaluru Authority (GBA) and earlier, the Bruhat Bengaluru Mahanagara Palike (BBMP) have introduced measures to improve lake health and prevent floods. The study found that interventions effective for one lake may fail in another.

"Sluice gates that drain lake volume before the monsoon were most effective upstream, creating buffer space for floodwaters before storms arrive. But for downstream lakes constantly replenished by sewage, sluice drainage offers little benefit; diverting treated sewage flows proves far more effective," the research said.

The study called for comprehensive planning across the interconnected lake network, noting that action at one lake affects others.

"The interconnected nature of the system means that what happens at one lake does not stay there. Treating or diverting sewage at upstream points has cascading benefits for water quality throughout the network. Conversely, pollution or mismanagement upstream degrades conditions for every lake below," the research said.

Diversion drains leave some lakes dry

Of the 44 lakes in the Hebbal-Nagavara Valley, 13 have engineered diversion drains that route dry-season sewage around the lake, allowing inflow only during heavy rains.

While intended to prevent pollution, this has left several lakes dry.

"While well intentioned, these drains mean some lakes now receive water only during peak rainfall events. Cut off from a constant sewage supply and with no other substantial inflow, they have become nearly perennially dry," the research said.

Nagavara Lake. DH FILE PHOTO