Understanding impacts of nutrient pollution: a case study of Jakkur Lake

Recent events in Bengaluru like the Bellandur Lake catching fire and fish kills in various lakes have put a spotlight on the pollution problem in the city’s lakes. Bengaluru generates 1400 million litres (MLD) of sewage every day, of which only about 40% gets treated. In the absence of proper infrastructure, both treated and untreated wastewater finds its way into the city’s lakes and rivers. Recent data indicate that as much as 90% of our lakes are polluted.

Dr Priyanka Jamwal, Fellow, ATREE recently published a paper titled "Fate of Nutrients in Human Dominated Ecosystems: A Case Study of Jakkur Lake in Bengaluru" in the Indian Academy of Sciences' "Resonance" magazine.

Jakkur Lake receives almost 10 MLD of treated water from Jakkur sewage treatment plant (STP) in addition to a small stream of raw sewage. These pass through the constructed wetland before entering the lake. For a span of one year, Dr. Jamwal's team collected samples from the two inlets of wastewater into the lake, two locations within the lake and the outlet of the lake to understand how the inflows influence the quality of water and thereby the health of the lake.

The inflows from the STPs carrying treated and raw sewage are rich in nutrients like phosphorus and nitrogen. High nutrient concentration leads to an increase in macrophytes, eventually causing hyper-eutrophication. The expansion of algal blooms depletes the lake of dissolved oxygen, which is crucial for the aquatic fauna and the primary cause of dead fish.

The study reveals that by conventional standards, Jakkur Lake suffers from excessively high nutrient levels. Specifically, the study suggests that the phosphorous concentration needs to be reduced by approximately 96% from the current levels to prevent algal blooms. However, despite this gloomy picture, the lake supports a thriving fish and bird population, recharges groundwater and provides highly valued green space. So on what basis do we judge the health of Jakkur lake?

Dr. Jamwal calls for a different approach when it comes to managing water quality in human dominated ecosystems. She argues that lake water quality cannot be managed in the absence of clear definitions or standards based on the anticipated end uses of the lake water. Her study can allow decision makers to devise policies and strategies for managing the nutrient concentrations from various sources that meet the lakes of Bengaluru.