mumbai

Updated: Jul 21, 2016 00:27 IST

Pollution levels at the Powai lake are five times the safe limit and the deteriorating water quality could endanger aquatic life, says a water quality study.

According to Central Pollution Control Board (CPCB) guidelines, levels of Biochemical Oxygen Demand (BOD) – the concentration of oxygen required for aquatic life – above 6 mg/L cannot sustain fish species. However, a study done between May and June by conservation group Naushad Ali Sarovar Samvardhini (NASS) found that BOD levels at five locations in the lake were between 27 and 34 milligrams per litre (mg/L).

As BOD levels above 3 mg/L make water unfit for human consumption, the 2.1 square kilometer lake can no longer be a source of potable water. It now supplies water to the surrounding industrial areas.

“The high pollution levels at the lake are due to heavy siltation, pollution due to human activities, untreated domestic waste and encroachments,” said PB Salaskar, water quality analyst and secretary, NASS, who tested the water samples at a union environment ministry and CPCB-approved lab and submitted the findings to the civic body.

Salaskar added that the growth of blue-green algae – cyanobacterial bloom that thrives in water bodies that receive nutrient-rich sewage and run-off – has contributed to the increase in BOD.

The growth of blue-green algae increased the lake’s oxygen supply, leading to a depletion of its aquatic life, said residents. (Badri Chatterjee/HT)

Residents said that the complete eradication of hyacinth – aquatic vegetation which chokes the water – was unnecessary and added to the problem. “The municipal corporation completely removed the hyacinth. Lakes require up to 10% water hyacinth as they absorb the heavy metals discharged into the water. Without them, the algal bloom increases, leaving a dense green colour to the lake and depleting the oxygen,” said Gordon Rodricks, vice president, Maharashtra State Angling Association.

Hyacinth in the lake was removed by the municipal corporation a few years ago. (Maharashtra State Angling Association)

Scientists said the lake becomes highly polluted in the summer when low water volumes and the concentration of sewage leads to high BOD. “The BOD levels will decrease during the monsoon as the rain will dilute the water,” said Rakesh Kumar, director, National Environmental Engineering Research Institute. “If the sewage from the surrounding areas is left untreated, the same situation will arise next year,” he said.

Municipal officials said that slums around the lake dump sewage into it. “However, over the past one year, cleaning operations have been carried out and the situation has improved,” said a senior official from the civic body’s hydraulics department. “Implementing a channeling technique in the area to treat waste before it enters the lake will take time,” he said.

“NGOs, residents and the civic body can be roped in to conduct regular cleanup drives. Only then can the quality of the lake be restored,” said Salaskar.

“At rivers or other large water bodies, faecal coliform – a bacterium found in human and animal faeces – and high BOD levels are the major concerns. BOD levels as high as 100mg/L have been noted. The main causes are open defecation and a lack of proper sewage treatment facilities. Sewage needs to be collected at a common point and then treated before it is discharged into the water body,” said A Sudhakar, additional director, Central Pollution Control Board.