The Poisar river, originating from the Sanjay Gandhi National Park, is polluted over 100 times the safety limit — making its water unsafe for human consumption and uninhabitable for both plant and animal life.
Researchers working on the Nallah Restoration Report conducted biochemical oxygen demand (BOD) tests — an analysis aimed at determining the quantity of oxygen required by plants and animals for surviving in water — only to find that the number was as high as 414.73 mg per litre at certain locations along the seven-km river.
The report was compiled by the Environmental Policy and Research India (EPRI) over 2014-15.
Central Pollution Control Board guidelines state that a BOD reading of more than 3 mg per litre means that the water is unfit for human consumption, and anything above 6 mg per litre makes it unsuitable for fish to survive.
The report comes days after thousands of city residents participated in a River March on Sunday to identify sources of pollution, besides ways to rejuvenate Mumbai’s four rivers — Poisar, Dahisar, Oshiwara and Mithi.
“The Poisar is highly polluted. While direct contact with this polluted water could cause severe skin problems in humans, it cannot support aquatic plants and animal life due to scarcity of oxygen below the water’s surface,” said Avick Sil, regional director, EPRI.
The study analysed samples of waste water collected from 10 equidistant locations between Damu Nagar and Hiranandani. It looked at physical and chemical parameters to identify sources of pollution and find ways to clean up the river.
“We found many children playing in the water. The water pollution needs to be treated and brought down to 10 mg per litre (to make it relatively safe for them),” Sil said.
The river was high on chemical pollution too – the chemical oxygen demand (COD) or amount of chemicals in the water was seven times higher than the permissible limit. The study said that though the COD level of normal sewage water is 100 mg per litre, its levels were as high as 767.1 mg per litre at certain areas along the Poisar.
The study has recommended lowering BOD and COD levels by increasing the amount of oxygen in the water. “Certain plants — common reed, spike rush, tube sedge and fan grass — can grow in polluted waters and absorb nutrients such as sulphate, nitrate and phosphate to aid their metabolism. In turn, their roots release oxygen, causing pollution levels to decrease substantially,” the report read.
The Poisar flows westward from the Sanjay Gandhi National Park to meet the Malad creek, before joining the Arabian Sea.