From being a well-bodied river flowing through some localities of the northern suburb of Dahisar, including those adjoining the Borivali National Park, the Dahisar river is now reduced to a sewer, reflecting prolonged neglect.
According to the latest pollution data, taken last summer, provided by environmentalist Anjana Pant, the biochemical oxygen demand (BOD) level — the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample — in the river was 420 mg/l, way above the prescribed standard 30 mg/l.
“Dahisar is a small river that flows from the Sanjay Gandhi National Park to the mangroves through habitations, dhobi ghat and tabelas, carrying huge loads of pollutants as is evident from the levels of BOD. During summer, the water in the river dries up and therefore the concentration of the pollutants becomes higher. The river also ceases to flow at some points. The river may be considered as highly polluted,” she told The Hindu .
“Encroachments on the dry areas of the riverbed as well as on the banks of this river have choked the water course aggravating the risk of flooding. Field observations indicate that the river is dumped with debris from construction activities and industrial wastes on the banks, and dumping of municipal solid wastes coupled with inadequate annual de-silting efforts add to the woe. Ingress encroachments from the banks (building, industries, and slums) as well as modification of river-course and local diversion of streams have compounded the risk of flooding,” Dr. Pant stated in a study she conducted in 2015.
Some of the glaring encroachments she noted included a bridge along the Dahisar River between Western Express Highway and SV Road, marble shops near the Western Express Highway, Leprosy Colony, slum pockets between Bhagwati Hospital and Rustomjee Park, and Ranchhoddas Marg.
She suggested bioremediation, a waste management technique that involves the use of organisms to remove or neutralise pollutants, to address the pollution in the river.
SR Mathew of the New Link Residents’ Forum said recent changes had added to deteriorating the quality of the river water.
“Walls have been built on both sides of the river, leading to a reduction in the rivers silt deposits. There is still a lot of untreated sewage and effluents emptying out in the river. The river water has turned black, enough to indicate its pollution level. There are nearly 400 buildings along the river, which are not connected to the BMC sewage lines as they are at a lower level and the sewage lines are at a higher level. Moreover, the flyover above the river has its pillars sunk in the river bank, affecting its flow. Check dams built in the national park have also affected the flow of the river,” Mr. Mathew said.
Chief Minister Devendra Fadnavis recently announced that the government would implement a programme to tackle river pollution in the State. However, experts say the project does not focus on urban rivers that are in dire straits.
“The rivers and drains in any Indian city have become surrogate sewer systems and these drains mainly receive grey water and sewage along with occasional runoff water and Mumbai is no exception. Ultimately, the drains outfall into creeks via mangrove forests, transmitting their heavy pollution load into them. In case we do not take any action to salvage these rivers now, these rivers will dry up and leave no signs of their existence leaving Mumbai with a huge water and environmental crisis. The groundwater levels of Mumbai are already quite low. The government needs to deal with this issue on an urgent basis and discuss various existing decentralised options with experts and other stakeholders and do it on a pilot test basis to move further,” Dr. Pant said.