Impact more in south Kolkata as innumerable new multi-storied buildings there run on groundwater
Groundwater now flows from west to east in Kolkata, according to a new study. Photo: iStock
Over-withdrawal of groundwater in Kolkata has created a ‘trough’ bigger than the city itself and turned three-fourths of its groundwater fully or partly brackish, a recently published study has stated.
The study is based on data from the mid-1980s to 2016. It was carried out across a 436 square kilometre area located on the east and west banks of the Hooghly river and covered the Kolkata and Howrah cities and their peri-urban areas.
The report, co-authored by Sumanta Banerjee and Pradip K Sikdar, said:
In the past two-and-a-half decades, due to over-withdrawal of groundwater, the groundwater trough in south-central Kolkata increased by 266 sq km and resulted in the deterioration of fresh water into blended and brackish water.
“The impact is more in the southern part of Kolkata as innumerable multi-storied buildings have mushroomed in the area compared to north Kolkata. Most of them run on groundwater,” it added.
The trend of water level decline was about 0.33 metre per year (about a foot) at the centre of the trough in Park Street and 0.11 m / year (about four inches) towards the periphery, the paper noted.
“We define an area as a trough when the groundwater level goes about 15 metres below the surface. The trough created below the city is bigger than its surface area,” Sikdar, the lead scientist in the study, said.
The Kolkata Municipal Corporation (KMC)’s surface area is about 200 sq km.
The study explained how the trough created due to over-extraction of groundwater, has completely changed the dynamics of groundwater flow in Kolkata.
“The flow of groundwater in Kolkata was from north to south in the mid-1950s. That changed during the mid-1980s when it entered the city from all directions,” Sikdar said.
This has now changed further after the creation of the large trough. The flow direction is now west to east, which has led to brackish groundwater from Howrah mixing with Kolkata’s groundwater, he added.
“A comparison of the mid-1990s and the present study reveals a reduction of the area of fresh water by 74 per cent and an increase in the area of blended and brackish water by 42 per cent and 32 per cent, respectively,” the report said.
The paper said the gap between the demand for treated water in Howrah and the supply has been mostly filled up by tapping groundwater indiscriminately. This, in turn, has caused rapid depletion of groundwater in Howrah since 1996.
“As a result of continuous extraction of groundwater, the poor quality groundwater in shallow aquifers had leaked into the middle and deeper aquifers,” the report said.
The experts have pointed out that shallow groundwater gets contaminated due to a range of reasons, natural as well as human-made. These include industrial water pollution, domestic sewage and others.
“In Kolkata, the built-up area increased from 100-162 sq km during 1980–2014, while the areas occupied by wetland and vegetation reduced by 40 sq km and 23 sq km respectively,” the report noted, underlining how concretisation had eaten up part of the city’s water recharging area.
The report also said the gradual lowering of groundwater levels was triggering subsidence in the city. “Apart from lowering of the piezometric surface, over-pumping of groundwater due to urbanisation has also resulted in land subsidence,” it stated.
Senior officials in the civic body have however pointed out that gradual discontinuation of groundwater extraction within the KMC area has slowed the speed of groundwater depletion in the city of late.
“The groundwater extraction by the KMC in the core city area has been almost stopped, though some extraction is going in the added area of the city,” a senior KMC official told this reporter.
The official however admitted that rampant extraction of groundwater remains the norm in most multi-storied buildings in the city.
“According to our findings, the rate of groundwater extraction has slowed down in the city as the KMC has mostly switched over to surface water,” Abhijit Mukherjee, groundwater expert from Indian Institute of Technology, Kharagpur, said.
The study is titled Hydrochemical fingerprinting and effects of urbanisation on the water quality dynamics of the Quaternary aquifer of south Bengal Basin, India
It has been carried out by groundwater experts from the Department of Environment Management, Indian Institute of Social Welfare and Business Management in Kolkata. It was published in the international journal Springer Nature February 15, 2022.
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