Excessive groundwater extraction causing parts of Delhi-NCR to sink: Study
Around 100 sq km of area in the national capital region has high risk of ground displacement, with the largest of these, of around 12.5 sq km, is in southwest Delhi’s Kapashera, barely 800m away from the airport, researchers have found using satellite data.
The alarming rate of groundwater depletion in the capital may be leading to a different kind of slow evolving crisis – parts of the surface in the city is sinking, a phenomenon known as land subsidence, potentially affecting the international airport.
Around 100 sq km of area in the national capital region has high risk of ground displacement, with the largest of these, of around 12.5 sq km, is in southwest Delhi’s Kapashera, barely 800m away from the airport, researchers have found using satellite data.
The rate of “sinking of land” in the neighbourhood near the airport is accelerating and the subsidence feature is rapidly expanding towards the airport, potentially threatening it, the researchers from IIT Bombay, German Research Centre for Geosciences, Cambridge and the Southern Methodist University, US said in a study titled Tracking hidden crisis in India’s capital from space: implications of unsustainable groundwater use, which was published in the peer-reviewed Nature journal.
“During the years 2014–2016, the subsidence velocity was found to be approximately 11 cm/year which rose significantly by almost 50% over the next two years to around 17 cm/year. The trend remained almost same during 2018–2019,” the study said.
Of all the areas that are under threat, Kapashera near the airport was the most vulnerable because the subsidence rate is extremely high, said Shagun Garg, a doctoral researcher from the University of Cambridge, who was part of the team that studied these land deformations.
“Airport requires stable ground because of the risk of major disruption if there’s significant ground movement,” Garg said. “Kuala Lumpur airport is an example to understand the implications of land subsidence where cracks on taxiways, and waterlogging emerged due to soil settlement. Continuous monitoring of IGI IIndira Gandhi International) airport and its connecting roads is therefore crucial.”
The study found another deformation in Mahipalpur, just 500m away from the airport where deformation of 15mm per year were observed in 2014-16, 30mm a year in 2016-18 and 50mm per year in 2018-19.
A widespread global problem, land subsidence occurs when large amounts of groundwater are withdrawn from some types of rocks and underground soil. When water is extracted from aquifers, the clay between pockets of water collapse gradually, leading to deflation of the land beneath. It isn’t like a sinkhole or a cave in, whose impacts are sudden, but occurs over a very large area in a gradual manner.
With a projected demand of 1,236 million gallons per day (MGD ), water-stressed Delhi has a 300 MGD demand-supply gap. According to the draft Master Plan 2041 , this demand is likely to increase to 1,746 MGD by 2031.
A large part of unmet demands are met through groundwater extraction. At certain locations in southwest Delhi, the water table is available at a depth of 80 m and this continues to deplete at the rate of 3-4 m a year. More than 80% of land subsidence across the world is caused due to excessive groundwater extraction, according to the US Geological Survey.
The study has found that an area of approximately 100 sq km in the national capital region is under “high risk of ground displacement.” The areas that require immediate attention include Bijwasan, Samalkha, Kapashera, Sadh Nagar, Bindapur, and Mahavir enclave in Delhi; Dundahera, Sector 22A, and Block C in Gurgaon; and pocket A, B, C of Sanjay Gandhi Memorial Nagar in Faridabad.
Despite its slow pace, land subsidence is known to cause losses worth billions of dollars all over world due to damage to infrastructure, roads, pavements, and underground utilities such as drainage system. The sinking also leads to aggravated flood risk and more frequent waterlogging.
The researchers also suspect that land subsidence may be responsible for poor condition of the 7.5 km stretch of the Delhi-Gurgaon road, which has subsided by more than 70 cm in the past five years.
All precautions were taken before construction of the airport, a Delhi airport official said.
“Studies have been done before carrying out construction work in and around the airport premises, with no impact of any land subsidence noted,” the official said, requesting anonymity. “All due checks are carried out before any construction work and, at the airport in particular, no impact of the groundwater table reducing has been seen.”
The Central Ground Water Board is studying the impact of land subsidence, an official of the agency said. “We have been looking at the land subsidence aspect and have also initiated a project on land subsidence with IIRS Dehradun,” he said, declining to be named. “Once the findings are clearer, we can comment on the ground reality.” IIRS is short for Indian Institute of Remote Sensing.
The matter needs further investigation, according to Shashank Shekhar, water expert and professor of geology at Delhi University.
“The research correlates minor land subsidence with water level decline. It may be that water level decline have triggered minor adjustments in the surface topography, and if so, then it should be visible in other similar localities,” Shekhar said. “However, in geology there is continuous landscape evolution, mostly linked to tectonic activity. As the area mentioned are on flanks of the Delhi ridge, which is also a fault zone, it will be good to additionally factor in the neo-tectonic activity for accounting the minor adjustments in the land surface.”
Land sinking is possible due to excessive groundwater extraction, said Vikram Soni, conservationist and emeritus professor at Jawaharlal Nehru University, who has been working on issues related to water sector in Delhi.
The study also highlights an “uplift trend” in Dwarka which has been attributed to a rise in the groundwater table and the consequent reduction of effective stress in the soil.
“The sub-city region was undergoing subsidence with an approximate rate of 3.5 cm/year in 2014-16 which changed to uplift of 0.5cm/year in 2016-18 and 1.2cm/year in 2018-20,” the study said. “The main reason for this change in the behaviour of subsidence pattern can be associated with the swelling behaviour of soil. Delhi government introduced several strict policies to improve the groundwater condition in the area.”
Can subsidence be reversed? Garg said that a detailed hydro-geological study is required to understand aquifer properties, and available data are limited.
“The Central Ground Water Board, the Geological Survey of India, and the Ministry of Urban Affairs need to investigate whether these trends are reversible,” he said. “We suggest the government and policymakers should have a detailed understanding of the geophysical properties of the areas undergoing subsidence, and incentivize rainwater harvesting while ensuring strict implementation of laws against illegal mining of groundwater. The building conditions in hazard zones should be evaluated and proper measures should be taken.”