Groundwater extraction causing parts of Delhi surface to sink: Study
NEW DELHI: The alarming rate of ground water depletion in the Capital may be leading to a different kind of slow evolving crisis-- parts of the ground surface in the city sinking, a phenomenon known as land subsidence.
Now, using satellite data, researchers have found an area of around 100 square kilometres in Delhi-NCR under high risk of ground displacement, with the largest of these, around 12.5 sqkm, in SouthWest Delhi’s Kapashera, barely 800m from the airport.
In a study titled “Tracking hidden crisis in India’s capital from space: implications of unsustainable groundwater use”, published in nature, the researchers from IIT Bombay, German Research Centre for Geosciences, Cambridge and the Southern Methodist University, US said that 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.
“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 added.
Shagun Garg, a doctoral researcher from the University of Cambridge, part of the team which studied these land deformations, said that of all the areas under threat, Kapashera (near the Indira Gandhi International Airport) was the most vulnerable because the subsidence rate is extremely high.
“Airport requires stable ground because of the risk of major disruption if there’s significant ground movement. 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 the airport and its connecting roads is crucial,” he added.
The study found another deformation in Mahipalpur, just 500m away from the airport. The area saw deformation of 15mm/year in 2014-16, 30mm/year in 2016–2018 and 50mm per year in 2018–2019.
A widespread global problem, land subsidence occurs when large amounts of groundwater are withdrawn from some types of rocks and underground soil. When large amounts of underground water are extracted from aquifers, the clay between pockets of water collapse gradually leading to deflation of land beneath. It isn’t like a sinkhole or a cave-in whose impacts are sudden, but occurs over a very large area gradually.
With a projected demand of 1,236 MGD (million gallons per day), water-stressed Delhi has a 300MGD demand-supply gap.
According to the draft MPD 2041 (Master Plan for Delhi) , this demand is further likely to increase to 1,746MGD by 2031. A large part of unmet demands are met through groundwater extraction. At certain locations in southwest Delhi, the groundwater table is available at a depth of 80m below ground and this continues to deplete at the rate of 3–4 m/year.
According to the US Geological Survey (USGS), more than 80% of land subsidence across the world is caused due to excessive groundwater extraction
The areas at high risk that need immediate attention include Bijwasan, Samalkha, Kapashera, Sadh Nagar, Bindapur, and Mahavir enclave in Delhi; Dundahera, Sector 22A, and Block C in Gurgaon; and pockets 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
Around 100 sq km of area in NCR has high risk of ground displacement, with the largest of these, of around 12.5 sq km, in S-W Delhi’s Kapashera 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 DelhiGurgaon road which has “subsided” by more than 70 cm in the past five years. “Civic authorities can look into the possibility of subsidence-induced gradient change of the sewer pipe resulting in leakage of sewage, breaking open the pavement, and deterioration of the road condition. However, further investigation is required.”
A Delhi airport official said all precautions were taken before constructing the airport. “Studies have been done before carrying out construction work in and around the airport premises, with no impact of any land subsidence noted. All due checks are carried out before any construction work and at the airport in particular, no impact of the groundwater table possibly reducing, has been seen.” the official said.
An official with the Central Ground Water Board said the agency is studying the impact of land subsidence. “We have been looking at the land subsidence aspect and have also initiated a project on land subsidence with IIRS Dehradun. Once the findings are clearer, we can comment on the ground reality.”
Prof Shashank Shekhar, a water expert and professor of geology at Delhi University said the matter needs to be further investigated.
“The research correlates minor land subsidence with water level decline. It may be that water level decline triggered minor adjustments in the surface topography. If so, it should be visible in other similar localities. However, in geology there is continuous landscape evolution mostly linked to tectonic activity. As the areas mentioned are on flanks of Delhi ridge, which is also a fault zone, it will be good to factor in the neo-tectonic activity to account for the minor adjustments in the land surface.”
Professor Vikram Soni, a conservationist and emeritus professor at JNU working on issues related to water in Delhi said land sinking is possible due to excessive groundwater extraction. “Imagine the crosssection of land to be a tumbler filled with sand. We can fill it with water up to certain level, and water occupies the space between the sand particles. If this water is withdrawn, the particles will also shrink by 15-20%. It is not surprising that the land is sinking.”
The study also highlights an “uplift trend” in Dwarka, attributed to a rise in the groundwater table and the consequent reduction of effective stress in the soil. “The 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 main reason for this change... can be associated with swelling behaviour of soil. Delhi government introduced several strict policies to improve groundwater condition in the area.”
Can subsidence be reversed? “Central Ground Water Board (CGWB), Geological Survey of India (GSI), and Ministry of Urban Affairs need to look into investigate whether these trends are reversible. We suggest the government and policymakers should have a detailed understanding of the geophysical properties of the areas undergoing subsidence and incentivise 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,” said Garg.