Groundwater pumping linked to land subsidence in India’s capital

Unregulated pumping of groundwater due to rapid urbanisation and population growth has been blamed for the steady sinking of major cities, such as Jakarta in Indonesia. A recent study¹ now shows that a similar crisis has beset parts of India’s capital Delhi.

Land subsidence, the caving in or sinking of an area of land due to natural events like earthquakes and human activity like underground extraction of water, oil, gas and minerals, is a global geological hazard.

The study’s authors found excess groundwater extraction had led to dangerous land subsidence in Kapashera, an urban village near Delhi’s international airport, and in Faridabad, a part of Delhi’s larger metropolitan area. Dwarka, in south west Delhi, was identified as experiencing uplift. Further analysis revealed that about 100 sq km in and around Delhi were at high risk of subsidence.

The team analysed and processed images captured by Sentinel-1, an Earth-orbiting satellite launched by the European Space Agency in 2014. Using differential interferometry, a remote sensing technique that exploits radar signals from microwave sensors onboard Sentinel-1, millimetre changes in the Earth's terrain are detected.

The subsidence was more than 11 cm/year in Kapashera and 3 cm/year in Faridabad. Both areas showed worsening subsidence between 2014 and 2020 that correlated with the rate of groundwater depletion. In contrast, Dwarka, which was previously undergoing subsidence, showed an uplift. The authors attribute this upturn to resilient groundwater management systems, such as incentivising rainwater harvesting.

Lead author Shagun Garg, a doctoral researcher at the University of Cambridge, told Nature India that this is the first extensive study on groundwater-induced subsidence in Delhi.

Ryan Smith, assistant professor at Missouri University of Science and Technology, and not part of the study, told Nature India, “This study provides a great first look at subsidence trends”.

Francesca Cigna, senior researcher in Earth observation at the Institute of Atmospheric Sciences and Climate, National Research Council, Rome, who was not involved in the research, called the study “timely” as India has the highest annual groundwater withdrawal rate globally making large regions prone to subsidence. The consolidated satellite-based technology used by the authors allowed them to “take the pulse” of the Earth’s landmass, she said.

“Another interesting element is that the subsidence hotspots in Delhi are located in fast-growing industrial centres, peripheral districts and slum settlements. This confirms that subsidence is often associated with urban development dynamics,” said Cigna.

An official from India’s Central Ground Water Board, who requested anonymity, told Nature India that there are no manifestations yet of subsidence, such as tilting of buildings, cracks in underground water pipes, and so on, in the two areas the authors cited. He said ‘ground truth’ studies—information collected on location by direct observation and measurement using advanced ground-level instruments —are vital to establish subsidence and if it correlates with groundwater depletion.

Study co-author Mahdi Motagh, professor at GFZ German Research Centre for Geosciences, Potsdam, said that advanced satellite technology can accurately identify hotspots afflicted by land deformation, which can prompt local authorities to carry out ground measurements. The authors hope the study prompts regulatory bodies to investigate and act on the data.

Preeti Tewari, associate professor of geography at University of Delhi, explained that the pressure that water exerts in aquifers (the rock or sediment layer that holds water) supports the ground above. Withdrawal of water creates a fall in this pressure making the soil compact, which leads to surface subsidence.

Associate professor of hydrogeology at IIT Kharagpur Abhijit Mukherjee said that subsidence is mostly observed in cities built on soft sediments. Coastal cities of Khulna and Kolkata in the Ganges-Brahmaputra delta, Hanoi in the Red River and Mekong delta, the California bay area, Mexico City, several basins in Iran and northeast China are experiencing significant subsidence.

On the vulnerability of Delhi to subsidence, Smith said that “aquifers that are pressurized and have lots of clay are most prone to subsidence.” The soil in Delhi is mainly thick alluvium, which is clay-rich and thus vulnerable. It can cause significant damage to buildings, bridges, pipelines, railways and canals. Even minor subsidence in densely populated areas can put lives at risk.

Solutions to offset the over-pumping of groundwater are recharging groundwater by rainwater harvesting, managed aquifer recharge, pond revival, curbing illegal groundwater pumping, and planting native species that can help in groundwater conservation. In rural areas check dams can help augment water resources.