Writing on the wall: Groundwater exploitation is triggering subsidence in Indo-Gangetic plain

All residential societies in Kapashera extract groundwater. But we never thought it could lead to the sinking of our land, said Rajesh Gera, president of Surya Vihar Housing society in southwest Delhi, which is less than 10 kilometres from the Indira Gandhi International Airport.

In 2014, one of the pillars in the parking lot of the society developed cracks. By 2019, the crack had widened so much that the entire building could have collapsed. This June, when Down To Earth (DTE) visited the society, Gera showed the repaired crack, which had reached more than 1.5 metres in height.

“We initially ignored it and then blamed the builder for poor construction. Now we know it is happening due to land subsidence triggered by excessive groundwater extraction,” he said.

Residents tell DTE that apart from the threat of a collapse, they are also seeing the groundwater levels declining rapidly. In early 2000, the society had four borewells at a depth of 200 m. By 2020, two of them had dried up.

Shagun Garg, who is currently at the University of Cambridge, the United Kingdom, had established the link between groundwater extraction and land subsidence in Delhi in 2020. He had found that land subsidence at Kapashera was happening at a rate of 17 cm per year, or roughly the length of a smartphone.

Garg came to this conclusion after he monitored the land subsidence rate at four locations in the National Capital Region in 2014-16, in 2016-18 and in 2018-20. Apart from the residential society, he found subsidence at Fun and Food waterpark in Surya Vihar and at Raja Nahar Singh International Cricket Stadium and Piyush Mahendra Mall in Faridabad.

“Groundwater generally resides in pores or aquifers within the soil. When large amounts of groundwater are extracted year after year, a void is created in the pores. This causes collapse or compaction of the soil, leading to land subsidence,” said Vineet K Gahalaut, chief scientist at the National Geophysical Research Institute, Hyderabad.

The Indo-Gangetic plain, which has stratified layers of sand and clay, is highly prone to subsidence, he added.

Besides Delhi, researchers have recorded land subsidence in several other cities including Chandigarh, Ambala, Gandhinagar and Kolkata. The only study to understand the impact of groundwater extraction in rural areas has been in Punjab and Haryana. The study, carried out by a team of five Indian researchers, was published in Springer in October 2020.

“After the high agricultural advancement and practices, the two states saw tremendous ground-water dependency for irrigation,” said Pranshu Pranjal, assistant professor at the Vellore Institute of Technology in Bhopal and one of the authors of the report. “We found villages after villages with cracked houses across Punjab and Haryana.”

Although the two states lie in an arid to semi-arid zone, which sees moderate precipitation in the monsoon months, this is not enough to recharge aquifers to their previous level.

This is causing land deformation, which is mostly witnessing tensional, compressional and shear cracks (rifts in concrete beams) with vertical, horizontal and diagonal orientations in nature, said the study.

Pranjal warns that while the problem is found in both urban and rural areas, the current focus is only on cities. “City administrators are slowly trying to ensure water supply from rivers and waterbodies to ensure less consumption of groundwater,” he said.

All the researchers that DTE spoke with suggest that there may be many more areas where subsidence is already happening but has not been recorded yet.

“Such events can be spread over a radius of a few km to several km. So, it requires studying of localised satellite images that show subsidence, corroborated with groundwater extraction rates. Finally, ground verification is required to understand the damage and scale. The entire process is complex and time-consuming,” said Pranjal.

DTE analysed state-wise changes in average groundwater levels between 2000 and 2022 and found it had worsened in 10 states and Union Territories. Punjab was the worst, with a groundwater table drop of 150 m — or twice the size of the Qutub Minar — between 2000 and 2022. It is followed by Meghalaya (13 m) and Uttar Pradesh (10.6 m) (see ‘Vulnerable stretch’).

A global crisis

The first case of land subsidence due to groundwater extraction was reported in California, the United States, in the early 1990s, where families were evacuated after certain regions recorded subsidence of up to 150 m over 50 years.

In California’s San Joaquin Valley, land continues to sink by 0.3 m per year due to excessive pumping of groundwater for a commercial orchard, which has caused permanent subsidence and landslides in the area.

In Southeast Asia, the rapid growth of megacities has led to a pressing design problem that many governments are only just starting to address.

Jakarta is considered the world’s fastest sinking city. With 40 per cent of the city already below sea level, it is predicted that by 2050, some 95 per cent of North Jakarta will be underwater.

Bangkok in Thailand and Ho Chi Minh City in Viet Nam are also sinking, with subsidence rates of up to 2 cm and 5 cm per year, respectively.

“Of all the cases of land subsidence worldwide, 77 per cent are caused by human activities, with groundwater extraction accounting for 60 per cent,” wrote Tamil Salvi Mari, programme director and senior lecturer in architecture at Taylor’s University, Malaysia, in a recent article titled Sinking cities: Two ways to fight land subsidence.

Subsidence damages critical infrastructure such as buildings and roads, changes drainage patterns and increases the risk of flooding, she added.

Simple solutions

Land subsidence cannot be reversed by groundwater recharge, according to experts. So, the only solution is to arrest the overextraction of groundwater. This can be achieved through a series of interventions, starting with water budgeting for high-risk regions.

In India, several villages have already adopted water budgets, where people compute the amount of water available and used. Cities should also implement similar budgets. Another step, suggests Pranjal, is reviving waterbodies that can aid groundwater recharge.

“Waterbodies are a part of the urban landscape in the US,” he said. In India, as per the recently released waterbody census, almost one out of every four urban waterbodies is defunct.

This was first published in Down To Earth’s print edition (dated July 1-15, 2023)

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