Groundwater experts urge further field investigations, noting that geological formation and seawater intrusion may limit Chennai's risk

Five Indian cities, including Chennai, face a rising threat of land subsidence that could increase the risk of damage to buildings due to groundwater over-exploitation, according to a recent study.

The study published in the journal, Nature Sustainability, noted that 2,406 buildings across Chennai, Delhi and Mumbai were at high risk of structural damage due to land subsidence. If the trend continued for five decades, an additional 23,529 buildings in Chennai, Delhi, Mumbai, Kolkata and Bengaluru could also face high risk.

Using InSAR, a satellite-based technique, and satellite radar observations from 2015 to 2023, the researchers found that people were exposed to gradual land subsidence, or sinking of the ground surface, of more than 4 mm per year.

However, groundwater experts say while the study provides an indication of the threat in major cities, the results need field validation for coastal cities such as Chennai, where depleting groundwater is often replaced by seawater. This would mean a lower risk of land subsidence. The findings should serve as a warning bell, highlighting the urgent need for sustainable groundwater management, they said.

The study, led by Nitheshnirmal Sadhasivam, United Nations University, Canada, found widespread subsidence in cities, including Chennai, where the ground is sinking by 31.7 mm per year.

Some hotspots of subsidence were observed around the floodplains of the Adyar and areas such as Valasaravakkam, Kodambakkam, Alandur, and Tondiarpet. The study also identified K.K. Nagar and Tondiarpet as major hotspots owing to groundwater over-extraction. It warned that nearly 958 buildings in Chennai could face high risk of damage in 30 years.

The compression of soft river alluvium deposits from Holocene fluvial sediments, especially in the floodplains of the Adyar, triggered by groundwater over-extraction, could be one of the causes of land sinking.

While land subsidence alone may not directly cause building collapse, the researchers said it could amplify the impact of other hazards, calling for land subsidence-resilient infrastructure.

Groundwater experts, however, emphasised the need for further field investigations. L. Elango, visiting faculty, Hydraulics and Water Resources Engineering Group, Department of Civil Engineering, IIT-Madras, said land subsidence was more likely in cities such as Kolkata. "We have also done a study on land subsidence threat in Kolkata, which has a confined aquifer overlaid by a thick layer of clay. When groundwater is over-extracted, the clay layer may lose water, leading to land subsidence," he said.

The scenario in Chennai is different, given its geological formation and predominantly unconfined aquifer. Groundwater withdrawal is less likely to cause land subsidence, as most areas recover their water levels during the monsoon. Additionally, areas close to the coast, such as Tondiarpet, experience seawater intrusion, and localities south of Adyar are rocky. The fluvial sediments of the Adyar river basin are of limited thickness.

"Data acquired from InSAR-based measurements may not always be accurate, as the map also shows regions with rising ground level," Prof. Elango said.