Editorial: Did the ground just move?

India’s urban planners normally see excessive extraction of groundwater as mainly a water supply-related problem. The other problem arising out of it, subsidence of land, is not given the importance it deserves. A recent study published in the journal Nature Sustainability has turned the spotlight on this creeping hazard in five of India’s largest cities.

Using satellite data covering more than 13 million structures, the study assessed risks posed to buildings, infrastructure, and populations in five Indian cities: the New Delhi National Capital Region, Chennai, Mumbai, Kolkata, and Bengaluru. It found that 878 square km of urban land in these cities is subsiding, mainly due to overextraction of groundwater. The maximum rate of subsidence was recorded in New Delhi (sinking 51.0 mm per year), followed by Chennai, Mumbai, Kolkata, and Bengaluru.

In Chennai (31.7 mm per year), 32 buildings are at “high damage risk” due to differential subsidence in the immediate term. And in the intermediate or far future, if no corrective or mitigating measures are taken, a staggering 97,946 buildings stand the risk of heavy damage. Subsidence risk is especially concentrated in the floodplain of the Adyar River and in the central built-up areas of Valasaravakkam, Kodambakkam, Alandur, and Tondiarpet, where there has been intense extraction of groundwater for decades.

Subsidence does not act uniformly or alone. Different localities sink at different rates, thereby weakening building foundations. Further, Chennai’s vulnerability is compounded by being a coastal city that regularly faces monsoon flood risk, which, in conjunction with subsidence, makes for double jeopardy.

The import of the study is clear. What’s happening unseen beneath one’s ground is not just a geophysical curiosity but a real hazard for thousands of buildings and millions of residents. The complacency that currently marks urban planning and policy implementation just will not do.

The first imperative is to commission a comprehensive survey of subsidence localities and identify buildings at risk. Fortunately, technology for this exists. High‐sensitivity sensors and high-res imagery allow us to map hotspots. In fact, Chennai ought to have already compiled a dataset of its entire inventory, recording the age of every structure, type of construction, maintenance status, etc. Such data would allow the municipality to integrate subsidence risk into building permitting processes.

The urgency of enforcing groundwater regulations, currently lying idle in the books, cannot be overemphasised. The need for compulsory borewell registration is obvious, but the requirement is to go much further: crack down on illegal extraction in all zones, make building permits willy-nilly conditional to groundwater licences and make aquifer recharge obligatory for all major users such as industries and large residential complexes.

The longer-term solution, of course, is to find water supply alternatives. All megacities, Chennai included, need to diversify water supply sources to reduce dependence on groundwater. While rainfall capture exists already as an option, alas largely unimplemented, cities need to do two simple things to begin with: promote water‐sensitive building design and return to permeable pavements. Plus, it’s time we mass adopted water reuse as an everyday option for everything except potable purposes.

The Nature study makes it plain that land subsidence is not a marginal issue in the five cities it studied. The ground is literally slipping beneath the weight of our urban agglomerations. If we do nothing, the problem won’t go away; it will take us down with it.