[Explainer] How falling groundwater levels compact the land above

Beneath the vast, fertile northern plains of India, groundwater has been vanishing at a rampant rate. According to the U.S. National Aeronautics and Space Administration (NASA), the groundwater in north India has decreased by 88 million acre-feet in the last decade.

The unsustainable use and decline of groundwater, leads to water becoming scarce for future generations, but it is not limited to that. Scientists are actively studying how the reduction in underground water is compacting the land above, causing irreversible changes in the aquifers. Experts warn, as the water from the aquifers keeps disappearing, the land may suddenly or gradually sink, leading to a phenomenon known as land subsidence.

How does underground water affect the land above?

The underground water exists deep in the earth surface, where it occupies the void left by soil pores or rock cracks. At those depths, water is pressurised and it pushes the earth up. “The weight of the earth is shared by both the water and the matter underneath,” says Shagun Garg, a civil engineer and researcher at University of Cambridge. “When the underground water is drawn out excessively, the matter underneath becomes the sole entity for managing the load.”

India is ranked number one for its excessive groundwater usage. Even within India, the northern Gangetic plains are exploited more than other parts. The impact of disappearing groundwater is accelerating changes on the shape of the land surface.

Garg studied the alluvial aquifers in India in the Delhi NCR region and found that the land has sunk and continues to sink at a substantial rate. Certain parts of Delhi like Kapashera, near the Indira Gandhi International airport, witnessed a land subsidence rate of 11 cm per year during 2014-2016, which grew to more than 17 cm per year during the two years that followed.

Apart from Garg, other researchers have also studied the Gangetic region from Punjab to West Bengal and Gujarat and have found prevalent evidence of land sinking.

Is land compaction caused only due to groundwater levels? 

The sinking of earth depends on many factors, including the fragility and type of earth. “If limestone presence in the ground is high, it can dissolve in the water,” further leading to compaction, explains Garg. “In some cases, other reasons like excessive mining of coal or petroleum can also play a role.”

But in cases where the earth matter is made up of thin soil particles, like the alluvial deposits of fertile Gangetic plains, the sinking of land becomes more prevalent when compared with the hard rocks, explains Vivek Grewal, a hydrogeologist, who also runs a Twitter microblog Groundwater Resources of India, discussing the groundwater scenario of the country.

However, Grewal emphasises that the compaction of land that we are witnessing is fully driven by anthropogenic activities. The movement of tectonic plates which may also result in land subsidence, happens over millions of years. “But,” he says, “changes as dynamic as movement of land by 1 cm per year is not geological time, it’s anthropogenic time.” Garg adds, “Over 80% of land subsidence happens due to groundwater loss.”

What does sinking land lead to? 

Sinking of land can mean many things. Garg elucidates that if the sinking of land is spread over a wide region, then it may mean that the area may become prone to flooding. But in cases where the rate of sinking is differential, it may impact the civil infrastructure like roads, buildings, and houses. It might lead to the weakening of foundations or develop cracks in the buildings.

To estimate the real impacts of land subsidence at a particular location, computational data, overlapped with detailed ground analysis is required. Nevertheless, it is estimated that by 2040, land subsidence will affect about 8% of the world’s surface and about 1.2 billion inhabitants living in 21% of the major cities worldwide. Like many other disasters, it is predicted that land subsidence will impact Asia more than other parts of the world. With 86% of Asian population exposed to effects of land subsidence, about US$8.17 trillion is potentially at stake.

How real and prevalent is land subsidence? 

Grewal highlights that one of the earliest reported cases of land subsidence was in California, USA — a region which has been witnessing a decline in groundwater since the 1930s, mainly due to agricultural practices.

Land subsidence has been witnessed in various parts of the world where exploitation of groundwater has been high. One of the most prominent cases of land subsidence is in Mexico, where buildings have been tilting due to differential rate of land sinking. In Indonesia, over the last decade, the capital city Jakarta has sunk more than 2.5 meters. The problem so grave that the government is planning to shift the capital. Countries like Iran and China too, have witnessed prominent land subsidence in the last few decades.

Can we fix the land affected by subsidence?

Recharge of ground water is a natural process. We can reduce land subsidence if the recharge of groundwater is equivalent to quantity exploited, or by using water sustainably.

However, research suggests that groundwater is exploited so much in the country that the rate of restoration far exceeds the consumption. With annual use of about 433 billion cubic metres of groundwater, reports have suggested that aquifers in India are in critical state.

In such cases, Grewal suggests Managed Aquifer Recharge (MAR), as a possible solution. MAR is often known as the process of hydrological banking — an artificial method of restoring groundwater levels.

But in other cases, if the aquifer has lost its storage capacity, the changes may be irreversible, mentions Garg. “Once the earth underneath loses its capacity to hold water and the land has sunk, it’s not possible to get the property of the aquifers back.” Grewal agrees and adds that the process is rather inelastic.

For example, Garg found in his research that the aquifer capacity in certain parts of Kapashera is lost.  But, we still have to conduct research and understand the whole phenomenon from a geological point of view. “Unfortunately,” Garg concludes, “India does not have enough research on the same.”

Read more: Clean tap water mission in Puri hopes to replenish groundwater

Banner image: Experts studying groundwater state that the compaction of land that we are witnessing is totally driven by anthropogenic activities. Photo by Anil/ Pixahive.