Studies on central Gangetic alluvial plains in Lucknow reveal that the city’s aquifers are unable to sustain extraction levels and may be disappearing permanently and rapidly.
The exploitation of groundwater resources and unsustainable land use were highlighted as major causes for countless cases of sinking land, or land subsidence.
Using remote sensing techniques, researchers found many regions to be sinking annually at a rate of up to four cm.
A recent study reveals that unsustainable urbanisation is driving intense groundwater depletion in Lucknow, Uttar Pradesh. As the city’s population continues to rise (62 percent from 2001 to 2020), the immense pressure on irretrievable aquifers, accompanied by heavy land use, will adversely impact water security and land subsidence rates in the years to follow.
Apart from storing water, aquifers are crucial in providing support to the land above them. When the extraction of groundwater is far higher than the aquifer’s ability to replenish itself (as is the case in most of urban India), the land above may sink suddenly, or gradually over time – a phenomenon termed land subsidence.
“Almost every Indian city is facing a water crisis and a threat of land subsidence.”Vivek Grewal, hydrogeologist who runs the Twitter microblog Groundwater Resources of India.
While the exploitation of groundwater across the country for agricultural use is a clear cause, scientists are realising that rampant urbanisation also aggravates the issue of land subsidence, especially in northern India. Moreover, weaker land threatens the foundation of infrastructure, thus posing severe hazards.
Water and Land, Hand in Hand
One of the earliest studies of land subsidence in India uncovered the issue in Kolkata in 2006.
"From 2009 to 2015, many studies were coming out — especially focusing on the Indo- Gangetic plains.”Ashwani Raju, researcher at Banaras Hindu University in Uttar Pradesh.
“During this period, urbanisation in Lucknow was at its peak,” said Raju, a co-author of the recent paper on land subsidence in Lucknow.
Raju, a resident of Lucknow, recalls certain parts of the city sinking every year, including the “Mahanagar region, near the Lucknow Municipal Corporation office in the north part of the city – but the response of the authorities has been to fill it.”
Many other regions have witnessed “a noticeable change,” according to Raju. This motivated him and other scientists to employ remote sensing techniques to quantify the sinking of land here. Their findings conclude that Lucknow is sinking at a rate of four cm every year.
According to Raju, while there persists a deficit of data, the number of government-approved tubewells in the city more than doubled from 300 (2005) to 672 (2015). Estimates suggest that potential groundwater withdrawals could be close to 750 million litres per day in Lucknow alone.
As groundwater is extracted from aquifers, their regeneration capacity depends on the type of soil present above. In the case of Lucknow, many of the sinking spots that Raju pinpointed in his research consist of thick, clayey layers.
“Once clay loses its moisture, it cannot regain its property of storing water, resulting in its inelastic deformation.”Ashwani Raju, researcher at Banaras Hindu University in Uttar Pradesh.
This phenomenon effectively destroys the aquifer permanently, and little can be done when the land in that region begins to collapse consequently.
Most of north India has alluvial aquifers, with a component of clay in it, while those in the south are made up of hard rocks. This means that despite groundwater depletion and aquifer destruction in south India, chances of land subsidence occurring in that part of the country are lower, owing to the sturdier support from the sediment underground, explains Grewal.
Unregulated Extraction is Rampant in India
Indian cities are not an exception to sinking land. Most major cities across the world like Beijing, San Francisco, Tehran, and Bali are sinking rapidly too, at as much 20 to 30 cm. each year, according to Raju.
Vivek Grewal, who was not associated with the study, also points out that one of the worst cases of land subsidence known worldwide is in California, USA, where inordinate groundwater extraction was performed to cater to intensive agricultural activities. In many parts of the country, “the domestic supply is not maintained,” says Grewal, “and there is no centralised system of water extraction from the ground.”
Since the majority of Indian households are not linked with a proper water supply system, people are often left to fend for themselves and end up digging a personal borewell, increasing unmonitored extraction.
In such cases, where there is no regulation or record of the exploitation of groundwater, maintaining aquifers and addressing the problem is not so straightforward.
Depending on the type of bedrock and the depth of aquifers, groundwater can take anywhere from a month to several decades to replenish. Nevertheless, in urban environments with no open ground, “less than 10 percent of the water collected during rains can infiltrate the soil,” mentions Grewal.
Concrete blocks the seepage of water underground, thus limiting the rejuvenation of groundwater into the earth. Furthermore, open drains, improper sewage systems, and industries releasing unchecked pollutants into their surroundings, all loom over the future of our cities’ water security.
The key aquifers — which can be rejuvenated easily with monsoon rains — are generally contaminated and users often drill till they reach the cleaner water in deep sitting aquifers, making rejuvenation far slower.
Both Raju and Grewal stress the need for well-managed urban water and sanitation systems. They emphasise reducing the groundwater dependency of our cities, along with allowing more natural land to absorb water to recharge the aquifers.
(This article was originally published on the Mongabay-India. It has been republished here with permission.)
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